Meiosis is a much more complex type of cell division, which is designed to halve the number of chromosomes so that each daughter cell will have just one chromosome from each of the homologous pairs in the original cell. In humans, the 46 chromosomes in 23 pairs, will become 23 chromosomes, one from each pair.

The whole idea here is that sexual reproduction requires an equal contribution of genetic material from each of the parents. This will keep the number of chromosomes in every generation the same.

The chromosomes appear and then join into homologous pairs. These pairs, are chromosomes which carry the same gene sequence along their length. The chromatids are fused at various points, called chiasmata, and the DNA chains may break at these points and become swapped from one of the homologous chromosomes to the other.

This swapping over of sections of the DNA allows new combinations of alleles to develop along the length of the chromosomes. The diagram illustrates the formation of chiasmata between an homologous chromosome pair and in stylised form the consequences of swapping DNA between the chromosomes. The original chromosomes had the allele combinations A-B and a-b, but after swapping, we get the new combinations a-B and A-b.

This will give rise to increased genetic variation in the offspring formed from the cells produced by meiosis. The new combinations are called recombinants.

The paired chromosomes line up along the equator and spindles attach to the centromeres and pull them apart. This means that one of each of the chromosome pairs will be moved to each pole of the cell. When the cell divides its cytoplasm into two the chromosome number is halved. The two daughter cells can now divide again, but this time the chromatids are pulled apart from each other just as they are in mitosis. Thus we get four cells each with a haploid (n) chromosome number.

The whole emphasis of meiosis is to increase variation of the genetic material. This is achieved by:-

1 Random assortment and selection of homologous chromosomes.

2 Chiasmata formation and the swapping of chunks of the chromosomes between homologous pairs.

3 The random selection of chromatids into the daughter cells.

The male gamete is designed to be motile, and its function is merely to reach an egg and fuse with it, supplying just its nucleus with the enclosed chromosomes to the next generation. The female gamete has a more difficult task since once fertilised it must divide repeatedly to form an embryo. It must contain a supply of nutrients big enough to last until another food source becomes available. The female gamete is therefore larger, and to ensure this happens the process of meiosis is modified. The process can lead to some massive cells, for example the ostrich egg is a single cell when first laid, its size is necessary to store the yolk needed to supply the developing embryo once it forms.