Why the Chromosome Number Matters
The key reason meiosis halves the chromosome number is to ensure that when two gametes fuse at fertilisation, the resulting zygote has the correct diploid number. In humans, body cells contain 46 chromosomes (23 pairs). Meiosis produces gametes with 23 chromosomes each, so fertilisation restores the count to 46. Without meiosis, the chromosome number would double every generation.
Mitosis, by contrast, maintains the chromosome number. Every new cell produced for growth or repair must be an exact copy of the original, so the full diploid set is preserved.
How Meiosis Creates Genetic Variation
Two mechanisms during meiosis produce genetically unique gametes:
- Crossing over: during prophase I, homologous chromosomes lie alongside each other and swap segments of DNA. This shuffles allele combinations onto new chromosomes.
- Independent assortment: during metaphase I, each pair of homologous chromosomes lines up randomly at the equator. With 23 pairs in humans, over 8 million different combinations are possible from this process alone.
These two sources of variation, combined with random fertilisation, explain why siblings (except identical twins) are genetically different from one another.
Exam Tip: How to Structure a Comparison Answer
When an exam question says "compare", always give paired statements. Do not describe mitosis fully and then describe meiosis. Instead, take one feature at a time and state what happens in both. For example: "Mitosis produces two daughter cells, whereas meiosis produces four daughter cells." Each paired statement typically earns one mark.