What process cuts the number of chromosomes per cell in half?

Meiosis is the process that reduces the chromosome number by half, creating gametes (sperm and egg cells) in sexually reproducing organisms. This reduction is essential for maintaining the species' chromosome number across generations. During meiosis, a single diploid cell undergoes two successive divisions—meiosis I and meiosis II.

In meiosis I, homologous chromosomes pair up and are separated into different cells. Each resulting cell is haploid, containing half the original number of chromosomes. Meiosis II then separates the sister chromatids, but the critical reduction in chromosome number occurs during the first division. This ensures that when fertilization occurs, the resulting zygote has the correct diploid chromosome number, restoring the full set.

The other processes listed do not lead to a reduction in chromosome number. Mitosis is the division of a single cell into two identical daughter cells, maintaining the same chromosome number. Fertilization is the union of two gametes, restoring the diploid number but not changing the number itself. Replication involves the copying of DNA but does not alter the number of chromosomes in a cell.