Big Idea 3.A
How does DNA replication take place?
Replication begins at an origin of replication, where two strands of DNA separate.
The enzyme DNA polymerase then catalyzes the antiparallel (DNA is antiparallel — parallel but moving in opposite directions) elongation of the new DNA strands.
DNA polymerase builds a new strand from the 5' to 3' direction by moving along the template strand and pushing the replication fork ahead of it.
DNA polymerase cannot initiate synthesis, it can only add nucleotides to the 3' end of a preexisting chain. This preexisting chain actually consists of RNA and is called RNA primer. An enzyme called primase makes the primer by joining together RNA nucleotides.
Both new strands are built in the 5' to 3' direction. One strand is formed towards the replication fork in an unbroken, linear fashion. This is called the leading strand. The other strand forms in the direction away from the replication fork in a series of segments called Okazaki fragments. The fragments will be joined into one continuous strand by DNA ligase.
Helicase untwists the double helix at the replication fork. Topoisomerases lessen the tension on the tightly wound helix.