Meiosis

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  • Created by: rosieevie
  • Created on: 20-05-17 18:57

Significance of Meiosis

Meiosis important = produces haploid gametic cells

Fertilisation causes mixing of genomes from two parents - increases ploidy and variability

All organisms have alteration between haploid and diploid geneations

Fungi - haploid generation dominant and transient diploid stage

Humans - diploid generation dominant and transient haploid stage

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Process of Meiosis

Somatic (diploid body) cells - pair of homologous chromosomes in each cell (1 maternal, 1 paternal)

Each haploid gamete has only 1 of each homologous choromosome

Fertilisation - 1 haploid homologous chromosome from each parent gamete = new diploid pair = restoration of somatic diploid level

Prophase 1

  • Leptotene - attachment plaque
  • Zygotene - synaptonemal complex
  • Pachytene - tetrad/bilavent recombination nodules chiasmata
  • Diplotene
  • Diakinesis

Metaphase 1, Anaphase 1, Telophase 1 

Prophase 2, Metaphase 2, Anaphase 2, Telophase 2

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Leptotene

  • Chromosomes condense
  • Chromosome ends attached to nuclear membrane by attachment plaque
  • Movement and recognition of homologous chromosomes = allows them to be brought together
  • e.g. C1 attatchment plaque meets with other C1 plaque, recognise and form a pair
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Zygotene

  • Initiation of synapsis formation
  • Chromosomes zipped together = synaptonemal complex
  • Genetic recombination requires close association
  • Attachment plaques move homologues around and brings ends together
  • Two chromatids zip together w/ ladder-like protein on either sde of homologues
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Pachytene

Chiasmata - point which paired chromosomes remain in cotact in first prophase of meiosis, crossing over and exchange of genetic material occurs between strands

  • When synapsis complete - bivalent (homologous chromosomes in pairs) tetrad formed
  • Tretrad - chromatids that make up 2 homologous chromosomes
  • Large recombination nodules appear - mediate chromosomal chaismata - 90nm diameter
  • Nodules bring non-sister chromatids 100nm together

Evidence for recombination nodule function:

  • Recombination nodule number same as ciasmata number in prophase 1
  • Recombination nodule distribution similar to chiasmata distribution
  • Drosophil mutations w/ reduced recombination frequency have fewer recombination nodules
  • Genetic recombination involves DNA synthesis at chiasmata
  • Radioactive precursors were preferentially incorporated in pachytene DNA at or near chiasmata

Cells may remain at pachytene for multiple days

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Diplotene

  • Synaptonemal complex dissolces
  • Homologues still attatched at chiasmata
  • Some relaxing of DNA and active RNA synthesis

Can last for years as oocytes

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Diakinesis

  • RNA synthesis stops
  • DNA becomes highly condensed 
  • Chromosomes detach from nuclear envelope
  • Chiasmata sites can be clearly observed
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Chiasmata

Chiasmata important in:

  • Chromosome orientation
  • Random assortment
  • Act as centromeres

Mutants that do not form chiasmata often fail to undergo normal segregation

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Meiosis 1

Chiasmata formation occurs - lasts until Anaphase 1 where chromosomes move like blocks around nucleus

Homologous chromosme pairs seperated in Anaphase 1 using 1 kinetochore attachment per centromere

1 homologous chromsome moves into one cell

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Meiosis 2

Results in 4 haploid cells

Each contain single chromatid from 1 or other homologue - may have exchanged material

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Differences between Mitosis and Meiosis

  • Meiosis 1 - kinetochore fibres attach only to 1 side of centromere - important for orientation, independent segregation and correct distribution
    • Mitosis - homologous chromosomes line up independently at metaphase plate
  • Meiosis 2 - independent line up but unlike mitosis homologoues are not present as seperated in meiosis 1
  • Meiosis 1 - chiasmata divide but in Meiosis 2 and mitosis centromeres divide
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Generation of Variation during Mitosis

For each pair of homologous chromsomes - 1 maternal, 1 paternal

Causes of variation:

  • Independent segregation of 23 non-homolgous chromosome pairs at Meiosis 1 (male and female do not line up on seperate sides)
  • Fertilisation - varied chance of genetics
  • Chiasmata
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