cell division ii

7/30/2019 Cell Division II

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Cell division II

By

Dr Ayesha Saba Naz

Senior lecturer; Department of Anatomy

BUMD&C


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Week 5

1. Mitoses

2. Meiosis

3. Significance and difference

Cell division


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Learning objectives

Revise the phases of the cell cycle and mitosis

Understand the different steps of mitoses I and II


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Reproduction

The process of sexual reproduction involves the fusion of

male and female gametes

Zygote forms as a result, 23 chromosomes donated by eachparent (total = 46 or 23 pairs)

Contain 22 autosomes and 1 sex chromosome

Gamates are haploid (haploid number n = 23 in humans)


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Reproduction

Fertilization results in zygote with 2 haploid sets

of chromosomes - now diploid.

Most cells in the body are produced by mitosis.

Only gametes are produced by meiosis.

Hence meiosis is also known as gametogenesis


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Meiosis key differences from

mitosis

Meiosis reduces the number of chromosomes by half

Daughter cellsdiffer from parent, and from each other

Meiosis involves two divisions, Mitosis only one


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Meiosis


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Meiosis 1

: Each chromosome duplicates and remains

closely associated. These are called sister chromatids.Crossing-over can occur during the latter part of thisstage.

: Homologous chromosomes align at theequatorial plate.

: Homologous pairs separate with sister

chromatids remaining together.

: Two daughter cells are formed with eachdaughter containing only one chromosome of thehomologous pair.

Prophase I

Metaphase 1

Anaphase 1

Telophase 1
http://www.accessexcellence.org/AB/GG/comeiosis.htmlhttp://www.accessexcellence.org/AB/GG/comeiosis.htmlhttp://www.accessexcellence.org/AB/GG/comeiosis.htmlhttp://www.accessexcellence.org/AB/GG/comeiosis.html


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Prophase I

Has further substages:

Leptotene

Zygotene

Pachytene

Diplotene

Diakinesis


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Meiosis I

Synapsis homologous chromosomes pair up. Chiasmata

form (crossing over of non-sister chromatids)

In Metaphase I, homologous pairs line up at metaphase plate

In Anaphase I, sister chromatids do NOT separate

Overall, separation of homologous pairs of chromosomes,

rather than sister chromatids of individual chromosome


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LeptoteneThe chromosomes

become visible in the

nuclei


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ZygoteneHomologous chromosomes come

together


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PachyteneChromosomes

become bivalent


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Prophase I

CHIASMATA


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Prophase I

Normally extended for 3 weeks in male

gametogenesis in humans

Oocytes arrest in first meiotic phase from fetal

period to maturity.. Could be to 12 years of

age to 5 decades!!!!


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Metaphase I:

Independent assortment


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Another Way Meiosis Makes Lots of Different

Sex Cells Crossing-Over

Crossing-over multiplies the already huge number of different gamete types produced by

independent assortment.


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Meiosis KM


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M i i II


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Meiosis II

Second division of meiosis: Gamete formation

Prophase 2: DNA does not replicate.

Metaphase 2: Chromosomes align at the equatorial plate.

Anaphase 2: Centromeres divide and sister chromatids

migrate separately to each pole.

Telophase 2: Cell division is complete. Four haploid

daughter cells are obtained.


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Mitosis vs. meiosis

Meiosis KM


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Meiosis KM


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Meiosis creates genetic

variation During normal cell growth, mitosis produces daughter

cells identical to parent cell (2n to 2n)

Meiosis results in genetic variation by shuffling ofmaternal and paternal chromosomes and crossing over.

No daughter cells formed during meiosis are

genetically identical to either mother or father

During sexual reproduction, fusion of the uniquehaploid gametes produces truly unique offspring.


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Revisiting learning objectives

Identify the phases of the cell cycle

Understand the different steps of mitoses I and II

cell division ii

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