DNA Replication

The structure of DNA remained a mystery until 1953 when Francis Crick and James
Watson articulated that it was a double helix structure. The double helix structure has two linear
strands that twist together and run to each other’s opposite. The double helix structure of DNA
gives a hint of the mechanism of DNA copying during replication process. Replication is a
process involving many stages during cell division. It is through this process that DNA makes a
copy of itself. Three models were initially used to explain DNA replication. These are
conservative, semi-conservative, and dispersive. The model that has been found to be most
accurate is semi-conservative.
In semi-conservative model the two strands that make up the double helix model
separate. Each of the two strands then acts as a template for copying new complimentary strands.
Thus, at the end of replication process, each double stranded DNA has a new daughter strand and
an old parent strand. The DNA molecules that emerge at the end of the this replication process
are, therefore, a hybrid of the regular and parental strand.
The semi-conservative model of DNA replication has been found to be the one that best
explains how DNA replication takes place. This process occurs with the help of various
enzymes. The first step in DNA replication is unzipping of the DNA molecule’s double helix
structure. This process takes place with the help of an enzyme called helicase which breaks
hydrogen bonds that hold the DNA’s complimentary bases together.
Once they have been separated the two strands form a Y-shaped structure called
replication fork. The two strands, however, have different orientations. One is oriented towards
the replication fork (leading) and another is oriented away from the replication fork (lagging).

DNA REPLICATION 3
The replication of the strands is, therefore, done differently because of their differing
orientations.
For the leading strand, primer, an RNA piece produced by primase enzyme bonds to the
strand’s end. DNA polymerase then binds and walks along the strand. As it walks along it adds to
the DNA strand new complementary nucleotide bases. For this reason, the replication method for
leading strand is continuous.
As for lagging strand, primase enzyme makes many RNA primers which then bind at
different points on the strand. Okazaki fragments, which are DNA chunks, are then added on the
strand. The replication process in lagging strands is, therefore, discontinuous because the
Okazaki fragments have to be joined together later in the process.
Exonuclease, an enzyme, strips away the primers once the bases have matched up.
Complimentary nucleotides then fill up the gaps left by the primers. Mistakes in the DNA
sequence are removed after proofreading the new strands. At the end of the replication process
two DNA molecules emerge. These molecules consist of one old and one new nucleotide chain.
The new DNA adopts the double helix structure automatically after it is formed.
DNA replication is, therefore, a process that involves a lot of enzyme action. Enzymes
are needed to unzip the double helix structure, produce RNA primer, and strip the primer away
when the replication process is nearing completion.