In which direction is new DNA synthesized?
Answer
5' to 3'
In which direction is new DNA synthesized? New DNA is synthesized from 5′ to 3′.
DNA polymerase adds each incoming nucleotide to the free 3′ hydroxyl group at the end of a growing strand. Because additions occur at that end, the new strand lengthens in the 5′-to-3′ direction. The template strand is read in the opposite, 3′-to-5′ direction.
This rule applies to both strands at a replication fork, even though their arrangements differ. The leading strand is made continuously toward the fork, while the lagging strand is built away from it in short Okazaki fragments. Those fragments are later joined by DNA ligase.
A common mix-up is to say that DNA is synthesized 3′ to 5′ because the template is read that way. Another useful neighbouring fact is that DNA polymerase cannot begin a strand from nothing: it needs a primer providing a free 3′-OH group.