The first
step in DNA replication is to ‘unzip’ the double helix structure of the DNA
molecule. This is carried out by an enzyme called helicase which breaks the
hydrogen bonds holding the complementary bases of DNA together. The separation
of two single strands of DNA creates a ‘Y’ shape called the replication ‘fork’.
The two separated strands will act as templates for making the new strands of
DNA. One of the strands is oriented in the 5’ to 3’ direction (towards the
replication fork), this is the leading strand. The other strand is oriented in
the 3’ to 5’ direction (away from the replication fork), this is the lagging
strand. As a result of their different orientations, the two strands are
replicated differently.
For the leading strand:
A short piece
of RNA called primer (produced b an enzyme called primase) comes along and
binds to the end of the leading strand. The primer acts as the starting point
for DNA synthesis. DNA polymerase binds to the leading strand and then ‘walks’
along it, adding new complementary nucleotide bases to the strands in the 5’ to
3’ direction. This is called continuous.
For the lagging strand:
Numerous RNA
primers are made by the primase enzyme and bind at various points along the
aging strand. Chunks of DNA called Okazaki fragments are then added to the
lagging strand also in the 5’ to 3’ direction. This type of replication is
called discontinuous as the Okazaki fragments will need to be joined up later.
Once all the
bases are matched up, an enzyme called exonuclease strips away the primer(s). The
gaps where the primer(s) were are then filed by yet more complementary
nucleotides. The new strand is proofread to make sure there are no mistakes in
the new DNA sequence. Finally, an enzyme called DNA ligase seals up the
sequence of DNA into two continuous double strands. During the zipping up of
the DNA molecule, there is a possibility that the pairing of bases get shuffled
and thus cause for mutation. The result of DNA replication is two DNA molecules
consisting of one new and one old chain of nucleotides. The answer is C.
