Video Transcript
In semiconservative DNA
replication, what is the primary role of DNA ligase? (A) DNA ligase adds nucleotides to
a growing DNA chain to synthesize a strand of DNA complementary to the template
strand. (B) DNA ligase joins the backbones
of fragments formed on a complementary strand during replication. (C) DNA ligase catalyzes the
breaking of phosphodiester bonds in the sugar-phosphate backbone, so the DNA can be
split into fragments that are ready for replication. (D) DNA ligase breaks the hydrogen
bonds between base pairs, separating the two strands of DNA that are ready for
replication. Or (E) DNA ligase joins RNA primers
to the five prime end of a single strand of DNA to indicate where replication should
begin.
This question is asking us about
the primary role of the enzyme DNA ligase during DNA replication. To answer this, let’s clear these
answer choices so we have more room to work with. In order for a cell to divide and
make new cells, its DNA needs to be replicated during a process called DNA
replication. DNA is replicated in a
semiconservative way, meaning that a new strand is synthesized from each of the two
old strands. Let’s look at this process in a bit
more detail. So here’s a molecule of DNA with
the nitrogenous bases in different colors and the sugar-phosphate backbone in
black. These two strands of DNA are held
together by complementary base pairing between the bases.
You’ll recall that DNA has
directionality and that one strand goes in the five prime to three prime direction,
while the other strand goes in the three prime to five prime direction. During DNA replication, these two
strands are separated to reveal these individual bases. Now, an enzyme called DNA
polymerase can bind to a strand of DNA and add new complementary bases to synthesize
a new strand of DNA, which we can see in pink here. If we look at the directionality of
this newly synthesized segment of DNA, we can see that it’s in the three prime to
five prime direction.
DNA polymerase can only move and
add nucleotides in the five prime to three prime direction. So when DNA polymerase is on this
top five prime to three prime strand, it’s making a new three prime to five prime
strand, but it’s moving in the five prime to three prime direction because that’s
the only way that it can add new nucleotides. On this strand, a new strand of DNA
can be synthesized continuously, while on the opposing three prime to five prime
strand, the new DNA strand being made is in the five prime to three prime direction,
which is the same direction as DNA synthesis. But in order to synthesize this
region indicated here, DNA polymerase needs to detach and bind up here.
After synthesis, we have these two
fragments as shown here that have a gap in the sugar-phosphate backbone of the newly
synthesized strand of DNA, which you can see here. This gap can be joined together by
the enzyme DNA ligase. We now have enough information to
answer this question. In semiconservative DNA
replication, DNA ligase joins the backbones of fragments formed on a complementary
strand during replication.
