Meiosis

- Meiosis is cell division to form haploid gametes for sexual reproduction: a diploid cell divides twice to form four gametes.

 - Meiosis begins with prophase 1 which is similar to mitotic prophase. However, when the chromosomes meet at the centre, the homologous chromosomes cross to form bivalents, where genetic information is exchanged between them in a process known as genetic recombination.

 - The rest of the first division in meiosis is similar to mitosis, except the chromosomes do not divide. Instead, the homologous chromosomes separate into two nuclei so that each cell contains only one allele for each gene.

 - The process then repeats itself; the nuclear envelopes disintegrate again and the cells divide once more. This time the chromosomes separate; four gametes are formed. 

- Genetic recombination – the genes recombine randomly to so each chromosome in the gametes is different to those of the parent.

 - Independent assortment: when the homologous chromosomes line up during the first division they segregate randomly – there are millions of possible combinations of chromosomes in each nucleus.

 - Random fusion of gametes: a single organism produces millions of gametes, which could hypothetically fuse with the gametes of any member of the same species, giving billions of possible combinations.