OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

During meiosis, the chromosome number halves. This process is essential for sexual reproduction, as it ensures that when two gametes (sperm and egg) fuse during fertilization, the resulting zygote has the correct diploid chromosome number.

Meiosis consists of two rounds of cell division: meiosis I and meiosis II. In meiosis I, homologous chromosomes (pairs of similar chromosomes from each parent) are separated into two daughter cells, reducing the chromosome number by half. Each daughter cell then undergoes meiosis II, where the sister chromatids of each chromosome are separated, but this does not further change the chromosome number.

In contrast, doubling the chromosome number or keeping it the same would disrupt the balance needed for proper genetic inheritance. Variability in chromosome number does not occur as each species has a characteristic number of chromosomes, making halving the number during meiosis a critical rule in the process of gamete formation.