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This type of cell division occurs in the formation of sex cells (gametes) in organisms that reproduce sexually. Sex cells are cells such as sperm, egg cells and pollen grains. Meiosis results in halving the number of chromosomes in a cell. So the daughter cells formed by meiosis (the gametes) therefore contain half the number of chromosomes of the parent cell. The effects of this are shown in the table below for humans.



Number of chromosomes in each set of chromosomes present
23
23

Number of sets of chromosomes present
2 (diploid)
1 (haploid)

Total number of chromosomes present in one cell
46
23

The process of meiosis is actually two consecutive divisions. The first is quite different to mitosis but the second is the same. Because there are two divisions, meiosis produces four daughter cells from one parent cell. The two divisions can be referred to as Meiosis 1 and Meiosis 2.

Meiosis 1

This division has the same stages as before (prophase, metaphase, anaphase and telophase) but the chromosomes behave differently in order to reduce the number present in the daughter cells

Prophase

During prophase 1 the chromatin condenses and chromosomes become visible as long, thin entangled threads which become shorter and thicker. The Nucleolus disappears. Centrioles begin to migrate to opposite poles. As they move apart a network of protein microfilaments develops between them (this is the spindle). Towards the end of prophase 1 the nuclear membrane breaks down.

Prophase

Metaphase 1

During metaphase 1 chromosomes come together in homologous pairs (bivalents). The bivalents touch at certain points which are called chiasmata. Bivalents arrange themselves on the equator of the cell and become attached by their centromeres.

Metaphase

Anaphase 1

During this stage the homologous chromosomes from each bivalent separate and one travels to each pole. This is the moment of reduction division. Each pole only has half the number of chromosomes of the original cell.

Anaphase

Telophase 1

The two groups of chromosomes come together at opposite poles. Each group of chromosomes uncoils partly or wholly and a new nuclear membrane forms around them.

Telophase

After telophase 1 cytokinesis occurs as in mitosis.

Daughter cells can go into a short period of interphase or they may proceed directly into meiosis 2.

Meiosis 2

Each of the two cells then undergoes a further division, which is identical to mitosis. So the two cells become four cells by going through prophase 2, metaphase 2, anaphase 2 and telophase 2.

Meiosis

The diagram shows how the four new cells then assume the non-dividing appearance. The colours show that each cell does not just have half the number of chromosomes of the original, but also has a different combination of chromosomes.

chromosomes

The genetic variation in the four daughter cells is produced by two main processes which occurred during meiosis.

  1. Crossing over

    This recombines alleles, which are present on the same chromosome

    This happens at prophase 1, when the bivalents first form. While the two homologous chromosomes are joined in a bivalent, bits of one chromosome "cross over" with the corresponding bits of the other chromosome. When the chromosome separate, the swapped bits do not go back to their original chromosome, but they become part of the other homologous chromosome.

    chromosome cross over
  2. Independent assortment of homologous chromosomes

    This recombines alleles, which are present on different chromosomes

    At metaphase 1 of meiosis the homologous chromosomes are assorted independently of each other. Separation of a pair of homologous chromosomes at anaphase 1 is therefore independent of the separation of other pairs. Therefore the alleles on one pair of homologous chromosomes separate independently of the alleles on others. Because of independent assortment a vast permutation of genes is possible in the gametes. However all the genes on a single chromosome must stay together

Summary

Meiosis results in the formation of gametes. The four daughter cells produced have half the chromosome number of the parent cell. All the daughter cells formed are genetically different from each other and parent cell.




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