At Biophilia:


At the very heart of our hereditary and developmental lives is DNA synthesis- replication that copies one duplex into two. But each beautiful, classic DNA duplex has two strands, each of which needs to be copied. Not only that, but those two strands go in opposite directions. The one is the complement, or anti-sense, of the other one, and in chemical terms heads the other way. This creates a significant problem, since the overall direction of replication can only go one way- the direction of the fork where the parental duplex splits into two.