7. What is the correct order of action of these enzymes during DNA replication?
a. helicase, primase, polymerase
b. helicase, polymerase, primase
c. primase, helicase, polymerase
d. polymerase, primase, helicase
The correct answer is option a: helicase, primase, polymerase.
This sequence reflects the initial steps of DNA replication at the replication fork.
Enzyme Roles
Helicase unwinds the DNA double helix by breaking hydrogen bonds, creating single strands for copying.
Primase then synthesizes short RNA primers on these exposed templates, providing a 3′ end needed for chain extension.
DNA polymerase binds to the primer and adds deoxyribonucleotides in the 5′ to 3′ direction to form new strands.
Option Analysis
Option a (helicase, primase, polymerase): Correct, as unwinding precedes priming, which enables polymerization.
Option b (helicase, polymerase, primase): Incorrect, since polymerase requires a primer and cannot start synthesis without primase acting first.
Option c (primase, helicase, polymerase): Incorrect, because primase needs single-stranded DNA from helicase to lay down primers.
Option d (polymerase, primase, helicase): Incorrect, as polymerase acts last among these and replication cannot begin without prior unwinding.
In DNA replication, the precise order of enzymes ensures accurate genetic copying before cell division. The sequence begins with helicase unwinding the double helix at the origin, followed by primase adding RNA primers, and DNA polymerase extending new strands – a key concept for CSIR NET aspirants mastering molecular biology.
This order of helicase, primase, polymerase forms the core of replication initiation, enabling semi-conservative duplication. Single-strand binding proteins stabilize the fork post-helicase, while later enzymes like ligase complete the process. For exams, recall that disruptions in this sequence halt replication entirely.
CSIR NET Relevance: Questions test this sequence to assess understanding of prokaryotic replication machinery. Practice reinforces retention for Unit 3.
