CELL DIVISIONDR.SUMAYYA BASHIRAM COLLEGE

MITOSIS• A process by which the parent cell divides into two

genetically identical daughter cells.• Each daughter cell receives a chromosomal set

identical to that of the parent cell. (46 chormosomes)

• Before a cell enters mitosis, each chromosome replicates its deoxyribonucleic acid (DNA).

• During this replication phase, chromosomes are extremely long, they are spread diffusely through the nucleus-chromatin network.

PHASES OF MITOSIS• PROPHASE • METAPHASE• ANAPHASE• TELOPHASE

PROPHASE• With the onset of mitosis, the chromosomes begin

to coil, contract, and condense; these events mark the beginning of prophase.( mitos-thread in greek )

• They keep on condensing until the very end of prophase when they become distinguishable.

• Each chromosome now consists of two parallel subunits, chromatids, that are joined at a narrow region common to both called the centromere.

• Other changes: nuclear membrane begins to disappear, nucleolus disintegrates, duplicated centrosomes migrate to each pole of cell & mitotic spindle appear between them.

PROPHASE

METAPHASE• The chromosomes line up in the equatorial plane.• The alignment is such that the centromeres come

to lie in a single transverse plane which in perpendicular to long axis of spindle.

• An additional set of microtubules is added to the mitotic spindle

• Microtubules is attached to chromosomes in the centromere region at kinetochore

• Each chromosome have two kinetochore microtubules, these exert the pull on chromosomes

METAPAHSE

ANAPHASE• Soon, the centromere of each chromosome

divides, marking the beginning of anaphase.• Followed by migration of chromatids to opposite

poles of the spindle. • During movement the centromere remains a little

ahead and arms trail behind.• At the end of anaphase the chromosomes are

clustered at the spindle poles.

ANAPHASE

TELOPHASE• Each telophase is characterized by the reappearance of

nuclei in the daughter cells. • The chromosomes revert to their semi dispersed state.• The nucleoli, chromatin, and nuclear envelope

reappear. • While these nuclear alterations are taking place, a

constriction develops at the equatorial plane of the parent cell, cleavage furrow.

• The cleavage furrow deepens, karyokinesis is followed by cytokinesis.

• Each daughter cell receives half of all doubled chromosome material and thus maintains the same number of chromosomes as the mother cell.

TELOPHASE

MEIOSIS• Meiosis is the cell division that takes place in the germ

cells to generate male and female gametes, sperm and egg cells, respectively.

• Because the ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional division. Meiosis II is an equational division analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (23 chromosomes, N) .

• As in mitosis, male and female germ cells (spermatocytes and primary oocytes) at the beginning of meiosis I replicate their DNA so that each of the 46 chromosomes is duplicated into sister chromatids.

PROPHASE-I• Prophase I is the longest phase of meiosis. During

prophase I, DNA is exchanged between homologous chromosomes in a process called crossing over.

• The new combinations of DNA created during crossover are a significant source of genetic variation.

• The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent.

• The process of pairing the homologous chromosomes is called synapsis.

PROPHASE-I• Leptotene: the chromosome become visible as

thin strands.• Zygotene: the homologous chromosomes come

together in close apposition along the entire length; synapses.

• Pachytene: chromosomes become thicker & shorter.

• Diplotene: chromosomes begin to separate along their length, each chromosome consist of two chromatids; except at chiasma-leading to crossing over.

• Diakinesis: separation continues,chiasmata & partly seperated chromosomes become more obvious, nucloelus & nuclear envelope disappear.

METAPHASE-I• A spindle of microtubules is produced by the

centrioles• The bivalent chromosome pairs gather in

alignment on the center of spindle & form the equatorial plate.

ANAPHASE-I• Chromosome of each homologous pair completely

separate & move to opposite poles of spindle.• No division of centromere occurs and whole

chromosomes move to opposite poles.• Random separation of homologous chromosomes

is another factor contributing to genetic diversity.

TELOPHASE-I• The nuclei are reconstituted & cytokinesis divides

the parents cell in two daughter cell• Each contain haploid chromosomes, but each

chromosome consists of two sister chromatids.

MEIOSIS-II• Meiosis II is the second part of the meiotic

process. Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different.

• The end result is production of four haploid cells from the two haploid cells produced in meiosis I.

• The four main steps of Meiosis II are: • Prophase II• Metaphase II• Anaphase II• Telophase II

PROPHASE-II• There is disappearance of the nucleoli and the

nuclear envelope.• The shortening and thickening of the chromatids. • Centrosomes move to the polar regions and

arrange spindle fibers for the second meiotic division.

METAPHASE-II• In metaphase II, the centromeres contain two

kinetochores that attach to spindle fibers from the centrosomes at opposite poles.

ANAPHASE-II• This is followed by anaphase II, in which the

remaining centromeric cohesin is cleaved allowing the sister chromatids to segregate.

• The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.

TELOPHASE-II• The process ends with telophase II, which is

similar to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle.

• Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.

• Meiosis is now complete and ends up with four new daughter cells.

CELL CYCLE• Cell belonging to renewing cell population undergo a sequence of

events that is repeated over & over again and is called cell cycle.• Its divided into two part: M-phase, (30-60 min) in which the

mitosis occur and Interphase, is the intervening period between two consecutive cell divisions.

• Interphase is further divided into• G1 phase: RNA & protein synthesis occur, cell volume is restored.

(8 hours)• S phase: synthesis of DNA occur, centrioles replicate. ( 8 hours)• G2 phase: production of energy & tubulin. (2-4 hours)• Static & stable cell leave cell cycle in G1 phase and enter G0

phase.• Cell proliferation is regulated by a group of genes called proto-

oncogenes.

TISSUE REGENRATION• Labile cells are those in

which replication is normally a continuous process.

• They include cells in: epithelium of e.g. skin, mucous membrane, secretory glands, ducts, uterus lining, bone marrow, blood, spleen and lymphoid tissue.

• Stable cells have retained the ability to replicate but do so infrequently.

• They include: liver, kidney and pancreatic cells, fibroblasts, smooth muscle cells, osteoblasts and osteoclasts in bone.

TISSUE REGENRATION• Permanent cells are unable to replicate after

normal growth is complete.• They include: nerve cells (neurons), skeletal and

cardiac muscle.

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