Question

In which direction, and on which strands does DNA replication proceed from a replication origin on a chromosome?

A. in both directions, on both strands

B. only on one strand in the 5′ à 3′ direction

C. only on one strand in the 3′ à 5′ direction

D. from the left to the right, on both strands

Answer 1

DNA replication proceeds in both directions from a replication origin, on both strands. The leading strand is synthesized continuously towards the replication fork, while the lagging strand is synthesized away from the fork in Okazaki fragments.

Step-by-step explanation:

In the process of DNA replication, replication proceeds from a replication origin in both directions on a chromosome. It occurs on both strands; however, due to the antiparallel nature of DNA, the mechanism is different for each strand. The enzyme DNA polymerase 'reads' the template strand from the 3' to 5' direction but can only add nucleotides in the 5' to 3' direction.

The leading strand is synthesized continuously in the direction of the replication fork, because it is oriented in a 3' to 5' direction, allowing the DNA polymerase to add nucleotides in a 5' to 3' direction as it moves towards the replication fork. Conversely, the lagging strand is synthesized in short stretches called Okazaki fragments, in a direction away from the replication fork. This occurs because the template strand is oriented in a 5' to 3' direction, which requires the polymerase to loop around and work in okward stretches.