DNA replication

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.

Table of contents

1 Initiation 2 Replication 3 Termination

Initiation

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.

At the beginning of replication, an enzyme called DNA polymerase binds to the RNA primase, which indicates the starting point for the replication. DNA polymerase can only travel from the 5’ to 3’. Because DNA has an unique antiparallel structure, the DNA polymerase can only travel on one side of the strand without any interruption. This strand, which goes from 3’ to 5’, is called a leading strand. The opposite strand, from 5’ to 3’, is a lagging strand.

Since the DNA replication on the lagging strand is not continuous, a new DNA polymerase has to be added each time as the helicase unwinds more DNA. As a result, the replicated DNA is fragmented, called Okazaki fragments. Another enzyme, DNA ligase, is used to connect the fragments.

Termination

When the polymerase reaches the end of replication, there is another problem due to the antiparallel structure. The RNA primer on the leading strand occupies a small portion of the DNA, which is not exposed to polymerase and therefore is not copied.

As a result, there would be a gap on the newly duplicated DNA at the original leading strand on the 5’ end. The solution is quite simple. The sticking out 3’ end consists of noncoding DNA called the telomere, which can be simply cut off.

Before the DNA replication is finally complete, enzymes are used to proofread the sequences to make sure the nucleotides are paired up correctly. If mistake or damage occurs, an enzyme called nuclease will remove the incorrect DNA. DNA polymerase will then fill in the gap.