Video Transcript
During DNA replication, which of
the following enzymes joins the backbones of the complementary DNA fragments of the
three prime to five prime strand? (A) DNA ligase, (B) DNA helicase,
(C) DNA polymerase enzyme, or (D) mRNA polymerase enzyme.
Every living cell carries genetic
material in the form of DNA or deoxyribonucleic acid. The genetic material controls
nearly every aspect of the cell by providing instructions about the particular
proteins and other molecules the cell needs to make. When a cell divides, each new cell
must contain a copy of the DNA in its nucleus for it to be able to function
properly. This is accomplished through DNA
replication, the process by which a dividing cell generates a copy of its DNA. It occurs in the nucleus of the
cell and is controlled by a set of enzymes.
Let’s have a brief review of the
role of each of these enzymes in order to arrive at the correct answer.
You likely already know that DNA is
composed of two strands that wind around each other to form a double helix. The first step of DNA replication
is to unwind and separate the complementary strands of the double helix. The enzyme DNA helicase performs
this job by breaking the hydrogen bonds between the complementary base pairs of the
two strands. This forms a replication fork,
which allows other enzymes to access the nitrogenous bases containing the genetic
code.
DNA polymerase now performs the
second step of DNA replication: synthesizing a new, complementary strand of DNA for
each original strand by using the free nucleotides that are found in the
nucleus. You may remember that DNA strands
are antiparallel. This means that one end of the
sugar–phosphate backbone, known as the five prime end, terminates in a phosphate
molecule. And the other end terminates in a
hydroxyl group and is known as the three prime end.
DNA polymerase requires a free
hydroxyl group in order to add a nucleotide. So it can only synthesize a new
strand of DNA while moving in a five prime to three prime direction. As a result, DNA is synthesized
continuously on the strand that runs five prime to three prime, called the leading
strand.
On the other strand, referred to as
the lagging strand, in order to work in a five prime to three prime direction, DNA
polymerase must jump forward and add nucleotides while moving away from the
replication fork. This process leads to multiple
fragments of DNA that are attached to the original strand by hydrogen bonds between
the base pairs but are not attached to each other along the new sugar–phosphate
backbone. These are called Okazaki
fragments.
An enzyme called DNA ligase
performs the third step of DNA replication by joining the sugar–phosphate backbones
of adjacent Okazaki fragments with phosphodiester bonds. We can remember the role of DNA
ligase because its name comes from the word “ligate,” which means to join
together.
We now have enough information to
answer our question. During DNA replication, the enzyme
that joins the backbones of the complementary DNA fragments of the three prime to
five prime strand is DNA ligase.
