Cell Division
Amitosis is
also called direct or incipient cell division. Here there is no spindle formation and chromatin
material does not condense. It consist of two steps: (Figure 7.2).
•
Involves division of nucleus.
•
Nucleus develops a constriction at the center and becomes
dumbell shaped.
•
Constriction deepens and divides the nucleus into
two.
•
Involves division of cytoplasm.
•
Plasma membrane develops a constriction along
nuclear constriction.
•
It deepens centripetally and finally divides the
cell into two cells.
Example: Cells of mammalian cartilage, macronucleus
of Paramecium and old degenerating
cells of higher plants.
(a) Causes
unequal distribution of chromosomes.
(b) Can lead
to abnormalities in metabolism and reproduction.
The most important part of cell division concerns
events inside the nucleus. Mitosis occurs in shoot and root tips and other
meristematic tissues of plants associated with growth. The number of
chromosomes in the parent and the daughter (Progeny) cells remain the same so
it is also called as equational division.
In closed mitosis, the nuclear envelope remains
intact and chromosomes migrate to opposite poles of a spindle within the
nucleus (Figure 7.3).
Example: Many single celled eukaryotes including
yeast and slime molds.
In open mitosis, the nuclear envelope breaks down
and then reforms around the 2 sets of separated chromosome.
Example: Most plants and animals
Some animals are able to regenerate
the whole parts of the body.
Mitosis is divided into four stages prophase,
metaphase, anaphase and telophase (Figure 7.6).
Prophase is the longest phase in mitosis.
Chromosomes become visible as long thin thread like structure, condenses to
form compact mitotic chromosomes. In plant cells initiation of spindle fibres
takes place, nucleolus disappears. Nuclear envelope breaks down. Golgi
apparatus and endoplasmic reticulum are not seen.
In animal cell the centrioles extend a radial array
of microtubules (Figure 7.4) towards the plasma membrane when they reach the
poles of the cell. This arrangement of microtubules is called an aster. Plant cells do not form
asters.
Chromosomes (two sister chromatids) are attached to
the spindle fibres by kinetochore of the centromere. The spindle fibres is made
up of tubulin. The alignment of chromosome into compact group at the equator of
the cell is known as metaphase plate.
This is the stage where the chromosome morphology can be easily studied.
Kinetochore is a DNA–Protein complex present in the
centromere DNA where the microtubules are attached. It is a trilaminar disc
like plate.
The spindle assembly checkpoint which decides the
cell to enter anaphase.
Each chromosome split simultaneously and two
daughter chromatids begins to migrate towards two opposite poles of a cell.
Each centromere splits longitudinally into two, freeing the two sister
chromatids from each other. Shortening of spindle fibre and longitudinal
splitting of centromere creates a pull which divides chromosome into two
halves. Each half receive two chromatids (that is sister chromatids are
separated). When the sister chromatids separate the actual partitioning of the
replicated genome is complete.
A ubiquitine ligase is activated called as the anaphase-promoting complex cyclosome (APC/C) leads to degradation of the key regulatory proteins at the transition of metaphase to anaphase.
APC is a cluster of proteins that induces the breaking down of cohesion proteins which leads to the separation of chromatids during mitosis (Figure 7.5).
Two sets of daughter chromosomes reach opposite
poles of the cell, mitotic spindle disappears. Division of genetic material is
completed after this karyokinesis, cytokinesis (division of cytoplasm) is
completed, nucleolus and nuclear membranes reforms. Nuclear membranes form
around each set of sister chromatids now called chromosomes, each has its own centromere. Now the chromosomes
decondense. In plants, phragmoplast are formed between the daughter cells. Cell
plate is formed between the two daughter cells, reconstruction of cell wall
takes place. Finally the cells are separated by the distribution of organelles,
macromolecules into two newly formed daughter cells.
A Culture of animal cells in which
the cell cycles were asynchronous was incubated with 3H-Thymidine for 10
minutes. Autoradiography showed that 50% of the cells were labelled. If the
cell cycle time (generation time) was 16 hrs how long was the S period?
Length of the S period = Fraction of cells
in DNA replication × generation time
Length of the S period = 0.5 × 16 hours = 8 hours
It is a contractile process. The contractile
mechanism contained in contractile ring located inside the plasma membrane. The
ring consists of a bundle of microfilaments assembled from actin and myosin. This
fibril helps for the generation of a contractile force. This force draws the
contractile ring inward forming a cleavage furrow in the cell surface dividing
the cell into two.
Division of the cytoplasm often starts during
telophase. In plants, cytokinesis cell plate grows from centre towards lateral
walls - centrifugal manner of cell plate formation.
Phragmoplast contains microtubules, actin filaments
and vesicles from golgi apparatus and ER. The golgi vesicles contains
carbohydrates such as pectin, hemicellulose which move along the microtubule of
the pharagmoplast to the equator fuse, forming a new plasma membrane and the
materials which are placed their becomes new cell wall. The first stage of cell
wall construction is a line dividing the newly forming cells called a cell plate. The cell plate eventually
stretches right across the cell forming the middle lamella. Cellulose builds up
on each side of the middle lamella to form the cell walls of two new plant
cells.
Exact copy of the parent cell is produced by
mitosis (genetically identical).
1.
Genetic
stability – daughter cells are genetically
identical to parent cells.
2.
Growth – as
multicellular organisms grow, the
number of cells making up their tissue increases. The new cells must be
identical to the existing ones.
3.
Repair of
tissues - damaged cells must be
replaced by identical new cells by mitosis.
4.
Asexual
reproduction – asexual reproduction
results in offspring that are identical to the parent. Example Yeast and
Amoeba.
5.
In flowering plants, structure such as bulbs,
corms, tubers, rhizomes and runners are produced by mitotic division. When they
separate from the parent, they form a new individual.
The production of large numbers of offsprings in a
short period of time, is possible only by mitosis. In genetic engineering and
biotechnology, tissues are grown by mitosis (i.e. in tissue culture).
6.
Regeneration
– Arms of
star fish
In Greek meioum
means to reduce. Meiosis is unique because of synapsis, homologous
recombination and reduction division. Meiosis takes place in the reproductive organs. It results in the
formation of gametes with half the normal chromosome number.
Haploid sperms are made in testes; haploid eggs are
made in ovaries of animals.
In flowering plants meiosis occurs during
microsporogenesis in anthers and megasporogenesis in ovule. In contrast to
mitosis, meiosis produces cells that are not genetically identical. So meiosis
has a key role in producing new genetic types which results in genetic
variation.
Meiosis can be studied under two divisions i.e.,
meiosis I and meiosis II. As with mitosis, the cell is said to be in interphase
when it is not dividing.
Prophase I is the longest and most complex stage in meiosis.
Pairing of homologous chromosomes (bivalents).
Prophase
I –
Prophase I is of longer duration and
it is divided into 5 substages – Leptotene, Zygotene, Pachytene, Diplotene and
Diakinesis (Figure 7.7).
Leptotene
–
Chromosomes are visible under light
microscope. Condensation of chromosomes takes place. Paired sister chromatids
begin to condense.
Zygotene – Pairing
of homologous chromosomes takes
place and it is known as synapsis.
Chromosome synapsis is made by the formation of synaptonemal complex. The
complex formed by the homologous chromosomes are called as bivalent (tetrads).
Pachytene
– At this
stage bivalent chromosomes are
clearly visible as tetrads. Bivalent of meiosis I consists of 4 chromatids and
2 centromeres. Synapsis is completed and recombination nodules appear at a site
where crossing over takes place between non-sister chromatids of homologous
chromosome. Recombination of homologous chromosomes is completed by the end of
the stage but the chromosomes are linked at the sites of crossing over. This is
mediated by the enzyme recombinase.
Diplotene
–
Synaptonemal complex disassembled
and dissolves. The homologous chromosomes remain attached at one or more points
where crossing over has taken place. These points of attachment where ‘X’
shaped structures occur at the sites of crossing over is called Chiasmata. Chiasmata are chromatin
structures at sites where recombination has been taken place. They are
specialised chromosomal structures that hold the homologous chromosomes
together. Sister chromatids remain closely associated whereas the homologous
chromosomes tend to separate from each other but are held together by
chiasmata. This substage may last for days or years depending on the sex and
organism. The chromosomes are very actively transcribed in females as the egg
stores up materials for use during embryonic development. In animals, the
chromosomes have prominent loops called lampbrush
chromosome.
Diakinesis
–
Terminalisation of chiasmata.
Spindle fibres assemble. Nuclear envelope breaks down. Homologous chromosomes
become short and condensed. Nucleolus disappears.
Spindle fibres are attached to the centromeres of the two homologous chromosomes. Bivalent (pairs of homologous chromosomes) aligned at the equator of the cell known as metaphase plate. Each bivalent consists of two centromeres and four chromatids.
The random distribution of homologous chromosomes
in a cell in Metaphase I is called independent
assortment.
Homologous chromosomes are separated from each
other. Shortening of spindle fibers takes place. Each homologous chromosomes
with its two chromatids and undivided centromere move towards the opposite
poles of the cells. The actual reduction in the number of chromosomes takes
place at this stage. Homologous chromosomes which move to the opposite poles
are either paternal or maternal in origin. Sister chromatids remain attached
with their centromeres.
Haploid set of chromosomes are present at each
pole. The formation of two daughter cells, each with haploid number of
chromosomes. Nuclei are reassembled. Nuclear envelope forms around the
chromosome and the chromosomes becomes uncoiled. Nucleolus reappears.
In plants, after karyokinesis cytokinesis takes
place by which two daughter cells are formed by the cell plate between 2 groups
of chromosomes known as dyad of cells
(haploid).
The stage between the two meiotic divisions is
called interkinesis which is
short-lived.
This division is otherwise called mitotic meiosis. Since it includes all the stages of mitotic divisions.
The chromosome with 2 chromatids becomes short,
condensed, thick and becomes visible. New spindle develops at right angles to
the cell axis. Nuclear membrane and nucleolus disappear.
Chromosome arranged at the equatorial plane of the
spindle. Microtubules of spindle gets attached to the centromere of sister
chromatids.
Sister chromatids separate. The daughter chromosomes
move to the opposite poles due to shortening of microtubules. Centromere of
each chromosome split, allowing to move towards opposite poles of the cells
holding the sister chromatids.
Four groups of chromosomes are organised into four
haploid nuclei. The spindle disappears. Nuclear envelope, nucleolus reappear.
After karyokinesis, cytokinesis follows and four
haploid daughter cells are formed, called tetrads.
·
This maintains a definite constant number of
chromosomes in organisms.
•
Crossing over takes place and exchange of genetic
material leads to variations among species. These variations are the raw
materials to evolution. Meiosis leads to genetic variability by partitioning
different combinations of genes into gametes through independent assortment.
•
Adaptation of organisms to various environmental
stress.
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