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.
Initiation of Dna Replication
DNA replication begins (initiates) at special sites called origins of DNA replication. Eukaryotic DNAs each contain multiple replication origins,spaced at intervals of approximately 100,000 base pairs (100 kilobase pairs, or 100 kb) along the length of the DNA.
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