Before a cell divides it copies its DNA. At the replication fork the double helix is unwound and each strand serves as a template. One new strand is made continuously, and the other in short pieces.
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Key points
Helicase unwinds the helix and single-strand binding proteins keep the strands apart.
DNA polymerase can only add nucleotides in one direction, so the lagging strand is made as Okazaki fragments.
Each daughter DNA molecule has one old strand and one new strand (semi-conservative replication).
