IGCSE Biology / Cell division
Meiosis and gametes revision notes
Edexcel International GCSE Biology (4BI1), chapter 17: Cell division, specification point 3.30, 3.31. Free lesson.
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Meiosis is a type of cell division that produces four genetically different haploid gametes. It halves the chromosome number so fertilisation can restore the usual diploid number, while also creating genetic variation.
How the two divisions of meiosis work
DNA is copied before meiosis begins, as it is before mitosis. Meiosis then has two divisions. The first separates homologous pairs of chromosomes, which halves the chromosome number. The second separates the strands of each chromosome, as in mitosis.
How meiosis creates genetic variation
Gametes are genetically different because alleles are rearranged into different combinations. Crossing over and independent assortment shuffle alleles that already exist. Mutation is different because it can create a new allele. During crossing over, homologous chromosomes exchange sections. A chromosome can then carry alleles from both parents. In independent assortment, each homologous pair lines up independently of the other pairs. This makes which chromosome goes into each cell random. With 23 pairs, this gives millions of combinations. At fertilisation, any sperm can join…
Why gametes must be haploid
A gamete has half the usual chromosome number. When a sperm and egg join at fertilisation, their chromosome sets combine and restore the diploid number. This keeps the chromosome number constant from one generation to the next.
What makes the first division different
The first division is the key stage that makes meiosis different from mitosis. Homologous pairs line up two abreast and whole chromosomes separate, halving the number. The second division separates chromosome strands, as in mitosis, so describing only that division misses the main feature of meiosis.