The DNAs that make up the genomes of bacteria and eukaryotic cells are double-stranded molecules in which each strand is composed of subunits called nucleotides. DNA nucleotides have a direction, in the same way that an arrow has a head and a tail. In DNA strands, the head is the 3′ ("three prime") end of the strand, and the tail is the 5′ ("five prime") end. As a result, each strand also has a direction, whose ends are referred to as the 3′ and 5′ ends. The two strands of DNA run in opposite directions, and are wound around each other in a double helix, with the strands held together by hydrogen bonds between paired bases of the nucleotides (A pairs with T, and G pairs with C).
During the process of DNA replication, the strands are unwound by an enzyme called DNA helicase, and a new strand of DNA is synthesized on each of the old (template) strands by an enzyme called DNA polymerase, which joins incoming nucleotides together in a sequence that is determined by the sequence of nucleotides present in the template strand. DNA replication is said to be semiconservative because each of the two identical daughter molecules contains one of the two parental template strands paired with a new strand. Prokaryotic replication can take as little as twenty minutes, while replication in eukaryotes takes considerably longer, approximately eight hours in mammals.