When is the chromosome number reduced by one-half during gametogenesis?

The chromosome number is reduced by one-half during the first meiotic division, which is a critical step in gametogenesis. Meiosis consists of two sequential divisions: meiosis I and meiosis II.

During meiosis I, homologous chromosomes are separated into different daughter cells. This reduction division is what leads to the halving of the chromosome number. Each homologous pair, which consists of one chromosome from each parent, is split so that each resulting gamete contains only one chromosome from each pair. This is essential for sexual reproduction because it ensures that when gametes fuse during fertilization, the resulting zygote will again have the full diploid chromosome number.

In contrast, meiosis II, which is similar to mitosis, does not change the chromosome number; it only separates sister chromatids. DNA replication occurs prior to meiosis I, ensuring each chromosome has two sister chromatids, but it does not affect the ploidy of the cells. Fertilization restores the diploid number but does not involve any reduction of chromosomes. Hence, the first meiotic division is the key stage where the chromosome number is halved during gametogenesis.