Cell division
Importance
of cell division:-
Cell
division is a means of multiplication in the unicellular organisms. In
multicellular organisms, it begins about embroyonic development and growth, and
also plays a role in repair and maintenance of the body, and also in
reproduction, both sexual and asexual.
Modes of
cell division:-
Cell
division occurs in three ways: amitosis, mitosis and meiosis. In each case,
division of the nucleus proceeds the division of the cytoplasm.
I.
Amitosis:
amitosis was firstly described by Robert Remak in the red bloods cells of chick
embryo.
Mechanism: Amitosis is very simple. It occurs without the formation of
spindle and appearance of chromosomes. It is therefore often called the direct
division. The nucleus of a cell elongates and develops a constriction round its
middle. The constriction gradually deepens and finally cuts the nucleus into two
daughter nuclei. A similar constriction appears in the cytoplasm between the
two nuclei and divides the cell into two daughter cells, each with a nucleus.
The daughter cells receive approximately equal amounts of nuclear and
cytoplasmic materials.
stages in amitosis
Example. Amitosis is rare, probably
because it is not an exact method of cell division. It takes place in certain
specialized cells, like those in the mammalian cartilage, in the degenerating
cells of the diseased tissues, and in the old tissues. Foetal membranes of some
vertebrates grow by amitotic cell divisions. The macronucleus of ciliates
divides by amitosis. Its chromatin remains attached to the nuclear envelope
throughout the division process.
II.
Mitosis: discovery:- mitosis was first described by a German
biologist Eduard Strasburger in 1875 and then by another German biologist
Walther Fleming in 1879. It was termed ‘’mitosis’’ by Walther Flemming in 1882.
Occurrence: mitosis is the common method of cell division in eukaryotes.
It takes place in the somatic cells of the body. Hence, it is also known as the
somatic division. It occurs in the gonads also for the multiplication of
undifferentiated germ cells. It takes place in the meristematic tissues in
plant cells.
Duration:- mitosis often lasts on an average fro 30 minutes to 3 hours.
Definition: mitosis is the division of a parent cell into two identical
daughter cells each with a nucleus having the same amount DNA, the same number
and kind of chromosomes and the same heredity instructions as the parent cell. Hence,
it is also known as the equational division.
Mechanism: mitosis is an elaborate process which involves a series of
important changes in the nucleus as well as the cytoplasm; therefore, it is
often called indirect division also. There are two main events in mitosis:
karyokinesis or duplication of the nucleus, followed by cytokinesis or division
of the cytoplasm. This is followed by separation of the daughter cells.
A. Karyokinesis: karyokinesis in eukaryotes is complex
due to the presence of many chromosomes. Through a continuous process,
karyokinesis may be divided into four stages: prophase, metaphase, anaphase and
telophase.
1. Prophase: the DNA molecules combined with
histone and nonhistone proteins from the chromosomes are greatly extended and
spread throughout the space in the nuclear compartment. The nucleus of a human
G2 cell has approximately 4 meter of DNA organized into 46
duplicated chromosomes. In the extended state, the chromosomes are
indistinguishable, and are together referred to as chromatin of the nucleus.
The extended state of interphase of transcription and replication, but not for processes of
transcription and replication, but not
for division into two daughter cells. To facilitate the separation of
duplicated chromosomes into different cells, evolution has provided a mechanism
by which the chromatin is condensed and compacted into the so called mitotic
chromosomes.
The transition from G2
phase to M phase is induced by some stimulatory agent produced in the cell.
This is indicated by the fact that fusion of a mitotic cell with a nonmitotic
cell, induces premature condensation of the chromatin in the latter’s nucleus.
The prophase is long and
complex. It lasts for about 50 minutes. It may be further divided into three sub
stages: early, middle and late.
a. Early prophase: the following events take place in
the early prophase of mitosis-
1. The
cell becomes more or less rounded, and its cytoplasm turns more viscous.
2. The
centrioles, already duplicates in the interphase, lie close to the nucleus.
Short radiating microtubules assemble around them by polymerization of the
tubulin dimmers. The two pairs of centrioles start moving to the opposite ends
of the cell. The microtubules surrounding each pair of centrioles (diplosome)
look like a star-like body called the aster. The microtubules, termed the
astral rays, are not in contact with the centrioles, but are separated from
them by an amorphous zone of cytoplasm known as pericentriolar cloud. As the
diplosomes move apart, the microtubules stretching between them increase in
number and elongate by incorporating more tubulin dimmers.
3. The role of
the asters is to shift the duplicated centrioles to the opposite ends of the cell.
In these location, the centriole pairs will pass into separate daughter cells
when cytokinesis occurs. The centrioles and asters have no role in the
formation of the spindle. The latter can assemble without asters and
centrioles.
The functions
of centrioles in mitosis are not clear. They may be concerned with orienting
the spindle.
4. Between
the separating asters, long microtubules assemble on one side of the nucleus
and form mitotic spindle. The latter is wide at the equator and tapers toward
the poles, hence the name spindle. This is so because the microtubules are more
closely spaced at the poles than at the equator. The microtubules are arranged
in bundles called spindle fibres. At each pole of the spindle lies the
mother-daughter centriole pair.
5. The
chromosomes first appear as long, thin threads in the nucleus. This thread-like
appearance of the chromosomes gives the cell division its name mitosis (mitos=thread,
osis=state).
formation of asters and primary spindle
6. The chromosomes gradually change into short, thick rodlets and become
visible. This change occurs by loss of water and progressive coiling of
chromosomes. The progressive folding and packing of the chromatin fibres into
thick chromosomes is called condensation. How this process occurs is not clear.
According to the most widely accepted hypothesis, the chromosomal protein
histon H1 forms crosslink between chromatin fibres that fold and hold them
together. Due to the duplication of DNA and chromosomal proteins during the
interphase, each chromosome appears longitudinally double, consisting of two
identical sister chromatids. The chromatids of each chromosome may be coiled
about each other in the beginning. They are held together at the narrow region
called primary constriction or cenrtromere. At this region, each chromatid has
a disc-like structure, the kinetochore, where the spindle microtubules join it.
It may be noted that the chromosomes are fully replicated and double at all the
point along their length, including the centromere. Earlier, centromere was
thought to be unreplicated region of the chromosome, joining its chromatids
together.
b. Middle prophase: the following events take place in
the middle prophase of mitosis:
i.
The
chromosomes further shorten and thicken, and their chromatids uncoil. Finally,
they assume their characteristic forms and sizes, and become distinguishable
individually. The chromosomes differ in the location of primary constriction
and in the presence of secondary constrictions, besides size and shape.
A metaphase chromosomes with the kinetochores jointed by spindles microtubules
ii.
Nucleoli
become progressively smaller and finally disappear. This happens because rRNA
synthesis drops or stops, and the nucleolar materials (partially processed
ribosomal subunits and processing enzymes) are dispersal into the nucleoplasm.
It may be reminded that ribosomal subunits are formed in the nucleoli by
association or rRNA and ribosomal proteins.
iii.
Nuclear
envelope begins to breakdown into small vesicles which disperse into the
cytoplasm, becoming indistinguishable from the ER elements. The lamina
dissociates into its protein subunits.
c. Late prophase: the following events take place in
the late prophase of mitosis:-
i.
The
nuclear envelope breaks down fully, releasing the chromosomes and other nuclear
contents into the cytoplasm. The chromosomes looking like short double rodlets
lie randomly at the site formerly occupied by the nucleus.
ii.
The
spindle assumes its proper form and size. It may contain 500-1000 or more
microtubules.
iii.
The
centriole pair are pushed to the opposite ends of the cell by the growing
spindle.
The spindle and the
asters are together referred to as mitosis apparatus, although a similar
structure is formed in meiosis also.
2. Metaphase: the metaphase is short and simple.
It lasts for 2-100 minutes. It involves the following events:-
i.
The
spindle moves into the region formerly occupied by the nucleus.
ii.
The
chromosomes move to the equatorial plane of the spindle.
iii.
Some
spindle microtubules extend to and join the chromosomes. These are called
chromosomal or kinetochore microtubules. Two chromosomal fibres (bundles of
chromosomal microtubules) are attached to each chromosome, one from one pole of
the spindle is jointed to the kinetochore of one chromatid and from the opposite pole of the second
chromatid. Other spindle microtubules extend from pole to pole of the spindle
and pass right by the chromosomes, not attached to them in any way. These are
termed polar or interpolar microtubules. The polar microtubules do not
necessarily extend from pole to pole; they may begin at one pole and end at
some distance or even exist free in the cytoplasm of the cell.
iv.
The
chromosomes soon get aligned at the middle of the spindle in the form of a
plate called equatorial or metaphase plate. This plate is at right angles of
the long axis of the spindle, and is actually formed by the kinetochores, the
arms of the chromatids trailing away on the sides. The directed movement of the
chromosomes into position at the metaphase plate is termed congression.
The events which connect
the chromosomes to the spindle fibres and bring them to the metaphase plate are
sometimes referred to as prometaphase. Metaphase represents the stage in which
chromosomes have fully aligned into a plate and await the separation of their
chromatids. The chromosomes are perhaps maintained in the equatorial plate by
the balanced tension exerted by the two chromosomal fibres that connect to the
sister kinetochore to the opposite poles.
The entire chromosomal
complement of a cell or species a seen in metaphase pf mitosis is called its
karyotype.
3. Anaphase : the anaphase is very short and simple. It
lasts for only 2 to 3 minutes. It comprise the following events:-
i.
The
sister chromatids of each chromosome slightly separate at the primary
constriction so that their kinetochores stretch toward the opposite poles of
the spindle. Separation of chromatids poles of the spindle. Separation of
chromatids occurs in all chromosomes almost simultaneously. The chromatids are
now referred to as chromosomes because they are no longer held to their
duplicates.
Behaviors of chromosomes in mitosis
ii.
After
a brief pause, the chromatids separate completely form their former mates, and
start moving to opposite poles of the spindle. As each chromosomes is bring
pulled by its attached microtubules, its kinetochore leads and arms trial
behind. With the result, the chromosomes are pulled V, J and I shapes,
depending upon the position of the kinetochore.
Separation of chromatids in anaphase
iii.
As
the chromosomes move toward their respectective poles, the two poles move
farther apart by elongation of spindle. The anaphase ends when all the
chromatids reach the opposite piles. Each pole of the spindle receives one
chromatid from every metaphase chromosome, the two groups of chromatids
(chromosomes) have exactly the same hereditary information.
Chromosomes, aster and spindle in mitotic anaphase
The movement of the
chromosomes is called anaphase A, and the extension of the poles is termed
anaphase B. the mechanisms of these movements are discussed below –
Chromosome movements: the forces responsible for the movements of
the chromosomes are not still clear. One view suggests that the chromosomal
microtubules generate the force for poleward movement of chromosomes. These
microtubules, progressively become shorter, and this separates and pulls the
chromatids toward the poles of the spindle. Shortening may be brought about by
active sliding between the chromosomal microtubules and the polar microtubules,
or by reduction in the length of chromosomal microtubules through the
separation of tubulin subunit at their tips (kinetochore ends) it has been
experimentally shown that disassembly can generate sufficient force to pull the
chromosomes to the poles. Also a spindle isolated form a cell about to divide contracts
when ATP is added. May be that both the
mechanisms play a role in shortening of the chromosomal microtubules.
Moving apart of
spindle poles: elongation
of the spindle takes place as under –
Some microtubules extend
from the poles to a little beyond the equator, but are not joined to the
chromosomes. These microtubules overlap in the middle of the spindle. In the
region of overlap, the microtubules activity slide past each other. This
shortens the region of overlap and increase the overall length of the spindle
by a distance equal to the decrease in overlap.
Anaphase movement. chromosomal microtubules shorten as anaphase proceeds. polar microtubules grow longer, increasing the length of the spindle
The spindle may also
elongate by the addition of tubulin subunits to the ends of the polar
microtubules, that is, by growth of microtubules makes the poles move farther
apart.
Shortening of chromosomal
microtubules and lengthening of polar microtubules by loss and addition of
tubulin subunits occur at the same time in an anaphase cell.
Advantages of
chromosome shortening:
Fictional significance of
chromosome condensation, that occurs in prophase, become clear in anaphase. It
is physically easier for a Shorty, compact chromosome to move through the
cytoplasm than it is for a ling, twisted interphase chromosome.
4. Telophase :- the telophase is long and complex.
It lasts for an hour or so. In this phase, nucleus is reconstructed from each
group of chromosomes. It involve the following events:
i.
The
chromosome at each pole unfolds, and become long and slender. Finally they
become indistinguishable as in an interphase cell.
ii.
Nuclear
envelope is reconstructed around each group of chromosomes. This occurs
gradually. First, membrane vesicles associate with the individual unfolding
chromosomes, partially enclosing each chromosomes. Then they fuse to form an
envelope surrounding the entire set of chromosomes (by now almost changed to
interphase chromatin) at each pole. The lamina proteins reassociate
simultaneously with the reconstruction of nuclear envelop and form a complete
lamina within nuclear envelope. How the nuclear pores are formed is not known.
iii.
Nucleolar
material (partially processed ribosomal subunits and processing enzymes)
dispersed into the cytoplasm in the prophase return to the nucleolar organizer
site and form a small nucleolus. Processing of this preexisting material then
continues. Transcription of new rRNA also begins at this time, it gradually
picks up until it attains the high level characteristics of interphase cell.
With this, the nucleolus reformed at telophase, thus, contains both old and new
rRNA and ribosomal proteins. If nucleoli form at more than one secondary
constrictions, these nucleoli may fuse together or remain separate, depending
on the cell type and the species. In humans, five pairs of chromosomes have
nucleolar organizer sites and form 10 separate nucleoli during telophase. These
usually fuse into a single nucleoulus during the subsequent interphase. Presumably
, the dispersal of nucleolar material in prophase is an adaption aimed at
reducing the chromosome mass required to be moved in anaphase. It probably also
reduces the chance that the chromatids bearing nucleoli will tangle or fail to separate
during anaphase.
Breakdown and reconstruction of nuclear envelope and lamina in mitosis
iv.
With
the transformation of chromosomes into chromatin and reconstruction of nucleoli,
transcription of all the three RNA types gradually becomes normal.
v.
The
spindle begins to disappear. This occurs by depolymerization of microtubules.
The asters become small till only a few and short microtubules are left. This
also occurs by depolymerization of microtubules. The centrioles then take up
their characteristics interphase position close to the side of the nucleus.
Short spindle microtubule persists for some time at the spindle equator. These
tubules mark the region where the cytoplasm will later divide.
Stages of mitosis in an animal
Accuracy of
karyokinesis:- the
two daughter nuclei formed in telophase are identical because they are formed
form identical sets of chromosomes. The accuracy of karyokinesis depends on two
basic features of the process:
i.
The
arrangement of spindle microtubules to form two distinct poles in the cell, and
ii.
The
connection of two chromatids microtubules chromosome to the opposite poles of
the spindle to ensure their delivery to opposite poles.
B. Cytokinesis: cytokinesis, the division of
cytoplasm, encloses the daughter nuclei formed by karyokinesis in separate
cells, thus completing the process of cell division. Cytokinesis is signaled at
the metaphase by cytoplasmic movements that bring about equal distribution of
microchondria and other cell organelles in the two halves of the cell. Division
occurs differently in animal cells and plant clls. However, the spindle remains
function similarly in both.
1. cytokinesis in
animal cells: animal cell typically divide by a process called furrowing or
cleavage.Short spindle microtubules that persist at the spindle midpoint,
become surrounded by patches of dense, apparently structure less material. This
material forms a layer called midbody, which soon extends completely across the
cell.
Cytokinesis by furrowing in an animal cell
Then a constriction of furrow appears in the plasma membrane all round
the cell at the level of the midbody. The constriction is caused by a
peripheral band of microfilaments that are formed by polymerization of actin
subunits just within the cell membrane. The microfilaments have their long axes
oriented parallel to the plane of thr furrow gradually deepens, following the
plane of the midbody, until the opposite edges meet at the centre of the cell.
Then the membranes fuse. With this, the original cytoplasm and the two daughter
nuclei form two independent daughter cells. The latter are about half size of
the original mother cell. They enter the G1 phase of the next cell
cycle.
As the furrow deepens,
the midbody is compressed and becomes smaller, and finally disappears as the
two cells are fully demarcated.
The above mechanism of
cytokinesis is called contractile ring theory.
Cytokinesis in a plant cell by cell plate formation
It is clear from the
above description of furrowing that the orientation of the spindle determines
the plane of cytoplasmic division. Generally, the spindle lies with its
midpoint at the cell equator so that daughter cells formed by cytokinesis are
of equal size. In some cases, such as developing animal’s eggs, spindle is
formed on one side of the cell and the furrow formed opposite the spindle
midpoint divides the cell into two unequal daughter cells. What determines the
alignment of the spindle is not known.
The division of the cytoplasm by
furrowing is not confined exclusively to the animal cells. It occurs in a few
kinds of plant cells and in some groups of protists. Pollen-forming meiotic
cells in some flowering plants divide by furrowing.
2. cytokinesis
in plant cells: a plant cell, due to the presence of a rigid cell wall,
cannot divide its cytoplasm by an invaginating cleavage furrow. Hence, plants
cells divide by cell plate formation. Short spindle microtubules persist in the
midpoint of the spindle in telophase. Membrane-bound vesicles appear among the
microtubules in the central region of the cell during late anaphase or early
telophase. They arise from the ER or golgi apparatus. They contain a dense
material that represents polysaccharides precursors of cellulose and pectin for
the cell wall. The vesicles gradually increase in number and form a continoues
layer cross the cell at the former spindle midpoint. This layer of vesicles is
called phragmoplast. It looks like the midbody formed in animal cell
cytokinesis.
Stages of mitosis in a plant cell
In the central
part of the phragmnoplast the vesicles fuse together. Their membranes from the
plasma membranes of the two adjacent daughter cells and the cell grow towards
the periphery (lateral walls of the cell) by the formation and fusion of more
vesicles till they meet the side walls of the cell. The fully formed cell plate
is called the middle lamella. Cellulose is then deposited on either side of the
middle lamella to form the primary cell wall.
The primary cell
wall is flexible to allow the growth of the cell. A full-grown cell later forms
a rigid secondary cell wall internal to the primary cell wall. The primary and
secondary cell walls are formed of secretions produced in the cell. At places,
the new cell wall retains canals through which cytoplasmic strands interconnect
the adjacent daughter cells. These connections are called plasmodesmata. They allow
movement of materials between the adjacent cells.
Orientation of
the spindle in metaphase determines the plane of cell division in plant cells
as in animal cells. Generally, the spindle is formed at the middle of the cell
so that the daughter cells formed by cytokinesis are equal in size. In certain
plant tissue, the spindle is formed on the side in the cell and the daughter
cells formed are unequal. As in animal cells, the factors that determine the
alignment of the spindle in plant cells are not known.
It should be
noted that the cytokinesis in an animal cell begins at the periphery and
proceeds inwards, whereas in a plant cell it starts centrally and proceeds
outward.
In prokaryotes
and some eukaryotes, cytokinesis occurs in a different way. The plasma membrane
invaginates around the middle of the cell, and new cell wall forms alongside. As
the plasma membranes pinches in closer toward the centre of the cell, the new
wall extends till both join at the centre and completely separate the daughter
cells. As the separation of the daughter cells proceeds from the outside
inward, the process resembles the animal cell furrowing.
Cytokinesis in bacteria, algae and fungi
The mitotic
process described above is for diploid cells, but the process is similar in
haploid cells such as those the gametophytic generation of plants.
C. Cell separation: the daughter cells soon separate by secretion
intercellular substance between themselves. To begin with this substance is
jelly-like hyaluronic acid, but later other materials may permeate it.
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