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Meiosis
Reproduction
Reproduction:


Asexual (Vegetative)


Many single-celled organisms
reproduce by splitting, budding,
parthe...
Reproduction
Reproduction:


Sexual (Generative)


Fusion of two gametes to produce a
single zygote



Introduces great...
Meiosis


Cell division to form the gametes:



sperm (male gamete) and
egg (female gamete)



Characteristic of eukar...
Overview


Start with a diploid cell (2n), with 2 copies of
each chromosome, one form each parent
 The two copies are ca...
Meiotic Division
Meiosis I
• Prophase I
• Leptotene



Meiosis I


• Zygotene



Leptotene
 Zygotene
 Pachitene
 Dip...
Prophase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene



Prophase I is virtually identical to
prophase in ...
Prophase I
Meiosis I

Leptotene:

• Prophase I



• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis

• Metaphas...
Prophase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis

• Metaphase I
• Anaphase I
...
Prophase I
Meiosis I
• Prophase I
• Leptotene

Pachytene:


• Zygotene
• Pachitene
• Diplotene
• Diakinesis



• Metapha...
Prophase I
Diplotene:

Meiosis I
• Prophase I



• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis

• Metaphase...
Prophase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis

• Metaphase I
• Anaphase I
...
Metaphase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis

• Metaphase I
• Anaphase I...
Anaphase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis

• Metaphase I
• Anaphase I
...
Telophase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis

• Metaphase I
• Anaphase I...
(Interphase/Interkinesis)
Meiosis I



Similar to interphase of
mitosis



Without DNA replication.

• Prophase I
• Lept...
Prophase II
Meiosis I



the nuclear envelope is
dissolved again



the spindle is set up
again



Prophase II is ident...
Metaphase II
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis

• Metaphase I
• Anaphase ...
Anaphase II
Meiosis I



At anaphase the
centromeres divide,
splitting the 2
chromatids



The one-chromatid
chromosomes...
Telophase II
Meiosis I



• Prophase I



• Leptotene
• Zygotene
• Pachitene



• Diplotene



• Diakinesis

• Metaphs...
Summary of Meiosis


2 cell divisions
 Start with 2 copies of each chromosome
(homologues), each with 2 chromatids
 In ...
Summary of Meiosis
Mitosis vs meiosis

Meiosis KM

23
Mitosis vs meiosis

Meiosis KM

24
Meiosis creates genetic variation


During normal cell growth, mitosis
produces daughter cells identical to
parent cell (...
Independent assortment

Meiosis KM

26
Independent assortment
Number of combinations: 2n

e.g. 2 chromosomes in haploid
2n = 4; n = 2
2n = 22 = 4 possible combin...
In humans

23 chromosomes in haploid
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!
Crossing over
Chiasmata – sites of crossing over,
occur in synapsis. Exchange of
genetic material between non-sister
chrom...
Meiosis and sexual life cycles


Life cycle = sequence
of stages in organisms
reproductive history;
conception to
reprodu...
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Bft1033 2 meiosis_print

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Bft1033 2 meiosis_print

  1. 1. Meiosis
  2. 2. Reproduction Reproduction:  Asexual (Vegetative)  Many single-celled organisms reproduce by splitting, budding, parthenogenesis  Some multicellular organisms can reproduce asexually, produce clones (offspring genetically identical to parent).
  3. 3. Reproduction Reproduction:  Sexual (Generative)  Fusion of two gametes to produce a single zygote  Introduces greater genetic variation, allows genetic recombination  Zygote has gametes from two different parents.
  4. 4. Meiosis  Cell division to form the gametes:   sperm (male gamete) and egg (female gamete)  Characteristic of eukaryotes only  Normal cells are diploid: 2 copies of every gene  Gametes are haploid: 1 copy of every gene  Need to choose 1 copy of each gene randomly  Why have sexual reproduction?  Shuffling of alleles between parents and offspring leads to new combinations.
  5. 5. Overview  Start with a diploid cell (2n), with 2 copies of each chromosome, one form each parent  The two copies are called homologues  Each chromosome with 2 chromatids attached at the centromere  Meiosis consists of 2 cell divisions:  Meiosis I separate the homologues  Meiosis II separate the 2 chromatids  Meiosis I is unusual and needs a bit of study, but meiosis 2 is just like mitosis.
  6. 6. Meiotic Division Meiosis I • Prophase I • Leptotene  Meiosis I  • Zygotene  Leptotene  Zygotene  Pachitene  Diplotene  Diakinesis. • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II Prophase I  Metaphase I  Anaphase I  Telophase I  Meiosis II  Prophase II  Metaphase II  Anaphase II  Telophase II.
  7. 7. Prophase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene  Prophase I is virtually identical to prophase in mitosis, involving  the appearance of the chromosomes…  the breakdown of the nuclear membrane (envelope)  the development of the spindle apparatus. • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II
  8. 8. Prophase I Meiosis I Leptotene: • Prophase I  • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II initial phase of condensation  appears as thin threads with irregular dense granules (chromomeres)  chromomeres have a characteristic  size and number for a given chromosome.
  9. 9. Prophase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II Zygotene:  lateral pairing of homologous chromosomes (synapsis)  bivalent: synapsed homologous chromosome.
  10. 10. Prophase I Meiosis I • Prophase I • Leptotene Pachytene:  • Zygotene • Pachitene • Diplotene • Diakinesis  • Metaphase I  • Anaphase I  • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II  further chromosome condensation tetrad: pairing of 4 chrommatids crossing over begins crossing over: event of genetic exchange between chromosomes chiasma: cross connection of two chromosomes caused by breakage and rejoining between chromatids
  11. 11. Prophase I Diplotene: Meiosis I • Prophase I  • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II Kiasma chromosomes begin to separate, crossing over visible.
  12. 12. Prophase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II Diakinesis:  the centromeres move away from each other  the chromosomes remain joined only at the tips of the chromatids.
  13. 13. Metaphase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II  Homologous chromosomes align at the equatorial plate.
  14. 14. Anaphase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II  Homologous pairs separate with sister chromatids remaining together.
  15. 15. Telophase I Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II  Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.
  16. 16. (Interphase/Interkinesis) Meiosis I  Similar to interphase of mitosis  Without DNA replication. • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphse I • Anaphse I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II
  17. 17. Prophase II Meiosis I  the nuclear envelope is dissolved again  the spindle is set up again  Prophase II is identical to prophase of mitosis except that there is half the amount of chromosomes • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II
  18. 18. Metaphase II Meiosis I • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphase I • Anaphase I • Telophase I Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II  Chromosomes line up individually on the equator plate.
  19. 19. Anaphase II Meiosis I  At anaphase the centromeres divide, splitting the 2 chromatids  The one-chromatid chromosomes are pulled to opposite poles. • Prophase I • Leptotene • Zygotene • Pachitene • Diplotene • Diakinesis • Metaphse I • Anaphse I • Telophase I Meiosis II • Prophase II • Metaphse II • Anaphse II • Telophase II
  20. 20. Telophase II Meiosis I  • Prophase I  • Leptotene • Zygotene • Pachitene  • Diplotene  • Diakinesis • Metaphse I • Anaphse I • Telophase I  Meiosis II • Prophase II • Metaphse II • Anaphse II • Telophase II  Cell division is complete Four haploid daughter cells are obtained spindle fibers disappear nuclear membrane forms around chromosomes at each end of cell each nucleus has half the # of chromosomes as the original (haploid) now there are 4 sex cells (daughter cells).
  21. 21. Summary of Meiosis  2 cell divisions  Start with 2 copies of each chromosome (homologues), each with 2 chromatids  In meiosis I, crossing over in prophase mixes alleles between the homologues  In metaphase of meiosis I, homologues pair up, and in anaphase the homologues are separated into 2 cells  Meiosis II is just like mitosis  The centromeres divide in anaphase, giving rise to a total of 4 cells, each with 1 copy of each chromosome, and each chromosome with only 1 chromatid.
  22. 22. Summary of Meiosis
  23. 23. Mitosis vs meiosis Meiosis KM 23
  24. 24. Mitosis vs meiosis Meiosis KM 24
  25. 25. 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 of maternal 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 unique haploid gametes produces truly unique offspring.
  26. 26. Independent assortment Meiosis KM 26
  27. 27. Independent assortment Number of combinations: 2n e.g. 2 chromosomes in haploid 2n = 4; n = 2 2n = 22 = 4 possible combinations
  28. 28. In humans 23 chromosomes in haploid 2n = 46; n = 23 2n = 223 = ~ 8 million possible combinations!
  29. 29. Crossing over Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids. Crossing over produces recombinant chromosomes.
  30. 30. Meiosis and sexual life cycles  Life cycle = sequence of stages in organisms reproductive history; conception to reproduction  Somatic cells = any cell other than gametes, most of the cells in the body  Gametes produced by meiosis. Generalized animal life cycle

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