Meiosis on variation (A level Bio)

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Sexual reproduction occurs only in eukaryotes. During the formation of gametes, the number of chromosomes is reduced by half, and returned to the full amount when the two gametes fuse during fertilization.

Firstly the number of chromosomes doubles and pairs of similar chromosomes lie next to each other, these are called bivalents. this process is called prophase 1. These bivalents then line up along the equator of the cell in a process called metaphase. According to their orientation independant assortment is present this where the different alleles could be on either side of the equator. A process called chiasmata also takes place at this stage, this is where the similar groups on the bivalents are crossed over and so genetic information is transferred between non sister chromatids. This develops variation in offspring. The centriols which are at the poles of the cell then attach spindle fibres to the bivalents and the bivalents are subsequently pulled to the two opposite poles of the cells, this is called anaphase. The final stage in meiosis 1 is where the cell divides called telophase. When the bivalents are at the poles cytokenisis occurs and the cytoplasm is is split and two seperate cells are formed. Despite the original duplication of chromosomes the amount of genetic information is still diploid.

Meiosis two is essentially similar to that of meiosis 1 apart from the primary stage of prophase, this lack of chromosome duplication means the amount of gentic information at the end is half that at the beginning.

Meiosis 2, metaphase; the sister chromatids line up along the equator of the cell this introduces independant assortment according to their orientation. Next the spindle fibres from the centrioles at the poles of the cell attach and the sister chromatids are pulled apart in anaphase. Finally cytokenisis occurs again and the gametes are formed with a haploid number of chromosomes.

Russell Vanhinsbergh & Ian Brown