Meiosis
In sexual reproduction the maternal and paternal gametes combine. Even after they combine, the chromosome number of a species has to be kept constant from generation to generation — and there is the problem.
The problem
If both gametes carried 2n chromosomes, the zygote would end up with 4n after fertilisation, and the generation after that with 8n. So when gametes are formed the chromosome number has to be brought down to exactly half.
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Meiosis
The division that reduces the chromosome number to half is called meiosis — that is, bringing the chromosome number of a cell down to n.
How the number balances again
Meiosis takes place when the gametes of higher organisms are formed. Sperm and ova therefore carry only one chromosome of each pair (2n → n). When the gametes are fertilised and the zygote is formed, the chromosomes come together again (n + n → 2n).
It happens in two steps
Meiosis takes place in two steps: first a reduction division and then an equational division. That is why four daughter cells are formed at the end of it.
The importance of meiosis
Because structural changes take place in the chromosomes during meiosis, variations — new characteristics — arise in living things. This is a very important phenomenon in biological evolution.
- Keeping the chromosome number constant from generation to generation
- Being important in evolution, because the changes in chromosomes give rise to variation
One division, two results
Meiosis both keeps the number constant and produces variation. The first keeps the species what it is; the second lets it change. That is where the power of sexual reproduction lies.