Cell Division II
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Transcript of 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