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Meiosis

Meiosis is a type of cell division used to create the gametes (sex cells) of sexually-reproducing organisms.

Cells in the testes and ovaries divide by meiosis to produce sperm and egg cells respectively.

Meiosis results in four daughter cells each containing only one set of chromosomes (haploid). Haploid cells do not have homologous chromosomes.

The halving of genetic material ensures that during fertilisation (fusion of gametes), a normal diploid cell with homologous chromosomes is formed.

If gametes were diploid, the amount of genetic material would double every generation. After just 10 generations, a human cell would carry over 10 000 chromosomes!

Meiosis allows for genetic recombination in offspring. Each haploid cell contains a unique combination of genes since genetic material is swapped between pairs of homologous chromosome.

Gametes (sex cells) are formed by meiosis.
Gametes (sex cells) are formed by meiosis.

Meiosis is split into two cell divisions. Both parts of meiosis have the same stages as mitosis (e.g. prophase, anaphase).

  • Meiosis I :
  • During meiosis I, pairs of homologous chromosomes are separated. This is different to mitosis, where sister chromatids are separated.

    Homologous pairs are pulled to opposite ends of the cell and the cell divides, forming two cells.

  • Meiosis II :
  • During meiosis II, a second round of cell division occurs without replication. Unlike meiosis I, sister chromatids are separated as in mitosis. Meiosis II forms four cells with half as much genetic information as the parent cells.

This diagram illustrates the main events of meiosis.
This diagram illustrates the main events of meiosis.

The first meiotic division involves the separation of homologous chromosomes. This does not occur in mitosis.

Prophase I Condensed homologous chromosomes cross over, i.e. swap equivalent sections of DNA. This occurs at a point called the chiasmata, which is where chromosomes form an intersection. New alleles form through this process.
Metaphase I The chromosomes now align along the equator of the cell. Spindles attach to their centromeres.
Anaphase I The pairs of chromosomes are now pulled to the poles of the cell.
Telophase I There is now a haploid set of chromosomes at each end of the cell, but each chromosome still contains two chromatids.
Cytokinesis This occurs after telophase I, forming two daughter cells

Following Meiosis I, two haploid daughter cells are created. However, each of these chromosomes carries two copies (duplicate) of DNA.

Meiosis II involves the separation of the sister chromatids so that each chromosome carries only one set of DNA.

This is similar to mitosis. However, in meiosis, this occurs with one set of chromosomes rather than two.

Prophase II Spindles reform and chromosomes align at the equator of the cell.
Metaphase II Chromosomes line up in a single row at the equator of the cell. This axis is perpendicular to the axis of arrangement in metaphase I.
Anaphase II Centromeres break down and the sister chromatids separate. These will be the chromosomes of the daughter cells.
Telophase II and cytokinesis The nuclear envelope reforms and the cells split to form four daughter cells.