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Question Video: Describing the Role of DNA Ligase Biology

In semiconservative DNA replication, what is the primary role of DNA ligase?

03:29

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.

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