DNA replication of one helix of DNA results in two identical helices. If the
original DNA helix is called the "parental" DNA, the two resulting helices can
be called "daughter" helices. Each of these two daughter helices is a nearly
exact copy of the parental helix (it is not 100% the same due to mutations).
DNA creates "daughters" by using the parental strands of DNA as a template or
guide. Each newly synthesized strand of DNA (daughter strand) is made by the
addition of a nucleotide that is complementary to the parent strand of DNA. In
this way, DNA replication is semi-conservative, meaning that one parent strand
is always passed on to the daughter helix of DNA.
Figure %: The Semi-Conservative Nature of DNA Replication
Replication Forks and Origins of Replication
The first step in DNA replication is the separation of the two DNA strands that
make up the helix that is to be copied. DNA Helicase untwists the helix at
locations called replication origins. The replication origin forms a Y
shape, and is called a replication fork. The replication fork moves down the
DNA strand, usually from an internal location to the strand's end. The result is
that every replication fork has a twin replication fork, moving in the opposite
direction from that same internal location to the strand's opposite end. Single-stranded
binding proteins (SSB) work with helicase to keep the parental DNA helix
unwound. It works by coating the unwound strands with rigid subunits of SSB that keep
the strands from snapping back together in a helix. The SSB subunits coat the
single-strands of DNA in a way as not to cover the bases, allowing the DNA to remain
available for base-pairing with the newly synthesized daughter strands.
Figure %: Replication Fork
As you can see in , when the two parent strands of DNA are separated
to begin replication, one strand is oriented in the 5' to 3' direction while the other
strand is oriented in the 3' to 5' direction. DNA replication, however, is inflexible:
the enzyme that carries out the replication, DNA polymerase, only functions in the 5'
to 3' direction. This characteristic of DNA polymerase means that the daughter strands
synthesize through different methods, one adding nucleotides one by one in the direction
of the replication fork, the other able to add nucleotides only in chunks. The first
strand, which replicates nucleotides one by one is called the leading strand; the
other strand, which replicates in chunks, is called the lagging strand.
Figure %: Replication Fork
The Leading and Lagging Strands
The Leading Strand
Since DNA replication moves along the parent strand in the 5' to 3' direction, replication
can occur very easily on the leading strand. As seen in , the
nucleotides are added in the 5' to 3' direction. Triggered by
RNA primase, which
adds the first nucleotide to the nascent chain, the DNA polymerase simply sits near the
replication fork, moving as the fork does, adding nucleotides one after the other, preserving
the proper anti-parallel orientation. This sort of replication, since it involves one
nucleotide being placed right after another in a series, is called continuous.
The Lagging Strand
Whereas the DNA polymerase on the leading strand can simply follow the replication fork,
because DNA polymerase must move in the 5' to 3' direction, on the lagging strand the enzyme
must move away from the fork. But if the enzyme moves away from the fork, and the fork
is uncovering new DNA that needs to be replicated, then how can the lagging strand be
replicated at all? The problem posed by this question is answered through an ingenious
method. The lagging strand replicates in small segments, called Okazaki fragments.
These fragments are stretches of 100 to 200 nucleotides in humans (1000 to 2000 in bacteria)
that are synthesized in the 5' to 3' direction away from the replication fork. Yet while
each individual segment is replicated away from the replication fork, each subsequent
Okazaki fragment is replicated more closely to the receding replication fork than the
fragment before. These fragments are then stitched together by DNA ligase, creating
a continuous strand. This type of replication is called discontinuous
Figure %: Leading and Lagging Strands
As you can see in the figure above, the first synthesized Okazaki fragment on the lagging
strand is the furthest away from the replication fork, which is itself receding to the
right. Each subsequent Okazaki fragment starts at the replication fork and continues until
it meets the previous fragment. The two fragments are then stitched together by DNA ligase.
Figure %: Patching Up Okazaki Fragments
In figure above, we can also see how replication on the lagging strand remains slightly
behind that on the leading strand. Because synthesis on the lagging strand takes place
in a "backstitching" mechanism, its replication is slightly delayed in relation to
synthesis on the leading strand. The lagging strand must wait for a patch of the parent
helix to open up a short distance in front of the newly synthesized strand before it can
begin its synthesis back to the end of the daughter strand. This "lag" time does not
occur in the leading strand because it synthesizes the new strand by following right
behind as the helix unwinds at the replication fork.
