S phase is that period of time in the cell-division cycle during which nuclear chromosomal deoxyribonucleic acid (DNA) is replicated (1 ,2 ). The time required for S phase depends on the size of the genome, the organism, and its developmental state. DNA replication requires only 15 to 20 min in budding yeast, but 6 to 7 h in mammalian cells. In organisms, such as frogs, fish, echinoderms, and flies that undergo rapid cell cleavage events at the beginning of their development, S phase takes only a few minutes during these initial cell cleavage events, but it takes several hours in late-stage embryos, adult animals, or cells cultured in vitro.
