Which enzymes facilitate unwinding of the DNA during initiation?

Which enzymes facilitate unwinding of the DNA during initiation?

• A. Topoisomerase and Helicase
• B. DNA Polymerase
• C. Primase
• D. Ligase

Answer: (A)

Unwinding DNA at the start of replication relies on enzymes that both separate the two strands and relieve the twisting tension created by that separation. Helicase binds the double helix and breaks the hydrogen bonds between base pairs, effectively pulling the strands apart to form the replication fork. As the strands separate, the DNA ahead of the fork tends to become over-twisted, which would stall unwinding. Topoisomerase steps in to relieve this torsional strain by making transient cuts in the DNA backbone, allowing the helix to swivel and then resealing the breaks. This combination—unwinding by helicase and tension-relief by topoisomerase—is what enables initiation to proceed. The other enzymes have roles later in replication, such as primase laying down RNA primers, DNA polymerase synthesizing new DNA, and ligase sealing nicks.

Unwinding DNA at the start of replication relies on enzymes that both separate the two strands and relieve the twisting tension created by that separation. Helicase binds the double helix and breaks the hydrogen bonds between base pairs, effectively pulling the strands apart to form the replication fork. As the strands separate, the DNA ahead of the fork tends to become over-twisted, which would stall unwinding. Topoisomerase steps in to relieve this torsional strain by making transient cuts in the DNA backbone, allowing the helix to swivel and then resealing the breaks. This combination—unwinding by helicase and tension-relief by topoisomerase—is what enables initiation to proceed. The other enzymes have roles later in replication, such as primase laying down RNA primers, DNA polymerase synthesizing new DNA, and ligase sealing nicks.