Question

What are the functions of the gyrase, primase, and ligase enzymes in DNA replication?

Short answer

Gyrase relieves DNA torsional strain; primase synthesizes RNA primers; ligase seals nicks and joins Okazaki fragments.

Step-by-step solution

- Understand Gyrase Function

Gyrase, also known as DNA gyrase, is an enzyme that introduces negative supercoils into DNA. During DNA replication, it helps to relieve the torsional strain generated ahead of the replication fork by breaking and rejoining the DNA strands. This action helps in preventing the DNA from becoming too tightly coiled.

- Understand Primase Function

Primase is an enzyme that synthesizes a short RNA primer on the DNA template strand to provide a starting point for DNA synthesis. The RNA primer is necessary because DNA polymerases, the enzymes that synthesize new DNA strands, can only add nucleotides to an existing strand. Therefore, primase lays down a short sequence of RNA that DNA polymerases can extend.

- Understand Ligase Function

Ligase is an enzyme that seals the nicks and joins Okazaki fragments on the lagging strand during DNA replication. It catalyzes the formation of phosphodiester bonds between discrete fragments of DNA, thereby ensuring the continuity of the DNA strand after the RNA primers are replaced with DNA.

Key concepts

DNA gyrase function

During DNA replication, one of the key challenges is the excessive twisting of the DNA double helix ahead of the replication fork. This can create supercoiling, which acts like a knot making the DNA harder to unwind. DNA gyrase is an essential enzyme that addresses this challenge.
DNA gyrase reduces the torsional strain by cutting both DNA strands and then rejoining them after passing another segment of the DNA through the break.
This introduction of negative supercoils not only relieves the stress but also helps maintain the DNA in a workable state for the replication machinery.
By ensuring that the DNA doesn't become too tightly coiled, gyrase plays a critical role in smooth and efficient DNA replication.

primase function

Primase is a specialized enzyme that kickstarts the process of new DNA strand synthesis. Unlike DNA polymerases, which can only add nucleotides to an existing strand, primase is capable of creating a new strand from scratch.
It synthesizes a short RNA primer on the DNA template strand to provide a free 3’ hydroxyl group that DNA polymerase can extend. Primase lays down a short RNA sequence, which acts as a starting point for DNA polymerase to begin elongation.
This RNA primer is essential because without it, DNA polymerase wouldn’t be able to initiate the synthesis of the new DNA strand.
Thus, primase bridges a critical gap in the DNA replication process by enabling the initiation of DNA synthesis.

ligase function

During replication, the lagging strand is synthesized in small fragments known as Okazaki fragments due to the antiparallel nature of DNA. These fragments need to be joined together to create a continuous strand, and this is where DNA ligase comes into play.
Ligase seals the nicks between the Okazaki fragments by catalyzing the formation of phosphodiester bonds, which are the covalent bonds that link successive nucleotides in the DNA backbone.
After DNA polymerase replaces the RNA primers with DNA nucleotides, gaps are left in the sugar-phosphate backbone of the DNA. DNA ligase fills these gaps, ensuring the newly synthesized DNA is continuous and intact.
This final step completes the synthesis of the lagging strand, making ligase indispensable for accurate and seamless DNA replication.