Meiosis

Meiosis 1

Prophase 1:

prophase1

  • Chromatin condenses and supercoils so chromosomes shorten and thicken.
  • Chromosomes come together in their homologous pairs to form a bivalent.
  • The non-sister chromatids wrap around each other and attach at chiasmata. Sections may be swapped in crossing over.
  • Nucleolus disappears and nuclear envelope disintegrates.
  • A spindle (made of microtubules) forms.

Metaphase 1

  • Bivalents line up across equator of spindle, attached to spindle fibres at the centromeres.
  • They are arranged randomly with each member of a homologous pair facing opposite poles.

Anaphase 1

anaphase1

  • The homologous chromosomes in each bivalent are pulled by the spindle fibres to opposite poles. The centromeres do not divide. The chiasmata separate (and swapped sections stay swapped).

Telophase 1

telophase1

  • In animals two new nuclear envelopes form and the cell divides by cytokinesis.

Meiosis 2 (division is in a plane at right angles to meiosis 1)

Prophase 2:

  • Reformed nuclear envelopes break down, the nucleolus disappears, chromosomes condense and spindles form.

Metaphase 2:

  • Chromosomes line up on equator of spindle and are attached to the spindle fibres at centromeres.
  • The chromatids of each chromosome are randomly assorted.

Anaphase 2:

  • The centromeres dvide and the chromatids are pulled to opposite poles by the spindle fibres. The chromatids randomly segregate.

Telophase 2

  • Nuclear envelopes reform around the haploid daughter nuclei
  • In animals the two cells now divide to give four haploid cells
  • In plants a tetrad of four haploid cells is formed.
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