Fig. 1: A replication fork. On the right, the parental double helix is drawn as two parallel lines; it is being opened at the fork, which moves to the right, as an arrow shows. To the left of the fork the two parental strands separate. The upper parental strand is marked 3 prime at its left end and 5 prime at its right end; the lower parental strand is marked 5 prime at its left end and 3 prime at its right end. A new strand X is drawn as a dashed line along the upper parental strand, and a new strand Y as a dashed line along the lower one. The figure does not show how X and Y are made.