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Cell CycleBy
Dr. Khaled Moustafa
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CELL CYCLE
Two major events: 7 phases
1-Mitosis:4 phases
1 hour
cell divides.
2-Interphase:3phases
20 hours
between 2 divisions.
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Mitosis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
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INTERPHASE
G1 (gap1):cell prepare for DNA duplication.
S (synthetic):DNA duplicatation.
G2 (gap2):cell prepare for mitosis.
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I. Prophase Chromosomes visible.
Each consists of 2 sister chromatids joined by 1centromere and 2 kinetochores .
Kinetochore:microtubule organizing center (MTOC).
Nuclear envelope start to disappear.
Each pair of centrioles moves to one pole of the cellto form mitotic spindle.
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II- Metaphase
Disappearance of nuclear envelope.
Migration of chromosomes to equatorial plane.
Mitotic spindle:
chromosomal microtubules (attached to kinetochores) helpin chromatids migration to cell poles.
Polar microtubules (not attached to kinetochores) maintainspacing between the two poles.
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IV- Telophase
Chromosomes uncoil into chromatin.
Reappearance of nuclei + nuclear envelope.
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Cytokinesis
Cleavage furrow deepen.
Polar microtubules + contractile ring of
filaments separation of daughter cells.
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MEIOSIS
cell division producing germ cells
ova and sperms
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In Spermatogenesis
1ry spermatocyte give 4 sperms.
In oogenesis
1ry oocyte give by meiosis I 2ry oocyte + lst polar body
2ry oocyte give by Meiosis II
ova + 2nd polar body
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Meiosis I
Prophase I
Metaphase I
Anaphase ITelophase I
Cytokinesis
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Prophase I
long time
five stages:
leptotene Zygotene
Pachytene
Diplotene
Diakinesis
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1-leptotene
Ch visible as long strands.
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2-Zygotene
d-ch (each formed of 2 chromatids)
arrange in 23 pairs of bivalents.
bivalent consists of 4 chromatids (tetrads) .
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3- Pachytene
Shortening, thickening and crossing overof ch.
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4- Diplotene
More condensation of ch occurs.
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5- Diakinesis
Maximum condensation of ch.
Disappearance of nucleolus.
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Metaphase I
Bivalents (tetrads) arrange in
equatorial plane.
Microtubules of mitotic spindle
attach to chromosomes.
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Anaphase I
Homologous ch migrate toopposite poles.
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Telophase I
ch reach opposing poles.
nuclei reform.
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Cytokinesis
2 daughter cells are formed.
each contains 23ch + 2n.
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Meiosis II
Prophase II
Metaphase II
Anaphase IITelophase II
Cytokinesis
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like mitosis.
But not preceded by S - phase.