The Cell Cycle

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The Cell Cycle

DNA replicates as mitosis.

cell cycle diagram

Prophase I

Homologous chromosomes condense (synapsis) to form bivalents. The chromatids become coiled around each other. As the chromosomes pair up (homologous), the twisting produces tension, and sometimes sections of chromatid may break and exchange new partners with corresponding sections of different chromatids.

These breakage points result in "cross-overs" or Chiasmata.

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Metaphase I

The bivalents move to the equator of the cell. Which pair of chromosomes orientates to which pole is completely random (called random assortment.)

cell cycle diagram

Anaphase I

The pairs of bivalents seperate into chromatid pairs, each pair of chromatid is pulled to a pole.

Note: Unlike mitosis, there is no division of the centromeres at this stage.

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Telophase I

The pairs of chromatids reach their respective poles, the cell divides.

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Prophase II

New spindle is formed and the centrioles have replicated. Nuclear membrane disintegrates.

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Metaphase II

The pairs of chromatids line themselves up on the equator as in mitosis, with sister chromatids orientated toward opposite poles.

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Anaphase II

The centromeres divide and the chromatids separate, migrating to opposite poles.

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Telophase II

  • The cell divides.
  • Nuclear membranes and nucleoli are reformed.
  • The chromosomes uncoil and go into interphase.
  • The daughter cells have half the number of chromosomes present in the original cell.
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