Meiosis Revision Cards

A set of revision cards for the Meiosis topic (2.1.8) on the OCR syllabus for A2 Biology, Unit 2.

Prophase I (Meiosis I)

  • Chromatin condenses -> chromosomes shorten and thicken by supercoiling
  • Chromosomes come together in homologous pairs to form a bivalent
  • Each pair contains; the same genes @ same loci and 1 maternal & 1 paternal chromosome
  • Non-sister chromatids wrap around each other & attach @ chiasmata
  • Here crossing over takes place - sections of the chromatids are swapped
  • The nucleolus disappears & nuclear envelope disentegrates
  • A spindle forms
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Metaphase I (Meiosis I)

  • Centromeres attach to the spindle fibres, chiasmata still present
  • The bivalents arrange themselves across the equator of the spindle, the chromatids arranged by random assortment
  • Each pair of homologous chromosomes faces the opposite pole
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Anaphase I (Meiosis I)

  • One of each pair of homologous chromosomes is pulled by spindle fibres to opposite poles
  • Chromatids independently segregate
  • Centromeres do not divide 
  • Chiasmata separate - lengths of chromatid stay on newly attached chromatid
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Telophase I (Meiosis I)

In animal cells:

  • Nuclear envelopes reform around the chromosomes at each pole
  • The cell divides by cytokinesis

In plant cells:

  • Cell goes straight from Anaphase I -> Meiosis II
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Prophase II (Meiosis II)

  • Nuclear envelope breaks down again
  • Chromosomes supercoil
  • Spindle fibres form
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Metaphase II (Meiosis II)

  • Chromosomes arrange themselves on the equator of each cell and are attached by centromeres to the spindle
  • Chromatids are arranged by random assortment
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Anaphase II (Meiosis II)

  • Centromeres divide
  • Chromatids are pulled to opposite poles by spindle fibres
  • Chromatids randomly segregate
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Telophase II (Meiosis II)

In animal cells:

  • Nuclear envelopes reform around the haploid daughter nuclei
  • The two cells now divide to give four haploid cells

In plants cells:

  • A tetrad of four haploid cells is formed
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  • Meiosis is now complete
  • The cell rests until the next meiotic division
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