DNA replication begins at special sites called origins of
replication where the two parental strands separate to form
replication bubbles.
Helicases unwind the DNA at the replication fork and
separate the two template DNA strands for copying.
An enzyme called primase joins RNA nucleotides to make a
short stretch of RNA called a primer. The primer serves as a
starting point for making new DNA.
Elongating (building) a new strand
Starting at RNA primers, DNA polymerase joins DNA
nucleotides to form new DNA strands along the old ones.
Note that DNA polymerase can only move along a template
strand in the 3' to 5' direction, making new DNA 5' to 3'. This
means that along one template strand, DNA polymerase can
work toward the widening replication fork, but along the
other strand it must work in the opposite direction, making
the new DNA in short segments. DNA polymerases can only
add a nucleotide to an existing nucleotide joined to the
template strand. DNA polymerases cannot start the making
of a new strand.
Only one primer is required for the leading strand however
for the lagging strand each fragment must be primed.
The energy used to join the nucleotides comes from the
breaking of bonds between extra phosphates that are
carried on the free nucleotide.
The problem of antiparallel strands
DNA polymerases can only add nucleotides to the 3’ end of
a DNA template. Therefore a new DNA strand can elongate
only in the 5’ → 3’ direction.
Along one template strand DNA polymerase moves along
the template making a continuous complementary strand in
the 5’ → 3’ direction as the fork progresses.
This is called the leading strand.
To make the other new strand, polymerase must work along
the template away from the replication fork.
This is called the lagging strand.
As a replication bubble opens, a polymerase molecule works
away from the replication fork & makes a short segment of
DNA. These pieces are called Okazaki fragments.
Joining Okazaki fragments
A different DNA polymerase replaces the RNA primers with
DNA. Then an enzyme called DNA ligase joins the short
segments of DNA to complete the new DNA strands.
The fragments are numbered in the order in which they were
made.
Priming DNA synthesis
DNA polymerase cannot initiate a polynucleotide strand; it
can only add to the 3’ end of an already started strand. The
primer is a short segment of RNA synthesised by the
enzyme primase. Each primer is eventually replaced by
DNA.