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Biology

Meiosis

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The divisions of meiosis

Meiosis [meiosis: type of cell division which forms sex cells (gametes) each with half the usual number of chromosomes is the type of cell division by which gametes [gametes: the sex cells -sperm in males, ova (eggs) in females, are produced. Meiosis involves a reduction in the amount of genetic material.

Prior to meiotic division

Prior to meiotic division, chromosomes can be visible

Each chromosome replicates

In the gamete mother cell, before meiosis begins, each chromosome [chromosome: One of the rod shaped bodies found in the nucleus of cells that contain genetic information (DNA). replicates.

  1. At this stage each chromosome exists as two genetically identical chromatids [chromatid: One of two daughter strands of a replicated chromosome. attached to their centromere. This shows four chromosomes within the nucleus of the cell. Each chromosome appears as two chromatids attached to a centrometre.
  2. In the first meiotic division chromosomes align in homologous pairs. Points of contact form between members of the same homologous pair. The shaded chromosomes are identical to those that were originally donated by one of the parents in the ovum. The white chromosomes are identical to those that were originally donated from the other parent in the sperm [sperm: the male sex cell that fertilised the ovum. The points of contact or crossing over between members of a homologous pair are the chiasmata.
  3. The homologous pairs move to the equator of the cell. Equal lengths of the chromatids of the same homologous pair have broken off and crossed over. The chromosomes align at random and independantly. Only one possible arrangement is shown.
  4. Homologous pairs align at random at the equator of the cell. This shows the early separation of the chromosomes of each of the homologous pairs.
  5. Homologous pairs are separated. This shows the cell at an early stage of meiotic division. The cell membrane [membrane: a very thin layer of tissue is starting to pinch inwards.
  6. The cell divides to form two cells each with a haploid set of chromosomes. Cell division is complete and two cells containing a haploid set of chromosomes are formed.
  7. In the second meiotic division the chromosomes in each cell align independently and randomly at the equator of the cell.
  8. The chromatids in each cell are separated. The chromatids are pulled towards the opposite poles of the cell.
  9. The cells divide. The cell membrane is starting to pinch inwards.
  10. Cell division is complete, resulting in the formation of four cells each with the haploid number.
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