A level Biology B · Edexcel 9BI0 · Topic 2E
Meiosis
Key points
- Meiosis has two divisions and produces four genetically different haploid cells from one diploid cell.
- In meiosis I, homologous chromosomes pair up and then separate; in meiosis II the sister chromatids separate.
- Crossing over: chromatids of homologous pairs exchange sections at chiasmata, making new combinations of alleles.
- Independent assortment: each homologous pair lines up independently, so there are possible combinations (where is the number of pairs).
- Random fertilisation adds further variation.
- Non-disjunction (chromosomes failing to separate) gives gametes with an extra or missing chromosome, for example causing Down's syndrome (trisomy 21).
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Meiosis is the cell division that makes gametes. It halves the chromosome number, so fertilisation restores it, and it produces cells that are all genetically different from each other.
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