Meiosis: making sex cells with genetic variety — Biology, 14–17 years
Meiosis makes sperm, eggs and other sex cells with half the usual chromosome number. It also reshuffles genetic material, so offspring receive combinations that are not identical to either parent.
What meiosis does
Meiosis starts with one cell containing chromosome pairs and ends with four sex cells, each carrying one chromosome from every pair. Two divisions follow one round of DNA copying. The cells are not identical because chromosome pairs are shuffled and matching chromosomes can exchange sections.
Why meiosis is needed
Sexual reproduction needs two sex cells to join without doubling the chromosome number in every generation. Meiosis solves this by halving the number first; fertilisation then restores the paired number. Its reshuffling also creates variation, giving populations different combinations on which selection can act.
Worked example: chromosome numbers
A human body cell has 46 chromosomes, arranged in 23 pairs. During meiosis, DNA is copied once, but the cell divides twice. Each resulting sperm or egg has 23 chromosomes; when two join at fertilisation, the new cell has 23 + 23 = 46.
Trap: meiosis makes four identical cells
The mistake is understandable because mitosis makes matching daughter cells, and both processes begin with DNA copying. Meiosis is different: chromosome pairs separate in one division, copied chromosomes separate in the next, and genetic exchange can occur. The four products therefore usually carry different combinations.
Where meiosis appears
Meiosis matters in fertility, plant breeding and the study of inherited chromosome conditions. In crop breeding, crossing two varieties can bring useful traits together, while meiosis creates new combinations among their offspring. Errors in chromosome separation can produce cells with an unusual chromosome number.
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