DNA replication is the process of copying a double-stranded DNA strand, prior to cell division (in eukaryotes, during the S phase). The two resulting double strands are identical (if the replication went well), and each of them consists of one original and one newly synthesized strand. This is called semiconservative replication. The process of replication consists of three steps, initiation, replication and termination.
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In the initiation step, several key factors are recruited to an origin of replication. This is a sequence that is rich in adenine-thymine base pairs, which are more easily separated than cytosine-guanine base pairs. Once the strands are initially unwound, several factors come into play. The partially unwound strands form a "replication bubble", with one "replication fork" on either end. Each group of enzymes at the replication fork proceeds away from the origin, unwinding and replicating the DNA strands as they move.
The factors involved are:
- A helicase, which unwinds the DNA ahead of the fork.
- A primase, which generates an RNA primer to be used in DNA replication.
- A DNA holoenzyme, which is actually a complex of enzymes that performs the actual replication. There are two holoenzymes at each replication fork, one to replicate each strand of DNA.