Replication is the process by which a nucleic acid molecule is duplicated. DNA replication is required before cell division, ensuring that each daughter cell contains a copy of the genome. In eukaryotes, chromosomal replication occurs in the cell nucleus, during the S phase of the cell cycle. DNA replication proceeds in a semiconservative manner in which each of the parental strands of the DNA double helix acts as a template for the nascent strand. A DNA polymerase enzyme catalyzes the formation of a phosphodiester bond between a free 3′-hydroxyl radical of a primer DNA chain and an incoming deoxyribonucleotide, so that replication proceeds in the 5–3′ direction. RNA replication occurs during the infective cycle of viruses with RNA genome (with some exceptions including retroviruses which retro-transcribe their genomic RNA into DNA).