Meiosis

In the syllabus and the exam

Meiosis is examined as the process specifically responsible for producing gametes, and candidates are expected to know it involves two successive divisions of the nucleus, producing four daughter cells, each with half the chromosome number of the original parent cell, which restores the normal chromosome number when two gametes fuse at fertilisation. A key idea tested repeatedly is genetic variation: because chromosomes are shuffled and exchanged during meiosis, the four daughter cells produced are genetically different from one another and from the parent cell, which the syllabus links forward to the chapter on variation and inheritance as a major source of genetic diversity within a species. A common error is stating that meiosis produces two daughter cells, confusing it with mitosis, or forgetting that meiosis specifically occurs in reproductive organs (the testes and ovaries in humans) to produce gametes, rather than throughout the body for growth and repair, which remains mitosis's role. Candidates should also be able to explain why meiosis is essential for sexual reproduction: without the halving of chromosome number during meiosis, the chromosome number would double every generation after fertilisation, so meiosis is what keeps the chromosome number constant across generations of an organism. Diagrams showing crossing over between homologous chromosomes during meiosis are frequently used to test whether candidates understand that this exchange of genetic material is a further source of variation on top of the random way chromosomes are separated into each gamete. Candidates should also note that meiosis I separates homologous chromosome pairs while meiosis II separates sister chromatids, a two-stage structure that is frequently tested by asking candidates to state which division a given diagram represents.

Example

During the formation of sperm cells in the testes, a diploid cell undergoes meiosis to produce four sperm cells, each haploid and each genetically different from the others.

Related terms

MitosisHaploidChromosome

Source:SRC-DSKP-EN