Meiosis

Meiosis

Meiosis takes place in the reproductive organs of plants, animals and some protoctistans, before sexual reproduction and it results in the formation of four genetically distinct haploid gametes.

The diploid number is halved to haploid and when two haploid gametes then fuse during fertilisation, the zygote then has two sets of chromosomes, one from each gamete. This restores the diploid number and if this halving didn't happen, the number of chromosomes would double every generation.

The stages of Meiosis

Prophase I

• Paternal and maternal chromosomes come together in homologous pairs called synapis. Each homologous pair is bivalent.
• Chromosomes coil up and condense, becoming shorter and thicker, visible as two chromatids. 
• In animal and some plants the centrioles separate and move to the poles of the cells and organise the polymerisation of microtubules which form the spindles
• The homologous chromosomes stay in their pairs - the bivalents
• The chromatids wrap around each other and then partially repel each other but stay joined at the points called the chiasmata

At the chiasma:

• A segment of DNA from one chromatid may be exchanged with the equivalent part from a chromatid of the homologous chromosome
• This swapping is called crossing over - a source of genetic variation - it mixes genes from the two parents in one chromosomes
• This genetic recombiniation produces new combinations of alleles
• A single crossover during meiosis I results in four haploid gametes having a different genetic composiiton but crossing over can happen at several places along the length of the chromatid and so there are huge numers of genetic combinations made

End of prophase I

• The nuclear envelope has disintegrated and the…