Final answer:
The antiparallel nature of DNA creates a complication for the simultaneous replication of the two exposed templates at the replication fork. Only one new DNA strand, the leading strand, can be synthesized continuously towards the replication fork, while the other strand, the lagging strand, is synthesized in small fragments called Okazaki fragments.
Step-by-step explanation:
The antiparallel nature of DNA creates a complication for the simultaneous replication of the two exposed templates at the replication fork. Because DNA polymerase can only extend in the 5' to 3' direction, and because the DNA double helix is antiparallel, there is a slight problem at the replication fork. The two template DNA strands have opposing orientations: one strand is in the 5' to 3' direction and the other is oriented in the 3' to 5' direction. Only one new DNA strand, the one that is complementary to the 3' to 5' parental DNA strand, can be synthesized continuously towards the replication fork. This continuously synthesized strand is known as the leading strand. The other strand, complementary to the 5' to 3' parental DNA, is extended away from the replication fork, in small fragments known as Okazaki fragments, each requiring a primer to start the synthesis. New primer segments are laid down in the direction of the replication fork, but each pointing away from it.