The importance of meiosis in producing cells which are genetically different. Within this unit, meiosis should be studied only in sufficient detail to show • the formation of haploid cells • independent segregation of homologous chromosomes. Gametes are genetically different as a result of different combinations of maternal and paternal chromosomes • genetic recombination by crossing over.

Meiosis is the process by which sex cells (gametes) are produced. They have half the genetic information of a normal (somatic) cell.

The 'normal' amount of chromosomes is called the diploid number (in humans it is 46 chromosomes/23 pairs of chromosomes) and gametes have the haploid number of chromosomes (23 in humans).

It is important that gametes have half the genetic information so that when two gametes meet the child gets a mixture of genes from its parents and therefore is very different. This is called variation and is what has enabled evolution to happen.

Haploid cells are made in two stages: meiosis 1; and meiosis 2:

Prophase 1: chromatin condenses to form chromosomes. Homologous pairs (the same chromosomes, one from your mum and one from your dad) come together, here some segments of DNA can transfer from one into another, so alleles will swap- this is called crossing over and increases variation. Nuclear envelope and nucleolus dissolve. Spindle fibres start to form and chromosomes move to the poles.

Metaphase 1: homologous pairs line up in the middle of the cell, some of the paternal chromosomes are on the right and some on the left, the same for maternal chromosomes.

Anaphase 1: the pairs are pulled apart towards the opposite poles by spindle fibres.

Telophase1 : the nuclear envelope and nucleus reform (and cytokineses happens) so two new cells have been made. Because some of paternal and maternal DNA was in a random order, the cells have different DNA, this is called independent segregation.

Meiosis 2 happens in both of the cells formed by meiosis 1, it is the same as mitosis:

Prophase 2: nuclear envelope and nucleolus dissolve. Spindle fibres start to form. Spindle fibres start to form and chromosomes move to the poles.

Metaphase 2: chromosomes move into the middle.

Anaphase 2: spindle fibres pull sister chromatids apart in towards opposite poles.

Telophase 2: nuclear envelope and nucleolus reform (and cytokineses happens) so that each cell has become two, resulting in four cells, each with half the number of chromosomes than a somatic cell.

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