CELL CYCLE AND CELL DIVISION

o Cell division, DNA replication, and cell growth have to take place in
a coordinated way to ensure correct division and formation of
progeny cells containing intact genome.
o The sequence of events by which a cell duplicates its genome,
synthesises the other constituents of the cell and eventually divides
into two daughter cells is termed cell cycle.

Phases of cell cycle

o The duration of cell cycle can vary from organism to organism. For
example- typical eukaryotic cells divide once in approximately
every 24 hours, whereas yeast cell divides only in 90 minutes.
o The cell cycle is divided into two basic phases:

INTERPHASE
M PHASE (MITOSIS PHASE)

INTERPHASE : the interphase represents the phase between two

successive M phases.
The interphase is divided into three further phases:
G1 phase (Gap 1)
S phase (Synthesis)
 G2 phase (Gap 2)

➢ G1 phase corresponds to the interval between mitosis and

initiation of DNA replication, where the cell is metabolically
active and continuously grows but does not replicate its DNA.
➢ During S or synthesis phase, the amount of DNA per cell
doubles i.e DNA replication occurs in this phase but there is
no increase in the chromosome number.
➢ During the G2 phase, RNA and proteins are synthesized in
preparation for mitosis while cell growth continues.

G0 PHASE: The cells that do not divide further exit G1 phase to

enter an inactive stage called quiescent stage (G0) of the cell cycle.

MITOSIS PHASE :
o The M Phase represents the phase when the actual cell division or
mitosis occurs
o The M Phase starts with the nuclear division, corresponding to the
separation of daughter chromosomes known as karyokinesis and
usually ends with division of cytoplasm known as cytokinesis.
o In animals, mitotic cell division is only seen in the diploid somatic
cells and the plants can show mitotic divisions in both haploid and
diploid cells.

MITOSIS : Since the number of chromosomes in the parent and

progeny cells is the same in mitosis , it is also called as equational
division.
o Mitosis is divided into the following four stages:

o Prophase

o Metaphase

o Anaphase

o Telophase
 Prophase
o Prophase is the first stage of mitosis,
o The centriole moves towards opposite poles of the cell.
o Chromatin material condenses to form compact chromosomes.
o Chromosomes are seen to be composed of two chromatids
attached together at the centromere.
o Initiation of the assembly of mitotic spindle, the microtubules (the
proteinaceous components of the cell cytoplasm) help in the
process of mitotic spindle formation.
o Nuclear membrane , nucleolus and cell organelles disappear.

Metaphase
o Chromatids attached with spindle fibres with kinetochores.(Small disc-
shaped structures at the surface of the centromeres are
called kinetochores)
o Chromosomes move to spindle equator and get aligned along
metaphase plate.
(The plane of alignment of the chromosomes at metaphase is
referred to as the metaphase plate.)

Anaphase
o centromere split of chromosome , and the two daughter
chromatids separate
o Chromatid move towards the two opposite poles.
o The centromere of each chromosome is towards the pole with
the arms of the chromosome trailing behind.
o Chromatid may be V, J or I shaped depending upon the position
of centromere.
Telophase
o Chromosomes cluster at opposite spindle poles, decondense, and
lose their individuality.
o Nuclear envelope assembles around the chromosome clusters.
o Nucleolus, ER, golgi complex and ER reform.

o Cytokinesis
o Cytokinesis is the division of cytoplasm of a cell after
karyokinesis (division of chromosome) into two
daughter cells.
o In animal cells, appearance of furrows in plasma
membrane that deepens gradually and joins to divide
cytoplasm into two parts.
In plant cells, wall formation starts at the centre and
grows outwards to meet lateral walls. The formation of
cell wall begins with formation of cell plate

SIGNIFICANCE OF MITOSIS
o Mitosis results in the production of diploid daughter cells with
identical genetic complement usually.
o Cell divides by mitosis to restore the nucleo-cytoplasmic ratio.
o Helps in cell repair.
o Mitotic divisions in the meristematic tissues result in a continuous
growth of plants throughout their life.
MEIOSIS

The cell division that reduces the number of chromosome into half and results in the
production of haploid daughter cells is called meiosis. It helps in production of haploid
phase in the life cycle of sexually reproducing organism. It involves following events.
 Meiosis I Meiosis II

Prophase I Prophase II

Metaphase I Metaphase II

Anaphase I Anaphase II

Telophase I Telophase II

1. Two sequential cycles of nuclear and cell division called meiosis I and
meiosis II but single cycle of DNA replication.
2. It involves pairing of homologous chromosome and recombination of
them.
3. Four haploid cells are formed at the end of meiosis II.
Meiosis is very significant has it provides genetic diversity among
a population.

Meiosis I (Reductional)

Prophase I
o Prophase I has been further subdivided into

1. Leptotene,
2. Zygotene,
3. Pachytene,
4. Diplotene and
5. Diakinesis

o In leptotene stage, the chromosomes

condense and become visible.
 o During Zygotene stage, chromosomes start pairing
together (synapsis). The paired chromosomes are
called homologous chromosome. Synaptonemal
complex formed by a pair of homologous chromosome is
called bivalent or a tetrad.

o During Pachytene stage, crossing over between

non-sister chromatids of homologous
chromosome occurs for exchange of genetic
materials. The crossing over is enzyme –mediated
process which involves enzyme recombinase..

o Diplotene stage is characterized by the dissolution

of the synaptonemal complex, .
o X-shaped structures called chiasmata get formed.
Homologous chromosomes separate from each
other except at the sites of crossovers.

o Diakenesis is marked by terminalisation of chiasmata.

the chromosomes are fully condensed .
o the meiotic spindle is assembled to prepare the
homologous chromosomes for separation
o the nucleolus disappears and the nuclear
envelope also breaks down.
Fig. Prophase I
o Metaphase I
o The bivalent chromosomes align on the equatorial plate.
o The microtubules from the opposite poles of the spindle attach to
the pair of homologous chromosomes.
o Anaphase I
o The homologous chromosomes separate, while sister chromatids
remain associated at their centromeres.

Fig. Anaphase I
o Telophase I
o The nuclear membrane and nucleolus reappear, cytokinesis follows
and this is called as diad of cells.
o The stage between the two meiotic divisions is called interkinesis,
which is followed by prophase II.

Fig. Telophase I
Meiosis II

o Prophase II
o In contrast to meiosis I, meiosis II resembles a normal mitosis.
o The nuclear membrane disappears by the end of prophase II and
the chromosomes again become compact.
o Metaphase II
o The chromosomes align at the equator and the microtubules from
opposite poles of the spindle get attached to the kinetochores of
sister chromatids.
o Anaphase II
o The centromere of each chromosome splits, allowing them to move
toward opposite poles of the cell.
o Telophase II
o The two groups of chromosomes get enclosed by a nuclear
envelope.
o Cytokinesis follows resulting in the formation of tetrad of cells i.e.,
four haploid daughter cells.
Fig. Meiosis II

https://youtu.be/c5hA0WCv1lg

SIGNIFICANCE OF MEIOSIS

o Specific chromosome number of each species is achieved across

generations in sexually reproducing organisms by meiosis.
o It also increases the genetic variability in the population of
organisms.
o It results in reduction of chromosome number by half.