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Question Video: Recalling the Enzyme That Joins DNA Fragments during Replication Biology

During DNA replication, which of the following enzymes joins the backbones of the complementary DNA fragments of the 3′–5′ strand? [A] DNA ligase [B] DNA helicase [C] DNA polymerase enzyme [D] mRNA polymerase enzyme

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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.

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