ABSTRACT
This chapter describes the cellular machinery of sexual reproduction. It focuses on the special features of meiosis that distinguish it from mitosis. Sexual reproduction, by contrast, mixes the genomes from two individuals to produce offspring that differ genetically from one another and from both parents. Sexual reproduction occurs in diploid organisms, in which each cell contains two sets of chromosomes, one inherited from each parent. Sexual reproduction, therefore, requires a specialized type of cell division called meiosis, in which a diploid precursor cell gives rise to haploid progeny cells, rather than to diploid cells as occurs in ordinary mitotic cell division. At fertilization, a haploid sperm fuses with a haploid egg to form a diploid cell, which contains a new combination of chromosomes. In many animals, including many vertebrates, the unfertilized egg contains specific molecules localized in a particular region of the cytoplasm that determine which cells will become the germ cells.
