AQA A-Level Biology: Meiosis and Genetic Variation
A clear revision guide to meiosis for AQA A-Level Biology: how it halves the chromosome number, how crossing over and independent segregation create variation, and non-disjunction.
Sexual reproduction needs a special kind of cell division that halves the chromosome number and shuffles the genes at the same time. That division is meiosis. This guide covers how it works, the three ways it creates variation, and what happens when it goes wrong.
Why meiosis is needed
When two gametes fuse at fertilisation, their chromosomes combine. If gametes had the full diploid number, the chromosome number would double every generation. Meiosis prevents this by producing haploid gametes, each with half the chromosome number, so that fertilisation restores the diploid number and keeps it constant from one generation to the next. Meiosis also introduces genetic variation, which is essential for evolution.
How meiosis works
Before meiosis begins, during interphase, the DNA replicates, so each chromosome becomes two identical sister chromatids joined at a centromere. Meiosis then involves two divisions.
Meiosis I separates the homologous chromosomes. The chromosomes first pair up into their homologous pairs, then one chromosome of each pair is pulled into each new cell. This is the division that halves the chromosome number.
Meiosis II separates the sister chromatids, much as in mitosis.
The result is four haploid daughter cells, and, thanks to the shuffling described below, they are all genetically different from one another and from the parent.
An overview of meiosis showing a diploid cell with replicated chromosomes, meiosis I separating homologous chromosomes, and meiosis II separating chromatids to give four haploid cells
Meiosis compared with mitosis
It is worth being clear how meiosis differs from mitosis, covered in the mitosis guide.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Daughter cells | Two | Four |
| Chromosome number | Maintained (2n to 2n) | Halved (2n to n) |
| Genetic result | Genetically identical | Genetically varied |
The differences follow from one thing: in meiosis the homologous chromosomes separate, and crossing over and independent segregation occur, none of which happen in mitosis.
How meiosis creates variation
Meiosis generates variation in two ways, and fertilisation adds a third.
Crossing over happens in meiosis I. The homologous pairs come together, and their non-sister chromatids make contact at points called chiasmata. Equal lengths of chromatid, carrying alleles, are exchanged between the chromosomes. This creates new combinations of maternal and paternal alleles on the same chromosome.
Independent segregation also happens in meiosis I. The homologous pairs line up at the equator in a random orientation, so which chromosome of each pair ends up in which daughter cell is a matter of chance. This produces many different combinations of maternal and paternal chromosomes. The number of possible combinations from independent segregation alone is 2 to the power n, where n is the number of homologous pairs.
Random fertilisation adds the final layer: any sperm can fuse with any egg, so the number of possible combinations becomes (2 to the power n) squared. Together these processes ensure that no two offspring (except identical twins) are ever genetically the same.
When meiosis goes wrong: non-disjunction
Occasionally the chromosomes fail to separate properly during meiosis, an event called non-disjunction. Either the homologous chromosomes fail to separate in meiosis I, or the sister chromatids fail to separate in meiosis II. The result is that some gametes end up with an extra copy of a chromosome (n+1) and others with one missing (n-1).
If such a gamete is fertilised, the zygote has an abnormal chromosome number. A missing chromosome is usually fatal, but an extra one can survive: an extra copy of chromosome 21, for example, causes Down's syndrome.