5 cell division.pptx
TRANSCRIPT
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CELL DIVISIONDR.SUMAYYA BASHIRAM COLLEGE
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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.
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PHASES OF MITOSIS• PROPHASE • METAPHASE• ANAPHASE• TELOPHASE
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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.
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PROPHASE
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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
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METAPAHSE
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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.
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ANAPHASE
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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.
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TELOPHASE
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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METAPHASE-II• In metaphase II, the centromeres contain two
kinetochores that attach to spindle fibers from the centrosomes at opposite poles.
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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.
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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.
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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.
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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.
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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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QUESTIONS
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