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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.