15 Homeostasis
The maintenance of narrow range of conditions in the body
despite changes in environment is known as homeostasis (homeo=same; stasis=state).
Due to this stable internal environment, cells can function efficiently even
when conditions outside the body become unfavorable.
Receptors are organs that detect changes in the environment.
Upon receiving a signal from the receptor, a control center (usually
brain etc.) generates output that triggers a response. This response is carried
out by effector organs.
Negative feedback is a control mechanism that reduces
the original stimulus. For example, excessive heat is produced during exercise.
Nervous system detects this increase in temperature and triggers sweating. Evaporation
of moisture from the skin cools the body, helping return the body temperature
to normal.
Osmoregulation
Control of solute concentration and water balance in the
body is called osmoregulation. All the chemical reactions occurring in
the cells take place in aqueous environment. Osmoregulation is necessary for
these reactions.
A hypertonic solution has a higher solute concentration
than the cell. When a cell is placed in hypertonic solution, water molecules
move out of the cell by osmosis. The cells become flaccid due to shrinking.
Isotonic solutionshave equal solute concentration to
that of the cell.Water does not enter or leave the cell, when the cell is
placed in isotonic solution.
Hypotonic solution has a lower solute concentration
than the cell. When a cell is placed in hypotonic solution, water enters the
cell through osmosis and the cell becomes turgid(very firm).
Osmoregulation in Plants
Plants can be divided into three categories on the basis of
availability of water.
Hydrophytes are plants that live in or around water
(e.g. in ponds and lakes). The cells of these plants are constantly flooded
with water. Stomata are present on the upper surface of leaves. Excessive
transpiration helps to get rid of water.
Mesophytes are plants that have moderate supply of
water. Upper surface of the leaves is covered in a waxy cuticle to prevent
excessive water loss. The lower surface of leaves have stomata which can be
opened or closed according to water availability.
Xerophytes are plants that live xeric (dry)
conditions. These have thick, small leaves to prevent water loss. The waxy
cuticle is thick and leathery. Stomata are sunken deep in the tissues to
prevent water loss. When water is available, it is stored in succulent stem.
Osmoregulation in Animals
There are two general strategies in the animal world in
terms of osmoregulation.
Osmoconformers are organisms that do not actively adjust
their internal salt concentration and water balance. Their internal conditions
change with environmental conditions. This strategy is good as it does not
require any energy. However, if the environment changes too much, the organisms
may die. These organisms live in environments that do not change much.
Osmoregulators are organisms that actively adjust
their internal salt concentration and water balance despite fluctuations in
external environment. They are capable of living in environments that may
change abruptly. However, to be able to live in such environments, they must
constantly spend energy.
Different land organisms are capable of tolerating different
levels of dehydration. This ability is called anhydrobiosis.
Excretion
During metabolism, many chemicals are produced which are not
required by the body. Elimination of these metabolic wastes from the body is
termed excretion.
Metabolism of carbohydrates and lipids produces CO2
and H2O. Metabolism of amino acids and nucleic acids, on the other
hand produces nitrogen containing compounds. These nitrogenous wastes
can become toxic for cells and are therefore major excretory product.
Excretion in Plants
In plants, CO2 and H2O are produced
during respiration and O2 is produced during photosynthesis. These
can be considered as waste products. Gases are usually exchanged through stomata
while most of the water is used to maintain turgor pressure and transpiration.
Plant cells have a single large central vacuole which stores
nutrients as well as wastes. Most of the plants store waste chemicals in their
leaves. These leaves become yellow due to accumulation of waste products and
later fall off. Leaves are therefore called excretophores as they have
important function in excretion.
Some plants, like ebony tree, stores waste materials in old
xylem of stem. These trees have very dark wood in the center due to storage of
these chemicals.
Some plants like conifers secrete waste substances in the
soil around their roots. This not only helps in excretion but also kills any
nearby plants that try to take ground in the same space, thus decreasing competition.
Excretion in Animals
Animals living in marine water must excrete excess salts.
Those living in freshwater must remove excess water from their body.
The major excretory product in animals is the –NH2
group produced during catabolism of amino acids. These amino groups are
transferred to other molecules for reuse or excretion. Most of amino groups are
converted to ammonia to be dissolved in water and excreted. Some animals
convert this ammonia into urea or uric acid to conserve water.
Some of waste nitrogen is excreted in the form of chemicals
like creatinine, trimethylamine oxide and in very dilute amounts as amino acids
and nucleic acids.
Organisms that excrete nitrogen in the form of ammonia are
called ammonotelic. One gram ammonia requires about 500mL of water for
its removal. This strategy can only work for organisms that live in freshwater
and have abundant supply of water.
Organisms that excrete waste nitrogen in the form of urea
are called ureotelic. One gram urea requires about 50mL water for its
removal. Most terrestrial animals with moderate supply of water are ureotelic.
Organisms that excrete waste nitrogen in the form of uric
acid are called uricotelic. One gram uric acid requires about 1mL of
water for its removal. Terrestrial animals that live under severe shortage of
water have adapted for this mode of excretion.
Excretion in representative animals
In simple freshwater organisms like hydra, the body is very
simple. Nitrogenous wastes (mainly ammonia) leave their body by simple diffusion.
In flatworms, like Planaria, the excretory system consists
of a network of branches called protonephridia. (proto=first;
nephros=kidney) These branched tubules extend throughout the body and each
branch is capped by a special cell, called a flame cell. The flame cell
absorbs wastes from the surrounding areas and drives them in tubules by a tuft
of cilia. Protonephridia of freshwater flatworms remove excess water while
those of parasitic flatworms excrete nitrogenous wastes.
In annelids like earth worm, the body is segmented. Each
segments contains a pair of tubules called metanephridia (meta=medium;
nephros=kidney). Each metanephridium has a ciliated opening called nephrostome
(stoma=mouth). The nephrostome absorbs coelomic fluid from its surrounding area.
As the fluid moves through tubules, water and important substances are
reabsorbed in blood capillaries that are present around the tubules. The
remaining concentrated urine is excreted through nephridiopore.
In terrestrial arthropods like insects, the excretory system
is connected directly with digestive system. These are specialized tubules
called Malpighian tubules. The bodies of these insects are filled with a
special fluid called hemolymph. All the nutrients as well as waste
products are dissolved in hemolymph. Malpighian tubules absorb excess salts and
waste materials from hemolymph and pass it on to hind gut. When it reaches
rectum, all the necessary salts and water are reabsorbed while uric acid
crystals are produced in the form of solid excreta.
In early vertebrate ancestors, like hagfishes, the kidneys
are segmented tubules like those in earth worms. In mammals, the function of
excretion is performed by two kidneys.
Excretion in Humans
Liver plays an important role in excretion as it
converts water insoluble substances into soluble ones. It helps to neutralize
toxic substances like food additives and pesticides into less harmful and water
soluble products which can be excreted by kidneys. Drugs are also metabolized
by liver for same purpose. Liver has special enzymes that perform this job.
The ammonia produced during metabolism is converted into
urea by the liver. This is done by a set of chemical reactions that constitute
the urea cycle. During this cycle, ornithine combines with a molecule of
ammonia and carbon dioxide to form citrulline. Another ammonia is added to
citrulline to convert it into argino-succinate. Argino-succinate is converted
to arginine. Arginase enzyme removes urea molecule from arginine to produce
ornithine. Thus, ornithine that was used at the start of cycle is generated
again while net result is combination of two ammonia and one CO2 to
form urea.
Liver also produces a number of blood proteins like albumin
(which maintains osmotic balance of blood), fibrinogen (used for blood
clotting), bile (for fat digestion) and a number of lipids and cholesterol. It
also functions in storage of iron (for hemoglobin) and glycogen (for energy
production). It also converts used hemoglobin into bilirubin for its removal
from body. Creatine (used by muscles for energy production) is converted into
creatinine by liver for excretion.
Gross Anatomy of Kidneys
Human body contains two bean shaped kidneys present
in abdominal cavity attached to dorsal body wall. Each kidney is supplied blood
by renal arteries and the blood leaves through renal veins. The
urine produced by kidneys is collected in a central cavity call pelvis.
Urine leaves kidney through a duct called ureter. Ureter from each kidney
empties in urinary bladder. Urinary bladder stores urine and empties by
an opening called urethra. The emptying of urinary bladder is controlled
by a ring of muscles called sphincter which is under voluntary control
of brain.
In longitudinal section of kidney, two regions can be seen.
The outer, lighter region is called cortex and contains most parts of
nephrons. The inner, darker region is called medulla and contains
collecting ducts and loop of Henle. The nephrons present near the junction of
cortex and medulla are called juxtamedullary nephrons and help to
produce concentrated urine.
Structure and Function of Nephrons
Each kidney is composed of millions of functional units
called nephrons. A nephron can be divided into five parts and each part
has a special function.
The first portion of nephron is called Bowman’s capsule.
It is a cup shaped structure that encloses a ball of capillaries called glomerulus.
Blood enters the glomerulus through Afferent arteriole and leaves it
through Efferent arteriole. The main function of Bowman’s capsule is filtration.
The pressure of blood provides the force required for filtration. When blood
passes through glomerulus, almost all water and soluble parts of blood (except
cells) enter Bowman’s capsule to produce Bowman’s filtrate.
The second portion of nephron is called proximal tubules.
These tubules are surrounded by blood capillaries. The main function of
proximal tubules is reabsorption of important materials like glucose and
salts from Bowman’s filtrate by active transport.
The third portion of nephron is Loop of Henle which
is a U shaped tubular portion. It runs down from the cortex into medulla and
comes back again. The function of this portion is reabsorption of water.
The concentration of salts is kept high in medulla due to which almost 99.5%
water from Bowman’s filtrate is reabsorbed by osmosis. Loop of Henle is
sometimes surrounded by blood capillaries called Vasa Recta.
The fourth portion of nephron is distal tubules. The
main function of this portion is secretion. It removes H+
ions from blood by active transport and helps to maintain blood pH.
The fifth portion of nephron is collecting duct. Main
function of this portion is determining the final concentration of urine.
If excess water is present in the body, the urine is diluted to remove water
from body. If water is not available, the urine is concentrated in this region
to conserve water.
Control of Urine Concentration
In humans, two mechanisms maintain the concentration of
urine: counter-current multiplier, and hormonal control.
The concentration of salts is kept high in medullary region
to increase the rate of osmosis. This helps in reabsorption of as much water
from the Bowman’s filtrate as possible. Moreover, the direction of blood flow
in surrounding capillaries is kept opposite to that of filtrate to maximize
transport of substances. This is called counter-current multiplier.
Aldosterone is a hormone that is secreted by adrenal
cortex. It promotes the active uptake of sodium ions in loop of Henle. Anti-diuretic
hormone is produced by posterior lobe of pituitary gland. This hormone is
produced almost all the time and promotes reabsorption of water from loop of
Henle. When water is in abundant supply, this hormone is not produced and
kidneys do not reabsorb water, due to which, dilute urine is produced.
Problems in Kidney
Hyper-calcemia (hyper=high) is a metabolic disease,
in which level of calcium is higher in blood than required. Hyper-oxaluria
is another disease in which level of oxalate ions is higher than body
requirement. Due to constantly elevated levels of calcium and oxalate ions, calcium
oxalate can precipitate in kidneys and produce kidney stones. High
amount of oxalate ions are present in green vegetables and tomatoes, which may
become a cause of kidney stones. 70% stones in kidney are formed of calcium
oxalate. While calcium phosphate stones are the cause in 15% cases and uric
acid stones are the cause in 10% cases.
Lithotripsy is a technique used for non-surgical removal
of kidney stones. In this technique, a concentrated beam of X-rays or
shockwaves is directed towards the stone. As a result, the stone is broken into
fine sand which is removed by urine.
Long-term damage to kidneys results in their failure to
perform their function properly. This is called Renal Failure. The main
reason of renal failure is the damage to glomerular part. Kidney failure
results in high levels of urea and other wastes in the blood which damage
different organs of the body and death may ensue. This condition is called uremia.
In case of kidney failure, urea is removed from the blood
through an artificial purification process called dialysis. Dialysis can
be performed in two ways. In most cases, the blood passes through a machine,
called a dialyzer, which removes waste materials from it. In peritoneal
dialysis, the blood is passed through a cavity in the body called peritoneal
cavity. This cavity is separated by a thin lining of calls. On one side of
cells a special fluid is filled which absorbs urea from blood passing from the
other side of cells. This fluid can then be removed from the body.
Dialysis is only a temporary solution while a permanent
solution of kidney failure or end stage renal diseaseis a kidney transplant.
Kidney from a suitable donor is transplanted in the body of the patient by
surgery which may clean the blood.
Thermoregulation
Maintenance of internal body temperature despite changes in
external environment is called thermoregulation. All chemical and
physical processes in the body require energy. Thermoregulation is therefore
very important for organisms. Very high heat can denature proteins, while cold
can result in freezing or inactivation of enzymes.
Thermoregulation in Plants
At high temperatures, plants use evaporative cooling
to regulate their temperature. Water is constantly lost during transpiration
which cools their body. If water is not available, plants may get damaged due
to heat.
Some plants store special proteins in their cells called heat-shock
proteins. These proteins embrace enzymes if temperature is very high to
prevent it from denaturation.
At very low temperatures, the lipids in plasma membrane may
solidify and all traffic across the membrane may stop. Plants prevent this from
happening by accumulating unsaturated fatty acids in their plasma
membranes. These unsaturated fatty acids do not solidify at low temperatures.
However, in case of rapid chilling, these plants may not be able to prepare unsaturated
fatty acids and may suffer.
At freezing temperatures, ice crystals may form
inside the cells. These crystals may perforate membranes and damage organelles.
To prevent this, plants store large amount of salts at the arrival of winter.
These salts work as anti-freeze in cells.
Thermoregulation in Animals
In animals, heat is gained either through metabolic
processes or from the environment (air / sunlight). However, the heat is
constantly lost to colder air or water in which an animal lives. An animal must
balance its heat loss with heat gain to maintain a relatively constant
temperature.
Animals can be divided into two groups on the basis of
stability of their body temperature.Poikilotherms(poikilo=variable) are
organisms that cannot maintain a stable body temperature. Their body
temperature changes with temperature of water or air in which they live. These
include all invertebrates, fishes, amphibians and reptiles.Homeotherms
(homeo=same) are organisms which can maintain their body temperature despite
changes in environmental temperature. Only birds and mammals are included in
this group.
On the basis of source of heat, animals can be divided into
three groups. Ectotherms (ecto=outside) are organisms which cannot
generate enough heat by metabolism and therefore absorb heat from their environment
to remain active. Most invertebrates, fish, amphibians and mammals are in this
category. Endotherms (endo=inside) are organisms which can generate
enough body heat from metabolism. These do not depend on heat from environment
and can spend more active lives. Heterotherms are organisms which have
the ability to generate body heat by metabolism but their body temperature is
not fixed and can vary to great extent. These include bats and humming birds.
Adaptations for Heat Regulation
Animals have adaptations at different levels that help them
maintain their body temperature. These include structural, physiological and
behavioral changes.
Structural adaptations include long term changes in
the body for protection from environment. For example some organisms store fat
under skin for protection from cold. Some mammals have a thick layer of fat under
their skin for insulation, called blubber. Some organisms have hair (pelage)
on their skin which traps air and protects from cold. Some organisms have sweat
glands that produce water when body is heated excessively. This water
evaporates and cools the body. Some organisms have respiratory surfaces adapted
for panting, which cools the body. Some mammals possess brown fat,
which helps to generate high amount of heat when required.
Some organisms can change their body physiology in
response to temperature change. For example, if excess heat is produced in a
mammal, blood flow to skin is increased which helps to dissipate heat.
In cold environment, blood flow to skin is reduced to prevent heat loss.
Production of sweat can also be considered as a physiological mechanism.
Birds are capable of fluffing their feathers to dissipate heat. Some mammals
can raise their fur to trap air between hairs and protect from cold
environment.
Some animals adapt to environment by changing their behavior.
They may hide at cool places during a hot day or bask in sun at cold days. They
can change their body posture to increase or decrease heat loss from the
body. Some organisms (like bats) may urinate on themselves to promote
evaporative cooling.
Thermoregulation in Humans
Humans and other mammals, maintain their body temperature
around 37oC. This helps them to work round the clock and live in a
variety of habitats around the world. The heat in their body is produced by two
mechanisms.
Shivering thermogenesis (thermo=heat; genesis=generation)
produces heat in the body by action of muscles. Non-shivering thermogenesis
produces heat by hormonal mechanisms. For example, hormones of thyroid gland
control the heat production in the body by increasing rate of metabolism.
Excess heath can be dissipated from body by increasing blood
flow to skin. Vasodilation (vaso=vessel; dilation=expansion) is the
increase in diameter of blood vessels to allow more blood flow. Vasoconstriction
(constriction=tightening) is the decrease in diameter of blood vessels to
decrease blood flow. Both of these mechanisms help to redirect blood flow to
different organs to body and to conserve or dissipate heat.
Pyrexia
Pyrogens are compounds that are produced by infectious
agents or by white blood cells detecting those infectious agents. These
chemicals change the set point of hypothalamus from 37OC to higher
temperature causing fever. Fever (or pyrexia) is an adaptive response
against infections that helps the body fight against infectious agents.
16 Support & Movement
Support in Plants
In young plants and herbaceous parts, the support is
provided by hydrostatic pressure, called turgor pressure, which keeps
them rigid and firm.
Plant cells contain a single large central vacuole. The
membrane of this vacuole is called tonoplast and contains many protein
pumps that transport ions inside the vacuole by active transport. The water
then moves inside the vacuole due to osmosis and exerts pressure on the walls.
The cell therefore becomes turgid. In the absence / shortage of water,
this pressure is lost. As the cells become flaccid, the plant wilts.
There are two types of tissues that provide support to
plants.
Collenchyma cells provide support to young parts of
the plant.They are elongated cells with thick primary walls.These cells are
alive and do not have secondary cell walls. These are elastic in nature and
therefore do not restrict growth of the plant.
Sclerenchyma cells (scleros=hard) are specialized
cells that have thick secondary cell walls.They are specialized to provide
support to the plant. These are very rigid and contain large amounts of lignin.Lignin
is the main component of the wood. Mature sclerenchyma cells are not alive and
do not show growth.
There are three types of sclerenchyma cells.
1.
Fibers or tracheids are long and
cylindrical cells that are present in the form of bundles or as bundle caps.
These are present in the xylem.
2.
Sclereids are shorter than fibers and
make up seed coat and nuts. Their main function is protection.
3.
Vessels or tracheae are long tubular
cells that are joined from end to end to form conduction vessels in xylem.
Secondary Growth in Plants
Growth involves increase in number of cells. Growth
in plants can be divided into two types: increase in length and increase in
girth or the stem.
Primary growthis the increase in length of the plant.
Apical meristem is involved in primary growth.
Secondary growthis the increase in girth / thickness
of the stem. Lateral meristem is involved in secondary growth. Two types
of tissues make up the lateral meristem: vascular cambium and the cork cambium.
Vascular cambium is present in the form of a ring between
xylem and phloem. Its cells are capable of active division. Each year, new
cells of xylem and phloem are produced by division of vascular cambium, called secondary
xylem and secondary phloem.When trees are cut, these rings can be
counted to determine the age of the tree. These are called annual growth
rings. Secondary xylem is produced towards the inside of the ring of
vascular cambium while secondary phloem is produced towards outside.
Only the newest layers of xylem and phloem are capable of
conduction as the older tissues die. The outermost, living portion of xylem
makes the sapwood, while the innermost, dead portion is called heartwood.
In most trees, waste substances and special chemicals are stored in heartwood.
These chemicals provide resistance against decay and insect attack.
In case of injury to the plant, the wounded portion is
filled by the cambium. It produces a parenchymatous tissue called callus.
It also helps to join two branches during grafting (a technique for producing
better plant varieties).
Cork is a special type of tissue produced by cork
cambium.Cork produced by plants like Quercussuber is of commercial importance.
Movements in Plants
All living organisms respond to environmental stimuli.
Plants usually respond to environmental stimuli by showing changes in their
growth patterns. However, other types of movements are also shown by plants.
A movement is called positive if it is towards a stimulus
or negative if it is away from a stimulus. Movements in plants can be divided
into two main groups: autonomic movements and paratonic movements.
Autonomic Movements
Autonomic movements (auto=self) are spontaneous
movements due to internal causes. These are of three types.
Tacticmovements(taxis=locomotion) are movements of an
entire organism due to internal causes. On the basis of type of stimulus, they
can be divided into two types:
1.
Phototacticmovement(photo=light; taxis=locomotion)
is the movement of an entire organism in response to light. Positive
phototactic movement is movement towards the source of light, while negative
phototactic movement is away from the source of light. Movement of unicellular
algae in response to light is example of phototactic movement.
2.
Chemotactic movement (chemo=chemical) is
the movement of an entire organism in response to chemical signals. Sperms of
lower plants (ferns, bryophytes) move towards archegonia in water in response
to chemical signals produced by ovum.
Turgor movements are due to changes in turgor pressure
and size of cells. Movement of leaflets of “touch me not” plant due to touch is
an example of turgor movements. These are of two types.
1.
Bean plants and members of legume family lower
their leaves at night. These are called sleep movements and are due to
changes in turgor pressure in pulvinus of the leaves. When turgor pressure
decreases in the lower part of pulvinus, the leaves lower and go to “sleeping”
position.
2.
Rapid movement of leaflets (as in case of
mimosa / touch-me-not) plant are also due to changes in turgor pressure. Experiments
have shown that movement of K+ ions out of the cells drives the
water out due to which turgor pressure decrease. This process is very quick.
Growth movements are due to unequal growth in on two
sides of an organ like stem, root, tendrils and buds. These are of three types:
1.
In Epinasty, upper layers of cells of a
bud show more growth than lower layers. This results in opening of the bud.
2.
In Hyponasty, lower layers of cells of a
bud show more growth than upper layers. This results in closing of the bud.
3.
Growing tip of a stem moves in a zig-zag fashion
due to alternate changes in growth on opposite sides of the apex. This pattern
is called nutation.
Paratonic Movements
Paratonic movements are due to external causes. These are of
two types.
Tropic movements(tropos=turn) are due to curvature of
whole organ towards or away from a stimulus. These are of several types.
1.
Phototropism is the movement of part of
plant in response to light. Stem shows positive phototropism while roots show
negative phototropism.
2.
Geotropism orgravitropism is the
movement of part of plant in response to gravity. Roots display positive
geotropism while shoots show negative geotropism.
3.
Chemotropism is the movement of an organ
towards or away from a chemical stimulus. The hyphae of fungi are chemotropic
(they grow towards decaying material).
4.
Hydrotropism is the movement of an organ
in response to water. Roots show positive hydrotropism, while shoots show
negative hydrotropism.
5.
Thigmotropism(thigmo=touch) is the
movement in response to touch stimulus. For example, when tendrils of climbing
vines come in contact with solid support, the growth in opposite side of
contact increases due to which the tendril bends around the support.
Nastic movements are non-directional movements in
parts of plants due to external stimuli. These are of two types:
1.
Haptonastic movements are in response to
contact. For example, action of Venus fly trap in response to insects.
2.
Nyctinasticmovements are shown by organs
in response to external stimuli leading to differential growth. These are due
to changes in turgor and growth and are of two types.
a.
Photonastic movements are in response to
changes in length of day (photoperiod). Flowers open and close due to changes
in light intensity.
b.
Thermonastic movements are in response to
temperature. Flowers of tulip open during day due to higher temperature during
day.
Support in Animals
Skeleton is a tough and rigid framework in animals
which provides shape, protection, and support to body. It can be organic and/or
inorganic in nature.
1.
Skeleton provides support to the body.
and maintain its shape.
2.
It provides protection to vital organs
like brain, spinal cord, hear, and lungs.
3.
It provides site of attachment for muscles,
thereby helping in locomotion.
4.
Bones are involved in mineral homeostasis.
They store excess minerals (like calcium, phosphorus) when available. When body
needs these chemicals, bones can release them.
5.
Bones contain bone marrow which is the site for production
of red and white blood cells.
There are three types of skeletons found in animals.
Hydrostatic Skeleton
Hydrostatic skeleton is a fluid filled cavity
(usually gastro-vascular cavity or coelomic cavity) that acts as a skeleton. It
provides support for contraction of muscles to help in locomotion. It is
present in cnidarians, annelids, and other soft bodied invertebrates.
Sea anemone has a large central cavity which is filled by
sea water. Anemone closes its mouth and contracts its circular muscles, due to
which pressure develops and the body elongates. It helps to maintain an upright
posture.
Hydrostatic skeleton is also present in earthworm. In these
animals, the body is divided into segments by septa. Each segment is a fluid
filled compartment surrounded by circular and longitudinal muscles. Contraction
of circular muscles elongates the body, while contraction of longitudinal
muscles shortens the body. Alternating waves of contraction and elongation
helps the animal in locomotion.
Exoskeleton
Exoskeleton(exo=outside) is a hard outer covering on
the body which has muscles attached on the inside. It is composed of two
layers. The outer epi-cuticle is made up of waxy lipoproteins. It is
impermeable to water acts and acts as barrier to microorganisms. Main part of
exoskeleton is pro-cuticle which is below epi-cuticle.It is composed of
chitin (a polysaccharide) and several proteins. It is tough and leathery and is
hardened by sclerotization.
Simplest exoskeleton is present in Mollusca and is composed
of one or two parts. Marine molluscs have shells made up of calcium carbonate.
Shells of land snails are relatively smaller and lighter. As the animal grows,
larger chambers are formed and appear as rings.
The exoskeleton of arthropods is made up of chitin. It is
very strong and versatile and performs a variety of functions. It protects the
animal from drying and predators.It has ridges and bars on the inside for
muscle attachment. Legs of insects also have joints, which are thin, soft and
flexible, allowing movement. Sensilla are small bristles made up of
exoskeleton which are involved in sensory perception. Lenses, bristles, claws
are also modifications of exoskeleton.
Due to presence of a hard exoskeleton, young insects face a
problem. The skeleton is hard and does not permit growth. To overcome this
problem, old exoskeleton is shed periodically in a process called ecdysis
or molting. A hormone, named ecdysone is involved in this
process. This process can be divided in to 4 steps:
1.
Enzymes secreted by hypodermal glands (hypo=below;
derm=skin) digest old endocuticle due to which exoskeleton is detached from
hypodermis.
2.
New pro-cuticle and epi-cuticle is secreted.
3.
Old exoskeleton is split and removed from body.
4.
New exoskeleton is hardened by deposition of
calcium carbonate.
Endoskeleton
The skeleton of vertebrates is called endoskeleton
(endo=inside). It is composed of two types of connective tissues, bones and
cartilage. In both of these, cells are embedded in a matrix of collagen
protein.
Cartilage is a soft connective tissue. The living
cells of cartilage are called chondrocytes (chondro=cartilage). These
cells secrete collagen that surrounds chondrocytes. Cartilage covers ends of
bones at joints, and also supports flexible portions of ears and nose. There
are no blood vessels in cartilage. Cartilage is of two types:
1.
Hyaline cartilage is most abundant for of
cartilage in the body and is present in movable joints.
2.
Fibro-cartilage contains bundles of
collagen fibers and forms external pinnae and epiglottis.
Bones are relatively hard form of connective tissue.
The collagen fibers of bones are hardened by calcium phosphate deposition. The
bones in arms and legs have an outer shell of compact bone which is hard and
provides support for muscle attachment. The inner mass of these bones is light
and spongy and contains bone marrow. Bones contain three types of cells:
1.
Osteoblasts (osteo=bone) are bone forming
cells.
2.
Osteocytes (cyte=cell) are mature bone
cells.
3.
Osteoclasts are bone dissolving cells.
Human Skeleton
Human skeleton can be divided into axial skeleton and
appendicular skeleton.
Axial Skeleton
Axial skeleton supports the main body and protects
vital organs. Skull, vertebral column and ribs form axial skeleton.
Skull is made up of cranium (cranium=head) and facial
bones which protect brain. Cranium consists of 8 bones, 4 are unpaired
and 2 are paired. There are 14 facial bones of which 6 are paired and 2
unpaired.
Vertebral column extends from skull to pelvis and protects
the spinal cord. It has 4 curvatures which provide strength and flexibility to
backbone. It consists of 33 vertebrae.
1.
Seven vertebrae of neck are called cervical
vertebra.
2.
Twelve vertebrae in the chest cavity are called thoracic
vertebra.
3.
Five vertebrae in the abdominal region are
called lumbar vertebra.
4.
Nine vertebrae in the pelvic region are
grouped into two sets:
a.
Sacrum is formed by fusion of 5
vertebrae.
b.
Coccyx is formed by fusion of four
vertebrae.
Rib cage consists of twelve pairs of ribs that
articulate with thoracic vertebra. Then ribs fuse in the front with sternum
(chest bone) while two pairs do not attach to sternum and are called “floating
ribs”. Ribs provide support to chest cavity.
Appendicular Skeleton
Appendicular skeleton is present in limbs and helps
in locomotion. It consists of pectoral girdle and pelvic girdle and limbs.
Joints
Joints are present at the junction of two bones. They
are responsible for movement. Joints can be immovable, slightly movable or
freely movable. On the basis of structure, joints can be divided into three
types:
1.
Fibrous joints are held together by short
fibers embedded in connective tissue. Teeth are connected in the jaw through
such joints.
2.
Cartilaginous joints allow little or no
movement. Hyaline cartilage is present between growing bones. Fibrous cartilage
is present between vertebrae and in front of pelvis.
3.
Synovial joints are present between
moving joints. They contain a cavity filled with fluid and help to reduce
friction between bones. The cavity is filled by a “fibrous capsule”. It
may also have ligaments to hold the joint.
On the basis of movements, joints can be classified into two
categories:
1.
Hinge joints are present in elbows and
knees and allow movement in two directions. A pair of muscles is responsible
for movement of bones at these joints.
2.
Ball and socket joints are present in
shoulders and hips and allow movement in many directions. At least two pairs of
muscles are responsible for about along these joints.
Deformities of Skeleton
Some diseases of skeleton are caused by genetic defects
while others may be due to hormonal imbalance or injury.
Cleft palate is a birth defect in which maxilla and
palatine fail to fuse and there is an opening between oral and nasal cavities.
It can lead to inhalation of food into lungs causing further complications.
Microcephaly is a genetic defect resulting in small
skull and mental retardation.
Arthritis is inflammation or degeneration of joints
that may be due to more than 100 different causes.Osteoarthritis is a
common form of chronic arthritis caused by genetic defect.
Some defects of skeletal system are caused by hormonal
imbalance. Osteoporosis is caused by resorption of bone without change
in chemical composition. It is common in aged women, due to decreased estrogen
levels. Insufficient exercise, poor exercise and lack of calcium may also cause
this disease. Estrogen replacement therapy may help in osteoporosis.
Osteomalcia is the softening of bones due to inadequate
minerals in the body. Calcium salts are not deposited in bones and therefore
bones become weak and soft. Bones may deform or bend due to this disease. A
common symptom is pain in bones when weight is put on them.
Rickets is a disease of children resulting in bowed
legs and deformed pelvis. It is due to dietary deficiency of calcium or vitamin
D. Children are given fortified milk or exposed to sunlight to cure this
disease.
Intervertebral discs are present between vertebrae
where they act as cushion-like pads. These discs act as shock absorbers during
walking and jumping. Sever or sudden trauma to spines results in herniation
of discs. Each disc is composed of an inner elastic, semi-fluid “nucleus
pulposus” and outer ring of fibrocartilage “annulus fibrosus”.
Annulus fibrosus might rupture and nucleus pulposus may come out. In some cases
this fluid may put pressure on spinal cord and cause severe pain. Disc slip is
usually treated with rest and in some cases by surgery.
Spondylosis is a disease of vertebral column in which
vertebral joints are fused together.
Sciaticpain results from injury to sciatic nerve. It
may get damaged due to a fall, herniated disk or an injection in buttock,
leading to severe pain. It may result in lower limb impairment. If sciatic
nerve is completely severed, limbs become useless.
Repair of Broken Bones
Although bones are strong and rigid structures, they may
break due to accidental trauma especially in young age. In old age, bones
become weak and may fracture. Broken bones are usually repaired by reduction
and realignment.
There are two types of reduction. In closed reduction,
the physician re-aligns the broken ends and coxes them together by hand. In open
reduction, a surgery might be performed and broken ends may need to be
joined by steel pins or wires.
After aligning the ends of broken bone, it is usually
supported by a cast or traction (relieving stress on bones).
It may take two to three months for a bone to repair completely.
However, large, weight bearing bones or bones of old people usually take longer
to repair. The repair process can be divided into four steps.
As a result of injury, the blood vessels inside the bone and
surrounding tissue get ruptured, resulting in hemorrhage. Within a short time,
a hematoma (heme=blood; toma=clot) forms in the area. The tissue
surrounding the injury becomes swollen and painful.
The next stage in bone repair is formation of cartilaginous
or soft callus. Capillaries penetrate in the area and help to remove
dead cells. Fibroblasts and osteoblasts also migrate to the area and start
formation of bone cells.
The soft cartilage is replaced by bony cartilage due
to infiltration of large number of osteocytes. Osteoclasts also help to remove
replace soft cartilage with bony cartilage. The process takes about 2 to 3months.
In remodeling, the original composition of bone is
restored. The bone assumes its actual shape and structure and now resembles an
unbroken bone.
Movement in Animals
Muscle is a special type of tissue that helps in
locomotion usually with the help of skeletal system. Muscle cells contain
special type of proteins called actin and myosin arranged in the form of
filaments.
Types of Muscles
Muscles are of three types:
Smooth muscles are found in almost all animals. These
are long and spindle shaped. Each cell has its own nucleus. Their surface
appears smooth and does not have any striations (stripes). These are usually
present in visceral organs (like blood vessels and digestive system) and
control the flow of materials. They are not under voluntary control of the body
and contract slowly.
Cardiac muscles are present in heart. They form long
chains of cells, each having its own nucleus. These chains form long fibers,
which are branched and interconnected. Under microscope, they appear striated.
These are also not under voluntary control. Their contraction is rhythmic and
relatively fast.
Skeletal musclesare attached with bones and help in
movement of bones. They appear striated under microscope, due to presence of
light and dark bands. They are usually under voluntary control of the body.Their
contraction can be from slow to very fast.
Structure of Skeletal Muscles
The plasma membrane of muscle is called sarcolemma
(lemma=sheet). The cytoplasm of muscle cells is called sarcoplasm. The
endoplasmic reticulum of muscle cells is called sarcoplasmic reticulum. Muscle
cells usually contain large granules of stored glycogen for their energy
requirement. A special protein called myoglobin is present in muscles and
is red in color. Its function is to strongly bind and store oxygen. A large
number of mitochondria are also present in muscle cells to provide ATP
for contraction.
Muscles are composed of many bundles of muscle fibers or
cells. Muscle fibers are actually long, cylindrical cells (joined end to
end) having multiple nuclei. These nuclei are usually present just beneath the
sarcolemma. Each muscle fiber is about 10–100µm in diameter.
Each muscle fiber contains large number of myofibrils,
arranged in parallel manner. These myofibrils are present along the entire
length of the cell.
A single contractile unit of myofibril is called sarcomere.
Under high magnification, a number of light and dark bands can be seen in a
sarcomere, due to which it appears striped.
·
Each dark band is called A band or
anisotropic band, which can polarize light.
o
A lighter region is present in the middle of A
band, called the H zone.
o
A dark line is present in the middle of H zone,
called the M line.
·
Each light band is called I band
or isotropic band, which cannot polarize light.
o
A line is present in the middle of I band, called the Z line.
The portion between two successive Z lines is the sarcomere.
Myofibrilsare made up ofmyofilaments. Each myofilament
is made up of thick (myosin) and thin (actin) filaments. One myosin filament is
surrounded by six actin filaments.
Thick filaments are composed of myosin. They
are about 16nm in diameter. Thick filaments extend the entire length of A band.
Each myosin molecule has a tail and two globular heads. The tail is composed of
two long polypeptide chains coiled together. The heads are sometimes called cross-bridges
because they link thick and thin filaments together during contraction.
Thin filaments are composed of actin. It is
about 7–8nm in diameter. Thin filaments extend the length of I bands and also
cover some area of A band. It appears like two strands of pearls coiled around
each other. Another chain of tropomyosin is twisted around these strands
of actin. Another major protein in thin filament is troponin. Troponin
is made up of three polypeptide chains: one chain binds to actin, second to
tropomyosin, andthird chain binds to calcium ions.
Sliding Filament Model
Sliding filament model was proposed by H. Huxley and
A.F. Huxley in 1954 to explain the mechanism of muscle contraction. According
to this model, there is certain degree of overlap between thick and thin filaments.
When a muscle contracts, the actin and myosin molecules slide past each other,
increasing this overlap. Z lines are brought closer due to this sliding and the
sarcomere shortens. During contraction:
·
H zone disappears
·
I band becomes short
·
Overall width of A band does not change.
Actin molecules have special site with which heads of myosin
can bind. When muscle is at rest, these sites are blocked by tropomyosin
chains.
The sarcolemma of muscle cell forms special infoldings,
penetrating deep in the sarcoplasm in the form of tubules called T-tubules.
Thousands of such tubules are present in a cell and form a network called T
system.
Sarcoplasmic reticulum of muscle cells does not have
ribosomes on it. It forms an intricate network of tubules present around
myofibrils. This network of tubules is called sarcotubules. Ca++
is stored in these tubules which is required for contraction.
The signal for contraction arrives from central nervous
system in the form of a “nerve impulse” through a “motor nerve”. The
place at which the nerve attaches to muscle fibers is called neuromuscular junction.
A single nerve usually innervates many fibers, which contract together. These
fibers are collectively called a motor unit.
When nerve impulse arises, Ca++ ions stored in
sarcoplasmic reticulum are released in the sarcoplasm. TheseCa++
ions bind to troponin. Troponin displaces tropomyosin from its position.
Binding sites on actin are exposed. Polar heads of myosin bind to sites on
actin (this is called cross-bridge formation). This binding induces a
conformational change in myosin, thereby pulling the actin molecules. An ATP
molecule is hydrolyzed to break the cross-bridge and bring them in their
original conformation.
Muscle contraction follows all or none response. Upon
arrival of impulse, all fibrils of a cell contract together. Degree of contraction
depends on number of cells contracting at a time.
Energy for Muscle Contraction
Energy for muscle comes from ATP. ATP is required to break
the cross-bridges. A contracted muscle cannot relax in the absence of ATP. The
amount of ATP is depleted in the body after death. Under such circumstances,
the cross-bridges cannot be broken, and the body becomes stiff. This is called rigor
mortis(mortality=death).
Mitochondria provide ATP for muscles by aerobic
breakdown of glucose. Glucose is produced from stored glycogen. Another energy
storing molecule, creatine phosphate, can also provide energy during high
metabolism.
If oxygen is not available in enough amount (e.g. during
strenuous exercise) glucose can be broken down anaerobically to produce
ATP. Lactic acid is produced during this process. Accumulation of lactic acid
results in muscle fatigue.
Effect of Exercise on Muscles
Regular exercise results in a number of changes in muscles.
The number of blood capillaries increase in such muscles. The amount of stored
glycogen and myoglobin is also increased. The number of mitochondria in each
cell also increases. The muscles themselves increase in size and strength. They
become more efficient and resistant to fatigue. Complete immobilization of
muscle leads to weakness and severe atrophy (degeneration of muscles).
Problems of Muscles
Inability of muscles to contract is called muscle fatigue.
This is due to lack of ATP molecules. In absence of ATP, muscles remain in
continuous contraction as cross-bridges cannot be broken. Accumulation of lactic
acid changes muscle pH and leads to muscle fatigue. Ionic imbalance can also
lead to fatigue.
Tetany is a disease caused by low calcium in blood.
It increases the excitability of neurons and results in loss of sensations. This
results in muscle twitches and convulsions. If left untreated, it leads to spasmof
larynx, respiratoryparalysis, and death.
Tetanus is a disease cause by a bacterium Clostridium
tetani. It is an anaerobic bacterium that causes painful spasms of
some skeletal muscles. It begins with stiffness of jaws and neck muscles. The
next stage is fixed rigidity of jaws (“lock jaw”) and spasms of trunk and limb
muscles. Death usually occurs due to respiratory failure.
Cramps are tetanic contractions of entire muscle.
They may last from a few seconds to several hours. The muscles become taut
(“pulled”) and painful. It is common in thigh and hip muscles and usually
occurs at night or after exercise. It may occur due to low blood sugar level,
electrolyte depletion, dehydration and irritability of spinal cord and neurons.
Arrangement of Muscles
Skeletal muscle has three parts:Origin is the end of
muscle that remains fixed when muscle contracts.Insertion is the end of
the muscle that moves the bone.Belly is the thick part between origin
and insertion that contracts.
Connective tissue binds tissues and helps to maintain
body form by holding various organs together.
·
Ligaments attach bones with bones and are
slightly elastic.
·
Skeletal muscles are attached to bone through a
non-elastic fiber of collagen, called tendon.
The skeletal muscles work by pulling tendons which in turn
pull the bone. Most muscles pass across a joint and are attached to the bones
that form joints. When such a muscle contracts, it draws one bone towards or
away from the bone with which it articulates.
There are 650 muscles in human body. Most of these muscles
occur in pairs. Muscles of each pair work against each other by contraction.
This is called antagonistic relationship. These muscles work on the principle
of lever. One muscle moves the lever, while the other muscle returns lever to
its original position.
Locomotion in Different Animals
Locomotion in Euglena
Contractile myonemes are present along the length of
euglena. When these myonemes contract, the organism can change its direction.
The shape of the body also changes due to this contraction. First the body
becomes short and wider at the anterior end then in the middle and later at the
posterior end. This characteristic movement is called Euglenoid movement.
Locomotion in Paramecium
A cilium consists of nine peripheral double fibrils and two
central fibrils. Each “double fibril” is made up of two thin fibers and appears
like 8. The cilium is covered in extension of plasma membrane.
The movement of cilium is brought about by sliding of double
fibrils in two groups.
·
Five out of nine double fibrils contract or
slide simultaneously, due to which the cilium bends or shortens. This is called
effective stroke and moves the organism in water.
·
When four out of nine double fibrils contract,
the cilium becomes straight. This is called recovery stroke.
All cilia beat together in the form of a wave. The energy is
provided by ATP.
Locomotion in Amoeba
Amoeba moves by means of pseudopodia. These are
finger-like projections thrown in the direction the flow of cytoplasm.
Consequently, the entire body flows in that direction.
Locomotion in Jelly Fish
Jelly fish moves by jet propulsion. Its body is
umbrella-like and is called “bell”. Water enters the bell and is then expelled
from it like a jet. Due to this, the body moves forward.
Locomotion in Earthworm
Earthworm shows accordion like movement. There are two sets
of muscles in earthworm: circular muscles and longitudinal muscles. When
circular muscles contract, the body becomes long and thin. The setae present on
lower side of anterior end come out and hold the ground. Longitudinal muscles
then contract to shorten the body and pulling the earthworm. The setae of
posterior end come out and attach to the ground. Circular muscles contract
again to make the body long and thin.
Locomotion in Cockroach
Cockroach shows both walking and flying locomotion.
During walking, only one side of legs is used to move the animal forward. The
foreleg pull the body forwards, hind leg also pushes it forward while the middle
leg acts as a prop (for support).
Cockroach has two pairs of wings. Only posterior pair of
wings brings about the flight. They beat in air to lift the body and move
through air.
Locomotion in Snail
Snails, mussels and other mollusks have a single muscular
foot which is used for “sliding” or crawling on the substrate.
Locomotion in Star Fish
The tube feet are filled with water to make them longer.
Suction cups are present on the lower end of tube feet which help to fix the
organism to the ground or an object. When tube feet shorten, the body is pulled
in that direction. Arms of star fish can also help in swimming.
Locomotion in Fishes
No structures usually project from the body except fins.
This also helps to reduce the drag, making the streamline more perfect.
The body of cartilaginous fish is covered with dermal
denticles and that of bony fish is covered by scales. They are kept
moist by slime produced by mucus or oil glands. This also considerably reduces
friction against water.
Fins are also an adaptation for swimming. There are three
set of fins in the body of the fish.
·
Dorsal and ventral fins are
unpaired and help to stabilize the fish in water.
·
Pectoral and pelvic fins are paired
and help in steering and balancing the animal. They also help in moving
forward.
·
Caudal or tail fins helps in forward
movement.
Bony fish have a special structure called swim bladder
that helps in buoyancy.
Locomotion in Amphibians
The body of amphibians is fish-like in appearance. Two types
of locomotion are present in amphibians.
They wriggle along their belly on the ground with the help
of segmentally arranged muscles. Their movement appears like “swimming on land”.
Some amphibians can raise their body on legs and help to propel them move like
“movable levers”.
In anurans (frogs and toads) the entire skeleton and
muscular system is specialized for swimming and jumping. Both limbs produce
extensor thrusts to propel the animal. Frogs and toads can walk and hop on land
due to its strong hind legs.
Locomotion in Reptiles
Reptiles have many adaptations for locomotion on land.
Reptiles can walk and run. Although the basic plan of skeleton is similar to
that of amphibians, the skeleton of reptiles is better evolved for movement.
The skeleton is hard (ossified) for better support.
Reptiles have cervical vertebrae which help in rotation of
the neck. The first two cervical vertebrae (atlas and axis) are modified for
rotation.
Ribs of reptiles are also highly modified. The ribs of
snakes are connected with large belly scales through muscles. This also helps
in locomotion.
Prehistoric reptiles (like dinosaurs) were bipedal (walking
on two legs). Their pelvis was narrow and large tail for balancing. This helped
to free the front legs, which became adapted to prey capture or flight.
Locomotion in Birds
Forelimbs of birds are evolved into wings with very strong
pectoral muscles. These muscles pull the wings up and down.
Sternum of birds is modified into keel. A large keel
is required for attachment of muscles.
The body of birds is covered with feathers which increase
the surface area for flight. Feathers also help to keep their bodies warm. Warm
bodies can generate enough energy for flight.
The body of birds is also streamlined to reduce friction in
air during flight. The flight of birds is of two types:
During passive flight, the birds glide and their
wings act as aerofoils. An aerofoil is any smooth surface which moves through
the air at an angle to the airstream. The air flows over the wing in such a way
that the bird is given lift. The amount of lift can be changed by changing the
angle of the wings.
When air currents are not strong, lift can be achieved by
flapping the wings. This is called active flight. The air moves more
quickly over the curved upper surface than over the lower surface. This results
in reduced air pressure above the wing, while more pressure below the wings.
This results in lift for the bird.
Locomotion in Mammals
Mammals have the most efficient skeleton for locomotion and
support. Three types of locomotion are found in mammals:
Plantigrade mammals walk on their soles with palms,
wrists, and digits all resting on the ground. This type of locomotion is
present in man, monkeys and bear.
Digitigrade mammals walk on their digits only. They
run faster than plantigrade animals. The first digit in these animals is
reduced or completely lost. Rabbits and rodents show digitigrade locomotion.
Unguligrade mammals walk on the tip of their toes.
These tips are modified into hooves. This is the swiftest type of locomotion
and is found in deer, goats, and horses.
20 Chromosomes & DNA
Chromosomes
Chromosomes are
thread-like structures visible during cell division. They were discovered by
Walther Fleming in 1882. They are present in all eukaryotic cells. Each
chromosome contains numerous genes that play important role in determining the
structure and function of the body.
The number of chromosomes
varies from species to species and is characteristic of each species. Presence
of all chromosomes is essential for survival. Loss of a chromosome or its part
may result in disease or even death.
Species
|
Pairs
|
Chromosomes
|
Penicillium
|
1
|
2
|
Ferns
|
500
|
1,000
|
Mosquito
|
3
|
6
|
Honeybee
|
16
|
32
|
Drosophila
|
4
|
8
|
Corn
|
10
|
20
|
Sugarcane
|
40
|
80
|
Frog
|
13
|
26
|
Mouse
|
20
|
40
|
Humans
|
23
|
46
|
Structure of Chromosomes
Each chromosome has a
central portion called a centromere, to which spindle fibers are
attached during cell division. This is also called the primary constriction.
The arms of a chromosome
are called chromatids. The length of these chromatids and staining
properties usually vary from one chromosome to another. On the basis of shape,
the chromosomes can be of four types:
·
Metacentric chromosomes have arms of
equal length with centromere present in the middle. It forms a “v” shape during
anaphase of cell division.
·
In sub-metacentric chromosome, on arm is
slightly longer than the other. It forms a “j” shape during anaphase.
·
In acrocentric chromosome, one arm is
very long while the other is very short. It forms an “j” shape during anaphase.
·
In telocentric chromosome, only one arm
is present while the other arm is missing altogether. It also forms an “i”
shape during anaphase.
The particular set of
chromosomes that an individual possesses is called its karyotype. Karyotype
of one organism is usually different from the other.
Composition of Chromosomes
Chromosomes are composed of DNA (40%) and proteins (60%). A considerable
amount of RNA is also present in chromosomes because they are the sites of RNA
synthesis. The DNA in a chromosome is a single, large, double stranded molecule.Average
length of DNA in a chromosome is 5cm. This large molecule is coiled tightly to
fit in the nucleus.
A special group of proteins, called histone proteins,
is involved in packaging of DNA. These proteins are highly basic in nature due
to abundance of amino acids lysine and arginine. Due to these amino acids, the
histones are positively charged and tightly bind with negatively charged DNA.
Every 200 nucleotides of DNA are coiled around a core of
eight histone proteins. This complex of histones and 200 bases of DNA is called
a nucleosome. The nucleosomes appear like “beads in a string”. Further
coiling occurs when the string of nucleosomes wraps up into higher order coils
called supercoils.
When the cell is in non-dividing (resting) stage, its nuclear
material is in the form of chromatin. Chromatin material controls all
properties and activities of the cell. It can be of two types.
Highly condensed portions of the chromatin are called heterochromatin.
Some of these portions remain permanently condensed, so that their DNA is never
expressed.
Relatively less condensed portion of chromatin is called euchromatin.
It condenses only during cell division whencompact
packagingfacilitatesthemovement ofthechromosomes. The genes in euchromatin form
can be expressed (can produce proteins).
Chromosomal Theory of Inheritance
Karl Correns suggested in 1900 that chromosomes may play a
role in inheritance.
Sutton’s Hypothesis – Chromosomal Theory
Walter Sutton, in 1902, formulated the chromosomal theory of
inheritance. He observed that chromosomes pair during meiosis. Several pieces
of evidence supported his theory:
·
Reproduction involves the initial union of only
two cells, egg and sperm. These two cells must make equal hereditary
contribution. Sperms contain little cytoplasm, suggesting that the hereditary
material must reside within the nuclei of the gametes.
·
Diploid individuals have two copies of each pair
of homologous chromosomes, while gametes have only one. This observation was
consistent with Mendel's model, in which diploid individuals have two copies of
each heritable gene and gametes have one.
·
Chromosomes segregate during meiosis, and each
pair of homologue orients on the metaphase plate independently of every other
pair.
Some problems are also associated with this theory. For
example, if the independent assortment of Mendelian traits reflects the independent
assortment of chromosomes in meiosis, why does the number of characters that
assort independently in a given kind of organism often greatly exceed the
number of chromosome pairs, the organism possesses?
Morgan’s Experiment – Sex Linkage
In 1950, T H Morgan discovered a mutant fruit fly (Drosophila)
which had white eyes instead of normal red eyes. When Morgan crossed this white
eyed male with red eyed female, all the offspring (F1) were red eyed,
indicating that white eyed gene is recessive.
Red eyed flies of F1 generation were then crossed with each
other. About 18% of their offspring were white eyed (which is close to the
expected 25%). However, Morgan noticed that all the white eyed individuals were
males. No white eyed female was produced in F2.
Morgan hypothesized that may be white eyed females are not
viable due to some reason and do not hatch from the eggs. To test this
hypothesis, Morgan test crossed red eyed females from F2 with the original parental
white eyed male. As a result of this cross, many white eyed females were also
produced, showing that white eyed females can also exist.
Morgan explained the absence of white eyed females in F2 on
the basis of difference in sex chromosomes. Gene causing the white eye trait in
Drosophila is present only on the X chromosome. It is absent from the Y
chromosome. Traits determined by genes on X chromosome are said to be sex
linked traits.
It was an important milestone in the history of genetics. It
showed for the first time that genes responsible for Mendelian traits actually
reside on chromosomes. The segregation of the white-eye trait has one-to-one correspondence
with the segregation of the X chromosome. In other words, Mendelian traits such
as eye color in Drosophila assort independently because chromosomes do.
Griffith’s Experiment – Transformation
Fredrick Griffith conducted some experiments on Streptococcus
pneumoniae bacteria. When a virulent strain of these bacteria is injected
into mice, the mice die of blood poisoning. This virulent strain is called S
type bacteria, because it produces smooth colonies when cultured in the
lab.
However, when a mutant strain of the same bacteria was
injected into the mice, the mice did not die. This mutant strain lacked a
polysaccharide coat around its cells. Griffith concluded that the coat is
somehow necessary for virulence (virulence is the capacity to cause disease).
This non-virulent or mutant form of bacteria is called R type bacteria,
as it produces rough colonies when cultured in the lab.
To determine if the polysaccharide itself is responsible for
causing the disease, Griffith injected dead bacteria of virulent strain into
the mice. The mice remained healthy. In another group of mice he injected a
mixture of heat killed S type bacteria and live R type bacteria. Both of these,
when injected separately, do not cause disease. However, when the mixture was
injected, the mice died and live S type bacteria were isolated from their
blood.
Somehow the information for polysaccharide coat had passed
from dead, virulent S type bacteria to live, coatless R type bacteria. The R
type bacteria were permanently transformed into S type bacteria due to this information.
Griffith called this process transformation. Transformation is the
transfer of genetic material from one cell to another cell in culture. It can
alter the genetic makeup of the recipient cell.
Avery, McLeod and McCarty
They prepared mixture of dead S type bacteria and live R
type bacteria like Griffith. They then removed as much protein from it as
possible by adding protein digesting enzyme. Even after removing almost all
proteins (99.98%), the transformation took place. This indicated that the
transforming principle is not protein.
In another mixture, they removed all RNA by adding RNA
digesting enzymes. The mixture was then tested for transformation. R type bacteria
were still transformed into S type, indicating that RNA is also not responsible
for transformation.
In the third mixture, the scientists added a DNA digesting
enzyme (called DNAse) to remove all DNA. When this mixture was tested for
transformation, no transformation was observed. This showed that DNA is actually
responsible for transformation.
Hershey &Chase Experiment
More convincing evidence in support of the idea that DNA is
the genetic material came from the work of Hershey and Chase in 1952. They were
working on T2 bacteriophages that infect bacteria. They used radioactive
isotopes of phosphorus and sulfur to label DNA and proteins of bacteriophages respectively.
They prepared two groups of viruses. One group was grown on 32P
containing media. This phosphorus became incorporated into the DNA genome of viruses.
The second group was grown on 35S containing media. This sulfur
became incorporated into the protein coat of the virus.
These viruses were then allowed to infect separate groups of
bacteria and the cells were agitated to remove the protein coats from bacterial
cells. The bacteria that were infected with viruses grown on 35S did
not show any radioactivity when the protein coats were removed. However, the
bacteria that were infected with 32P containing viruses showed signs
of radioactivity in their cells. This showed that hereditary material that
enters the bacterial cell and directs synthesis of new viruses is in fact DNA, not
protein.
Chemical Nature of DNA
Friedrich Miescher was a German chemist who discovered DNA
in 1869. He extracted a white substance from the nuclei of human cells and fish
sperm. He called this substance “nuclein” because it seemed to be associated
with the nucleus. Since nuclein was acidic, it came to be known as nucleic acid.
Levene’s Work – Chemical Composition
The chemical composition of nucleic acids was discovered by P.A.
Levene in 1920s. He showed that nucleic acidsare made of repeating units called
nucleotides. A nucleotide has three main components:
·
A pentose sugar
·
Phosphate groups
·
Nitrogen containing bases
Nitrogen bases can be classified into two groups: purines
and pyrimidines. Adenine and Guanine are purinesbases having a large
molecule with two rings. Thymine and cytosine are pyrimidines which are
smaller molecules containing a single ring. RNA contains uracil instead of
thymine.
In a nucleotide, a nitrogen base is attached to carbon
number 1’ of the pentose sugar. Phosphate group is attached to carbon number 5’
of the sugar. A free OH at carbon number 3’ is available. This 3’ OH group
allows nucleotides to join together and form long chains of DNA and RNA.
Thereactionbetweenthephosphategroupofonenucleotideand
thehydroxyl group of another isa dehydration synthesis. A water molecule
is eliminated and a covalent bond is formed between the two groups. This bond
is called phosphodiester bond because the phosphate group is now linked
to the two sugars by means of a pair of ester bonds (–O–P–O–). Many thousands
of nucleotides can join together in long chains by phosphodiester bonds.
Chargaff’s Rule
Erwin Chargaff proposed Chargaff’s rule. He showed that in
DNA isolated from any species:
·
amount of adenine is always equal to amount of
thymine
·
amount of cytosine is always equal to amount of
guanine
It also implies that there is always equal proportion of
purine (A+G) and pyrimidine (C+T).
Rosalind Franklin – X-Ray Diffraction
The diffraction pattern of DNA suggested that the DNA
molecule had a shape of a helix (spring) with a diameter of 2 nm and a complete
helical turn every 3.4nm.
Watson & Crick – Model of DNA
Watson and Crick proposed a model of DNA structure on the
basis of Chargaff’s rule and work of Rosalind Franklin in 1953. They proposed
that DNA molecule is a double helix.
·
The two strands (backbones) of double helix are
composed of alternating pentose and phosphate groups.
·
Nitrogenous bases of two strands are pointed
inward toward each other, forming base-pairs.
·
Purines are always base paired with pyrimidines.This
is called complementary base pairing.
·
Adenine cannot pair with cytosine and thymine
cannot pair with guanine.
·
The base pairs are held together due to weak hydrogen
bonds between bases of the two strands.
·
Adenine forms two hydrogen bonds with thymine.
·
Cytosine forms three hydrogen bonds with
guanine.
·
The overall diameter of DNA is constant 2.0nm.
·
The two strands of DNA are antiparallel to each
other, one chain running 3` to 5` and the other 5` to 3`.
·
The base pairs are planar (flat) and are 0.34nm
apart. Hydrophobic interactions among base pairs stabilize the overall
molecule.
Replication of DNA
Replication is the process by which copies of a DNA
molecule are made to be passed on to daughter cells. Watson and Crick model
suggested that the basis for copying the genetic information is
complementarity. The two strands of DNA can unwind (like a zip) and nucleotides
on each strand can direct which nucleotides to add to new strand. This form of
DNA replication is called semi-conservative meaning that one strand of
DNA comes from original molecule while the other strand is synthesized newly.
Other people also proposed different models of DNA
replication. The conservative model stated that the parental double
helix would remain intact and generate DNA copies consisting of entirely new
molecules. The dispersive model predicted that each strand of all the
daughter molecules would be a random mixture of old and new DNA.
Meselson& Stahl Experiment
Meselson and Stahl tested the three models of DNA
replication in 1958. They grew bacteria in medium containing heavy isotope of
nitrogen (15N). This nitrogen becomes part of nitrogenous bases and
is incorporated in the DNA. When bacteria are grown on this media for many
generations, their entire DNA contains this 15N.Since 15N
is heavier than the normal isotope (14N), the DNA containing 15N
is denser than normal DNA.
Meselson and Stahl isolated DNA from these bacteria and
performed cesium chloride density gradient centrifugation. In this
technique, the DNA is dissolved in cesium chloride and then centrifuged at a
very high speed in an ultra-centrifuge. The enormous centrifugal forces
generated by the ultracentrifuge cause the cesium ions to migrate toward the bottom
of the centrifuge tube, creating a gradient of CsCl, and thus of density. DNA
molecules of different densities also get separated. DNA molecule floats or
sinks in the gradient until it reaches the position where its density exactly
matches the density of cesium there.Since 15N molecules are heavier
than 14N molecules, they sink further down in the tube as compared
to 14N.
Bacteria growing in 15N media for several
generations were transferred to 14N media and were allowed to divide
only once. When DNA was isolated from these bacteria, its density was found to
be intermediate between that of 14N and 15N.
This meant that after first round of cell division, one of
the strand of DNA was made up of 15N (which was present in parents)
while the other strand was made up of 14N (newly synthesized in
fresh media). Such a molecule is called a hybrid.
When the cells were given time to divide for the second
time, two bands of DNA were observed on cesium chloride. The hybrid duplex
contributed one heavy strand to form another hybrid duplexandone light strand
toforma light duplex.
This experiment clearly confirmed the prediction of the
Watson - Crick Model that DNA replicates in a semi-conservative manner.
The Replication Process
DNA replication starts at a specific sequence of nucleotides,
called origin of replication. The enzyme that catalyzes synthesis of new
DNA strand is called DNA polymerase. There are three types of DNA
polymerases in bacteria called DNA polymerase I, II, and III.DNA polymerase
Iis a small enzyme that plays a supporting role during DNA replication (removal
of primer).
DNA polymerase III is the actual replicating enzyme
in E. coli and is about 10 times larger than DNA polymerase I. It is a
dimer and catalyzes replication of DNA. It adds about 1000 nucleotides to the
new strand in one second. DNA polymerase III has two limits. It cannot initiate
synthesis of a new strand and it cannot add nucleotides at the 5` end of the
strand.
At the beginning of synthesis, an enzyme called Primase
can construct an RNA primer. Primer is a sequence of about 10
nucleotides that are complementary to the parent strand. DNA polymerase III can
then use this primer as a starting point to add nucleotides at its 3` end. The
RNA nucleotides are later removed and replace by DNA nucleotides.
Synthesis of DNA only proceeds in 5` Ã 3` direction because DNA
polymerase III cannot add nucleotides at 5` end. BecausethetwoparentstrandsofaDNAmoleculesare
antiparallel,thenew strandsare oriented inoppositedirections. The strand which
elongates towards the replication fork is called leading strand.
Nucleotides are added continuously to 3` end of the leading strand.
The strand which elongates away from the replication fork is
called lagging strand. Synthesis of lagging strand takes place in the
form of short fragments. These fragments are called Okazaki fragments.
In bacteria, these strands are about 100–200 nucleotides long while in
eukaryotes these are about 1,000–2,000 nucleotides long. The fragments are later
joined by an enzyme called DNA ligase.
Concept of Gene
Garrod and Bateson
Garrod and Bateson observed in 1902 that some diseases are
more common in certain families. By examining several generations of these
families, they concluded that some of these diseases behaved as if they were
simple recessive alleles. He said that these disorders are Mendelian traits and
arise due to changes in hereditary information.
Alkaptonuria is a disease in which a substance called
homogentisic acid is produced in urine. When it is exposed to air, it is
quickly converted into a black substance, turning the urine black. In normal
individuals, homogentisic acid is broken down into simpler substances. Garrod
suggested that patients of alkaptonuria lacked the enzyme necessary to catalyze
this breakdown. He speculated that many other inherited diseases might also
reflect enzyme deficiencies.
Beadle and Tatum – One Gene One Enzyme
In 1941, two scientists, Beadle and Tatum provided the
experimental proof that the information encoded within the DNA acts to specify
particular enzymes. They deliberately created Mendelian mutations in chromosomes
and then studied the effect of these mutations on the organism.
They exposed Neurospora spores to X-rays. DNA in some
of these spores was damaged due to X-rays in the regions encoding the ability
to make compounds needed for normal growth. DNA changes of this kind are called
mutations.The organisms that have undergone such changes are called mutants.
They allowed the progeny of the irradiated spores to grow on a defined medium
containing all of the nutrients necessary for growth; so that any growth
deficient mutants resulting from the irradiation remain alive.
The progeny of these spores was then transferred to minimal
media. This type of media contains only sugar, ammonia, salts, a few vitamins
·and water. Mutants that have lost the ability to synthesize important compounds
from these simple ingredients will not be able to survive on this media. This
approach can help to identify and isolate many growth deficient mutants.
Various chemicals were then added to this minimal media to findonethatwould
allow amutantstraintogrow. This procedure allowed them to pinpoint the nature
of the biochemical deficiency that strain had. The addition of arginine, for
example permitted several mutant strains, dubbed arg mutants, to grow.
When their chromosomal positions were determined, the arg mutations were
found to cluster in three areas.
Beadle and Tatum were able to isolate a mutant strain with a
defective form of each enzyme in arginine biosynthetic pathway. These mutations
were always located at one of a few specific chromosomal sites. They also found
that there was a different site for each enzyme. Each of the mutants they
examined had a defect in a single enzyme, caused by a mutation at a single site
on one chromosome.
Beadle and Tatum concluded that
·
genes produce their effects by specifying the
structure of enzymes
·
each gene encodes the structure of one enzyme
This is called one gene – one enzyme hypothesis.
One Gene – One Polypeptide
Many enzymes are made up of many protein subunits or more
than one polypeptide chains. Each of these polypeptides is encoded by a separate
gene. This relationship is now called one gene – one polypeptide.
Enzymes are responsible for catalyzing the synthesis of all
the parts of an organism. They are also responsible for the assembly of nucleic
acids, proteins, carbohydrates and lipids. Therefore, by encoding the structure
of enzymes and other proteins, DNA specifies the structure of the organism
itself.
Sanger and Ingram
Fredrick Sanger, in 1953, determined the complete sequence
of insulin. It was the first protein to be sequenced. It was shown for the
first time that proteins have a definite sequence. It was soon shown that all
proteins and enzymes are made up of chains of definite sequence of amino acids.
VernonIngram, in 1956, determined the molecular basis
ofsicklecellanemia. It is aprotein defect inheritedasaMendeliandisorder.·Byanalyzingthestructureofnormalandsickle
cellhemoglobin, he showed that sicklecellanemiaiscausedbyachangefrom glutamic
acidtovalineatasingle positionintheprotein. The alleles of the gene encoding
hemoglobin differed only in their specification of this one amino acid in the
hemoglobin amino acid chain.
Modern Concept of Gene
The characteristics of sickle cell anemia and most other
hereditary traits are defined by changes in protein structure. These changes
are brought about by an alteration in the sequence of amino acids that make up
the protein. This sequence in turn is dictated by the order of nucleotides in a
particular region of chromosome. For example, the critical change leading to
sickle cell disease is a mutation that replaces a single thymine with an
adenine at the position that codes for glutamic acid converting the position to
valine. The sequence of nucleotides that determines the amino acid-sequence of
a protein-is called a gene.
Central Dogma of Molecular Biology
The overall process of production of polypeptides from genes
is called gene expression. The information required for synthesis and
expression of proteins is present in the DNA. This information is passed on to
RNA which is then converted to proteins. This is called “central dogma”
of molecular biology.
Making an mRNA copy from DNA is called transcription.
The enzyme that catalyzes the synthesis of RNA is called RNA polymerase.
Each gene has a special sequence present upstream (before) of the gene, called
a promoter sequence. RNA polymerase can recognize and bind to this
sequence. The enzyme then moves along the gene, reading it and making an mRNA
copy during the process. When RNA polymerase reaches the stop signal, it separates
from the DNA and the newly synthesized mRNA is released. This mRNA is a complementary
transcript of the gene from which it was copied.
The second step in central dogma is the synthesis of
polypeptide chain using the information contained in the mRNA. This process is
called translation. Ribosomes present in the cytoplasm perform this
task.
Types of RNA
There are three types of RNA present in the cell.
The type of RNA that is present in ribosomes is called ribosomal
RNA (or rRNA). It provides the sites of amino acid assembly during
translation.
Molecules of transfer RNA (or tRNA) are responsible
for transfer of amino acid molecules to ribosomes for synthesis of proteins.
They alsoposition each aminoacidatthecorrectplaceon the new polypeptidechain.
There are about 45 different types of tRNA molecules in humans.
Messenger RNA (or mRNA) is the complimentary copy of
the gene. They are long strands of RNA that are produced after transcription of
DNA. They travel to the ribosomes to direct precisely which amino acids are
assembled into polypeptides.
The Process of Transcription
Transcription is the process during which an mRNA copy of
the DNA sequence (gene) is produced. The enzyme that catalyzes the synthesis of
RNA is called RNA polymerase. Only one type of RNA polymerase is present
in prokaryotes. On the other hand, there are three types of RNA polymerases in
eukaryotes:
·
RNA polymerase I synthesizes rRNA
·
RNA polymerase II synthesizes mRNA
·
RNA polymerase III synthesizes tRNA
There are two strands in DNA. Only one strand is transcribed.
The strand which is transcribed is called template strand or antisense
strand. The opposite strand is called coding strand or sense
strand.
Like DNA, RNA is also synthesized in 5`Ã 3` direction. Transcription
begins when RNA polymerase binds to a specific sequence in DNA. This RNA
polymerase binding site is called promoter. There are two binding sites
present in the promoters of prokaryotes:
·
First site has the sequence TTGACA and is also
called –35 sequence. In eukaryotes this sequence is present at –75.
·
Second site has the sequence TATAAT and is also
called –10 sequence. In eukaryotes this sequence is present at –25.
A subunit of RNA polymerase, called sigma factor, is
involved in proper binding of RNA polymerase and initiation of transcription.
After initiation, sigma factor is released. The remaining part of the enzyme
moves on the template strand and completes the transcription of the gene. The
two DNA strands are separated and a “transcription bubble” is formed.
A stop signal is present at the end of the gene. The
simplest stop signal is a series of GC base pairs followed by a series of AT
base pairs. The RNA in this region forms a GC hairpin. Four or more U
ribonucleotides are also present after it. This hairpin stops the RNA polymerase.
There is no nucleus present in bacteria. The process of
transcription takes place inside the cytoplasm. The new mRNA is directly used
for polypeptide synthesis.
In eukaryotes, the mRNA has to travel long distance from
inside the nucleus to cytoplasm where ribosomes are present. Eukaryotic mRNA is
further modified to help in this journey. A cap and tail are added to mRNA
molecules. This cap and tail save the mRNA from digestion by nucleases and
phosphatases.
·
A cap is added to the 5` end of the mRNA.
It is a molecule of 7-methyl GTP, which is linked to mRNA by a 5` to 5`
linkage.
·
A tail is also added to the mRNA. It is in the
form of long stretches of Adenine nucleotides. It is therefore called poly-A
tail.
The Genetic Code
The information in mRNA is in the form of sets of 3
nucleotides. This three nucleotide set is called a codon. Each codon specifies a particular amino acid. The reading of
genetic code is continuous, without punctuation between codons.
There are only four types of nucleotides (A, G, U, C) in
RNA. If each codon was only two nucleotides long, it will make only 42
or 16 different combinations. Since each codon has three nucleotides, it makes
43 or 64 different combinations. These are more than enough to code
for 20 amino acids.
Nirenberg and coworkers made artificial mRNAs with triplet
codons. They used these artificial mRNAs to synthesize polypeptides in cell
free systems. The resulting polypeptides showed which codons coded for which
amino acid. The complete genetic code was determined in 1960s.
Every gene starts with an initiation codon or start codon (AUG). It encodes for the amino
acid methionine.
Three codons (UAA, UGA, and UAG) do not code for any amino
acid, so they are called non-sense
codons. They are usually present at the end of the gene, so they are also
called stop codons. They give “stop”
signal to the ribosomes.
The genetic code is universal.
It is same in almost all organisms. For example AGA specifies the amino acid
arginine in bacteria, humans and all other organisms. Due to this universality,
genes can be transferred from one organism to another and be successfully transcribed
and translated.
The genetic code of mitochondria is somewhat different from
that of other organisms, showing that genetic code is not absolutely universal.
For example, UGA is normally a stop codon, but in mitochondria it codes for
tryptophan. AUA codes for isoleucine in all organisms but in mitochondria it codes
for methionine. Similarly, AGG and AGA code for arginine in all organisms but
in mitochondria it is a stop codon.
Process of Translation
The process of synthesizing a polypeptide chain from mRNA is
called translation. Ribosomes perform the function of translation in the
cytoplasm. Each ribosome has three sites in it. The first site is called A (aminoacyl)site,
from where tRNA enters the ribosome. The second site is called P (polypeptide)
site, which holds the growing polypeptide chain. The third site is called E
(exit) site, from where tRNA leaves the ribosome.
The process of translation begins when rRNA of ribosome
binds to initial portion of mRNA. The mRNA lies on the ribosome in such a way
that only one codon is exposed at the polypeptide. A tRNA molecule possessing
the complementary three nucleotide sequence or anticodon, binds to this
exposed codon on mRNA.
The ribosome then moves forward on mRNA and the next codon
is exposed. Series of tRNA molecules bind one after another to the exposed
codons. Each tRNA has an attached amino acid molecule which is added to growing
polypeptide chain.
Amino acid molecules are attached to tRNA molecules by a
special enzyme called Aminoacyl-tRNAsynthetase. There are 20 types of
these enzymes, one for each amino acid.
Initiation
In prokaryotes, polypeptide synthesis begins with the
formation of initiation complex. A tRNA molecule carrying a chemically
modified methionine (called N-formyl methionine) binds to the small ribosomal
subunit. Special proteins, called initiation factors, position thetRNAontheribosomalsurfaceatthePsite.
This initiation complex, guided by another initiation factor, binds to AUG on
the mRNA.
Elongation
After formation of initiation complex, large ribosomal
subunit also binds to mRNA. Proteins called elongation factors assist in
binding of a tRNA to the exposed mRNA codon at the A site. The two amino acids
which now lie adjacent to each other undergo a chemical reaction. This reaction
is catalyzed by large ribosomal subunit. Initial methionine is released from
its tRNA and attached the second amino acid by peptide bond.
The ribosome now moves (translocates) three more nucleotides
along the mRNA molecule in 5' Ã
3' direction. This movement is also guided by elongation factors. The initial tRNA
is also moved to E site during this process and is ejected from ribosome. The
amino acid chain is also translocated to P site. A new codon is exposed at A
site. A tRNA molecule recognizing that codon appears and binds to the codon at
the A site, placing its amino acid adjacent to the growing chain. The chain
then transfers to the new amino acid, and the entire process is repeated.
Termination
Elongation continues in this manner until a
chain-terminating non sense codon is exposed (for example UAA). Nonsense codons
do not bind to tRNA, but they are recognized by release factors. Release
factors are proteins that release the newly made polypeptide from the
ribosomes.
Mutations
Changes in sequence of DNA are called mutations. These
changes may occur due to mistakes during replication or due to some kind of
damage. Mutationsin somatic cells are not passed on to next generation.
They have little evolutionary consequences. Germ line mutations are
passed on to next generations and provide raw material for evolution.Mutations
can be divided into two types:
1) Chromosomal Aberrations
Chromosomal aberrations are large-scale in number or
structure of chromosomes. An extra chromosome may be added or lost due to
mistake during meiosis. Sometimes a small part of chromosome may get deleted,
duplicated, or inverted. These types of mutations may cause diseases like
Down’s syndrome or Klinefelter’s syndrome.
2) Point Mutations
Pointmutations aresmall mutations in the sequence of DNA
nucleotides. They affect the message itself. They involve changes in one or a
few bases only. Some point mutations may arise due to spontaneous errors during
DNA replication. Some mutations occur due to damage caused by mutagens(like
radiation or chemicals). Modern industrialized societies releasemanychemical mutagensintotheenvironment.Sickle
cell anemia and phenylketonuria are well known examples of point mutation.
Examples of Mutations
Sicklecellanemiais caused by a point mutation that
leads to change of amino acid (glutamic acid to valine) at position 6 of
N-terminal end in beta chain of hemoglobin. This alters the tertiarystructureof
thehemoglobinmolecule, reducingitsabilityto carryoxygen.
In phenylketonuria an enzyme phenylalanine hydroxylase
is defective. Due to this defect, phenylalanine is not degraded, and
accumulates in the cells. Brain fails to develop during infancy leading to
mental retardation. This disorder is also because of a point mutation.
21 Cell Cycle
The cell undergoes a sequence of changes during its life,
called cell cycle. These changes include period of growth, DNA synthesis
and cell division. Average cell cycle in humans is 24 hours while in yeast it
takes only 90 minutes. The cell cycle can be divided into two phases:
·
Interphase is the period of non-apparent
division.
·
Mitosis is the period of cell division.
Interphase
The period of life cycle of cell between two consecutive
divisions is called interphase. It is sometimes misleadingly called
“resting phase”. However, it is the period of great biochemical activity.
During interphase, the DNA is present in the nucleus in the
form of network of very fine threads. This network is calledchromatin
material. Chromosomes are not visible even under electron microscope during
interphase.
Interphase can be further divided into three stages. At each
stage, there are specific check-points, which determine the fate of new
phase according to cell’s internal make up.
Gap 1 Phase or G1 (Post Mitotic phase)
It is a period of extensive metabolic activity. The cell
grows in size, synthesizes many enzymes, and accumulates nucleotides for DNA
synthesis during this phase. G1 phase lasts for about 9 hours.
Sometimes, the post-mitotic cells exit cell cycle during G1
phase and enter a phase known as G0 phase. They can remain in
G0 for days, weeks, months or even for lifetime of the individual
(e.g., nerve cells and cells of the eye lens).
Synthesis (S) Phase
The cell enters S phase after G1 phase. During
this phase, DNA is synthesized and the number of chromosomes is doubled. This
phase takes about 10 hours.
Gap 2 Phase G2 (Pre Mitotic Phase)
After the DNA has been synthesized, the cell enters G2
or pre-mitotic phase. The cell prepares itself for division during this phase.
The cell stores energy for chromosome movement and synthesizes many mitosis
specific proteins. Cell also synthesizes RNA and microtubule subunits for
spindle formation. G2 phase requires about 4.5 hours after which the
cell enters mitosis.
Mitosis
Mitosis is the type of cell division in which the
number of chromosomes remains constant in parent and daughter cells. It can
take place in haploid as well as in diploid cells in nearly all parts of the
body if and when required. Although it is a continuous process, conventionally mitosis
can be divided into two phases.
Karyokinesis
Karyokinesis (karyon=nucleus) is the division of the nucleus.
Following events take place during karyokinesis.
In the beginning of karyokinesis, partitioning of centrioles
takes place. These centrioles had been duplicated in the interphase but were
present in the same centrosome. At the start of mitosis, the two pairs of
centrioles separate and move to separate sides of nucleus.
Centrioles organize special sets of microtubules, called mitotic
apparatus. It is larger than nucleus and is designed to attach and capture
chromosomes. There are three types of microtubules in mitotic apparatus:
·
Astral microtubules radiate outward from
the centrioles and attach them to plasma membrane.
·
Kinetochore microtubules attach to
chromosomes at kinetochores. Kinetochore is a special protein complex at
centromere of chromosomes. Kinetochore fibers can attach to chromosomes through
these proteins.
·
Polar microtubules do not interact with
the chromosomes but instead interdigitate with polar microtubules from the
opposite pole.
Karyokinesis can be further divided into four phases.
Prophase
The following changes take place during prophase:
·
The chromatin material gets condensed by folding
and the chromosomes become visible as thin threads. They become more and more
thick, ultimately each chromosome is visible having two sister chromatids,
attached at centromere.
·
Nuclear envelope disappears and nuclear material
is released in the cytoplasm.
·
Nucleoli disappear.
·
Mitotic apparatus is organized.
·
Cytoplasm becomes more viscous.
Metaphase
A metaphase chromosome is a duplicated structure which
consists of two sister chromatids.
The kinetochore fibers of spindle attach to the kinetochore
region of chromosome.Two spindle fibers are attached to each kinetochore, one
from each pole.
Chromosomes are aligned at the equator of the spindle
forming equatorial plate or metaphase plate.
Anaphase
It is the most important phase of mitosis, which ensures
equal distribution of chromatids in the daughter cells.
The kinetochore fibers of spindle contract towards their
respective poles. At the same time polar microtubules elongate and exert force.
Sister chromatids are separated from centromere.Half of the
sister chromatids travel towards each pole.
Telophase
The chromosomes unfold and decondense and ultimately
disappear in the form of chromatin.
Mitotic apparatus disorganizes nuclear membrane. Two nuclei
are thus formed two poles of the cell.
Nucleoli also reappear.
Cytokinesis
During late telophase the astral microtubules send signals
to the equatorial region of the cell. Actin and myosin are activated in this
region which form contractile ring. This ring later becomes cleavage
furrow, which deepens towards the center of the cell, dividing the parent cell
into two daughter cells.
Mitosis in Plants
Major steps of mitosis are similar in plants and animals
despite slight differences:
Most of the higher plants lack visible centrioles. Instead,
they have an analogous region from which the spindle microtubules radiate.
Shapeoftheplantcelldoesnotchangegreatly as compared
to ananimalcellbecauseitis surrounded by arigidcellwall.
During cytokinesis in plants, a membranous structure is
formed at the place of cleavage furrow, called phragmoplast. It is
formed by fusion of small vesicles secreted by Golgi complex. These vesicles
line up in the center of the cell (during metaphase) and then fuse (at the end
of telophase). Themembraneofvesiclesbecomestheplasmamembraneofdaughtercells.
These vesicles also contain materials for future cell wall such as precursors
of cellulose and pectin.
Importance of Mitosis
1.
Inmitosisthehereditarymaterialisequallydistributed
inthedaughtercells.
2.
There is no crossing over or recombination. The
genetic information remains unchanged generation after generation. Continuity
of similar information is ensured from parent to daughter cell.
3.
Some organisms, bothplantsandanimals, can undergoasexual
reproduction by mitosis.
4.
Regeneration, healing of wounds and replacement
of older cells all are the gifts of mitosis.
5.
Development and growth of multicellular
organisms depends upon orderly, controlled mitosis.
6.
Tissuecultureandcloning techniques seekhelpthroughmitosis.
7.
An organism requires managed, controlled and
properly organized process of mitosis, which otherwise may result in malfunction,
unwanted tumors and lethal diseases like cancer.
Cancer – Uncontrolled Cell Division
The process of cell death and duplication are very strictly
regulated and both processes are balanced in a normal individual. Sometimes the
control, that regulates the cell multiplication, breaks down. A cell, in which
this occurs, begins to grow and divide in unregulated fashion without body's
need for further cells of its type.
A tumor is formed when cells continue to proliferate
in uncontrolled fashion. It is an unwanted clone of cells, which can expand
indefinitely. They are of two types:
Benign Tumor
Some tumors are of small size and localized (not transferred
to other parts) called benign tumors. The cells in this type usually
behave like the normal cells and have little deleterious effects. They may
interfere with normal cells by producing hormone-like secretions.
Malignant Tumor (Cancer)
The cells composing a malignant tumor or cancer
divide more rapidly and invade surrounding tissues. They get into the
circulatory system, and set up areas of proliferation, away from their site of
original appearance. This spread of tumor cells and establishment of secondary
areas of growth is called metastasis.
Characteristics of Cancer Cells
Cancer cells can be distinguished from normal cells because
they are less differentiated than normal cells and exhibit the characteristics
of rapidly growing cells. They have high nucleus to cytoplasm ratio, prominent
nucleoli, and continuously undergo mitosis.The presence of invading cells in
otherwise normal tissue is indication of metastasis.
Cause of Cancer
Cancer may result from as many as 3 to 20 different
mutations in genes that regulate cell cycle. Metastatic cells break contact
with other cells and overcometherestrictionsoncellmovement provided by basal lamina.
Ultimately metastatic cells can invade other parts of the body. Secondly, they
proliferate unlimitedly, without considering the checks or programs of the
body.
Meiosis
Meiosis is a special type of cell division in which
the number of chromosomes in daughter cells is reduced to half as compared to
parent cells. It takes place in diploid cells only at the time of gamete
formation. It produces eggs and sperms in animals and spores in plants.
Each diploid cell after meiosis produces four haploid cells,
because it involves two consecutive divisions after single replication of DNA.
These two divisions are called Meiosis I and Meiosis II.Both divisions can be
divided into further sub-stages.
Meiosis I
The first meiotic division is reduction division as
number of chromosomes is reduced to half during this phase. It can be divided
into four stages like mitosis.
Prophase I
Each diploid cell contains two chromosomes of the same type.
One member of this pair comes from each parent at the time of fertilization.
Each chromosome has two sister chromatids, because chromosomes have been replicated
during interphase. These similar but not necessarily identical chromosomes are
called as homologous chromosomes.
Prophase I can be further divided into followingfive stages:
·
During Leptotene, chromosomes become
visible, short, and thick. The size of the nucleus increases and homologous
chromosomes start getting closer to each other.Leptotene and zygotene can last
only for few hours.
·
During Zygotene, pairing of chromosomes
begins. This process is called synapsis. This pairing is highly specific
and exactly pointed, but with no definite starting point. Each paired but not
fused, complex of chromosomes is called bivalent or tetrad.
·
During Pachytene, the pairing of
homologous chromosomes is completed. Chromosomes become more and more thick.
Each bivalent has four chromatids, which wrap around each other.Non-sister
chromatids of homologous chromosomes exchange their segments due to chiasmata
formation, during the process called crossing over. Reshuffling ofgeneticmaterialtakes
placewhichproducesrecombination.Pachytene may last for days, weeks or even
years.
·
During diplotene, paired chromosomes
repel each other and start separating. The separation is not complete as
homologous chromosomes are still united at chiasmata.Each bivalent has at least
one such point, the chromatids otherwise are separated.
·
During Diakinesis, the condensation of
chromosomes reaches its maximum. At the same time separation of the homologous
chromosomes (started during diplotene) is completed, but still they are united
at one point, more often at ends. Nucleoli disappear.
Metaphase I
Nuclear membrane disorganizes at the beginning of this
phase.
Spindle fibers originate. Kinetochore fibers attach to the
kinetochore of homologous chromosome from each pole and arrange bivalents at
the equator.
The sister chromatids of individual chromosome in bivalent
behave as a unit.
Anaphase I
The kinetochore fibers contract and the spindle or pole
fibers elongate, which pull the individual chromosome (each having two
chromatids) towards their respective poles.
It may be noted here that in contrast to anaphase of
mitosis, sister chromatids are not separated. This is actually reduction phase
because each pole receives half of the total number of chromosomes.
Telophase I
Nuclear membrane reorganizes around each set of chromosomes
at two poles, nucleoli reappear thus two nuclei each with half number of
chromosomes are formed, later on cytoplasm divides thus terminating the first
meiotic division. It is also to be noted that chromosomes may decondense during
this state.
Meiosis II
After telophase I two daughter cells experience small
interphase, but in contrast to interphase of mitosis there is no replication of
chromosomes. Second meiotic division is just like mitosis.
Prophase II
Nuclear membrane disintegrates. Chromosomes condense and
become visible. Mitotic apparatus is organized.
Metaphase II
Chromosomes arrange themselves at equator. Each chromosome
is attached with kinetochore fibers from opposite poles.
Anaphase II
Sister chromatids are separated and move apart. Separated
sister chromatids reach at opposite poles.
Telophase II
Chromosomes decondense and nuclear membrane reappears.
Cytokinesis takes place and four haploid cells are formed.
Each cell has half the number of chromosomes (chromatids) as compared to parent
cell.
Importance of Meiosis
During crossing over, parental chromosomes exchange
segments with each other which results in a large number of recombinations.
The segregation of homologous chromosomes during
anaphase is random, which gives very wide range of variety of gametes.
Boththesephenomenacausevariationsand modificationsinthegenome.Thesevariationsarenotonlythebasesofevolution,but
alsomake every individualuniqueinhischaracteristics.Even the progeny of very
same parents,i.e.,brothersandsistersare not identicaltoeach other.
Meiosis also helps to maintain number of chromosomes constant,
generation after generation. If there was no meiosis, the number of chromosomes
would double after each generation.
Errors During Meiosis (Non-Disjunction)
Sometimes, abnormalities (aberrations) may arise during
meiosis producing unexpected results. One such abnormality is chromosomalnon-disjunction,
in which, the homologous chromosomes fail to separate during anaphase and telophase.
Unequal distribution of chromosomesresults in increased or decreased number of
chromosomes in gametes.
Many such abnormalities result in abortion or death at early
stage of life. Surviving individuals usually suffer from serious physical,
mental and social disorders.
Downs Syndrome
In Down’s syndrome, chromosome number 21 fails to segregate
during meiosis. The resulting gamete has one extra copy of chromosome 21. When
this gamete fertilizes a normal gamete, the new individual will have 47
chromosomes(2n +1).
The cause of Down’s syndrome is non-disjunction in ova
(egg). It is usually related to age of mother. The chances of a young mother
producing a baby with Down’s syndrome are one in several thousands. If the
mother is near 40 years of age, the chances increase to one in hundred. After
45 years of age, chances increase three times.
Individuals with Down’s syndrome usually have flat and broad
face, squint eyes, and protruding tongue. Abnormal development of nervous
system results in mental retardation. This disease is also called Mongolism.
Klinefelters Syndrome
Individuals with Klinefelter’s syndrome have an extra sex
chromosome (44 autosomes + XXY). These individuals look like males but have
abnormally large breasts. They are tall and obese, while testes are small. They
cannot produce sperms and have underdeveloped secondary sexual characters.
Turners Syndrome
Individuals with Turner’s syndrome have one missing X
chromosome (44 autosomes + X). These individuals are usually aborted during
pregnancy. Those who survive are like females in appearance. They have short
stature and webbed neck. They do not have ovaries and cannot produce eggs.
Necrosis and Apoptosis
The division, pattern formation, differentiation, morphogenesis
and motility of a cell usually depend on a number of intracellular and
extracellular signals. The responsibility of each cell is pre-determined. This
is called the fate of the cell.
Eventhe death of the cell ispre-planned. Programmed cell
death helps in proper control of multicellular development. Sometimes a
structure or its parts need to be removed (e.g.,thetailofdevelopinghumanembryos
and tadpoles and tissue between developing digits).
The cells need survival signals from surrounding cells to
remain alive. In the absence of these signals, the cell commits suicide. These
signals are called trophic factors. Alternatively, surrounding cells may
send a death signal which may trigger suicide in receiving cell.
Internalprogramofeventsand sequenceofmorphological
changesbywhich cells commit suicide are collectivelycalledas apoptosis.
During apoptosis the dying cells shrink and condense,and ultimately
split up.Small membrane bounded vesicles are produced, calledapoptotic bodies.
These apoptotic bodies are later removed by phagocytosis by other cells (like
macrophages). Intracellular constituents are not released freely in extracellular
atmosphere which otherwise might be dangerous for other cells.
The cell death due to tissue damage is called necrosis.During
necrosis, a typical cell swells and bursts, releasing the intracellularcontents,which
can damage neighboring cellsandcause inflammation.
22 Variation and Genetics
Geneisthebasic unit of biological information. DNA
stores biological information coded in the sequence of its bases in a linear
order.
Hereditary characteristics pass from parents to
offspring through genes in their gametes. Genes are responsible forproducingstartlinginherited
resemblance aswellasdistinctivevariationsamonggenerations.
Position of a gene on the chromosome is called locus.
Genes are present in the form of pairs on homologous
chromosomes. One member of a gene pair is located on one homologue, and the
other member on the other homologue. These partners of agenepairarecalledalleles.
Each allele of a gene pair occupies the same gene locus.
Phenotype istheappearanceof a trait.Genotype
is the genetic component responsible for a single trait.
A flower may be red or white in color. Color of flower is a
trait and red and white are· its two phenotypes. Each form· of expression is
determined by a different allele of the color gene. Allele "R" is the
determiner for redness, while "r" is the determiner for whiteness.
A group of interbreeding organisms of the same species that
exist together in both time and space is called a population. All the
genes/alleles found in a population at a given time are collectively called the
gene pool. It is the total genetic information present in the total
genes in a population existing at a given time.
Mendel’s Laws of Inheritance
Mendel performed series of breeding experiments on garden
pea (Pisumsativum).It is normallya self-fertilizing plant,but can alsobecross-fertilized.
Peahadmanysharplydistincttraits. Each trait had two clear
cut alternative forms or varieties; e.g., seed shape had a round or wrinkled
phenotype, plant height was either tall or short, seed color could be yellow or
green. Mendel called them contrasting pair of a trait.
Mendel established true-breeding lines or varieties for each
trait. A true-breeding variety always produces offspring identical to
the parents upon self-fertilization. For example, a true breeding
"round" seed plant produces only "round" seeds. Similarly,
a true breeding "wrinkled" seed plant produces only
"wrinkled" seeds.
Law of Segregation
Mendel cross-fertilized a true breeding round-seeded plant
with a true breeding wrinkled-seeded plant. He called it first parental
generation (P1).Their offspring were called F1 or first filial
generation. All F1 offspring were round like one of the parents. Wrinkled
phenotype did not appear at all.
Cross in which the individuals differ in only one trait is
called monohybrid cross. The trait that appears in the F1 is called dominant
trait. The trait that is masked in the F1 is called recessive trait.
Mendel allowed self-fertilization among F1 individuals to
produce F2 generation. About ¾ seeds produced this time were round while ¼
seeds were wrinkled. He obtained similar results for different contrasting
pairs of traits.
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