Video Transcript
The diagram provided shows a simplified outline of how DNA is replicated on the
leading strand and the lagging strand. Which enzyme joins the fragments that are formed during the replication of the
lagging strand?
In order to grow and reproduce, the cells of living organisms must go through cell
division. Because each daughter cell must receive a copy of the nuclear DNA in order to
function properly, the DNA must be copied or replicated before the cell divides. DNA replication is carried out by numerous enzymes and cofactors.
Let’s discuss some of these enzymes in order to determine the correct answer to our
question.
The first step of DNA replication is to unwind and separate the complementary strands
of the double helix. This forms a replication fork, making the nitrogenous bases containing the genetic
code accessible to other replication factors. The enzyme DNA helicase, which is not shown in the diagram, accomplishes this step by
breaking the hydrogen bonds between the complementary strands. Once the DNA is unwound, the new complementary strands can be synthesized.
An enzyme called DNA polymerase adds nucleotides to the new strand, ensuring that the
rules of complementary base-pairing are followed. You likely remember that DNA strands are antiparallel, meaning they’re read by the
cell in opposite directions. This antiparallel property arises because one of the sugar–phosphate backbones
terminates in a phosphate group. This end is called the five prime end. The other end terminates in a pentose sugar molecule bonded to a hydroxyl group and
is known as the three prime end. DNA polymerase can only synthesize a new strand of DNA while moving in a five prime
to three prime direction. On the leading strand, this is no problem. DNA is synthesized continuously, with DNA polymerase following behind DNA helicase in
a five prime to three prime direction.
On the lagging strand, however, DNA polymerase must jump forward along the strand and
work moving away from the replication fork. This process results in multiple fragments of DNA that are attached to the lagging
strand by the hydrogen bonds of the bases but are not attached to each other along
the new sugar–phosphate backbone. Therefore, the third step of replication is to complete the newly synthesized strand
by joining the fragments to each other. An enzyme called DNA ligase performs this job by creating phosphodiester bonds
between the sugar–phosphate backbones of adjacent fragments.
We can now answer our question. The enzyme that joins the fragments formed during the replication of the lagging
strand is DNA ligase.
