During DNA replication, what is the primary function of helicase?

• A. It adds DNA nucleotides to the growing strand in the $5^{\prime }\to 3^{\prime }$ direction.
• B. It seals nicks between Okazaki fragments by forming phosphodiester bonds.
• C. It synthesizes short RNA primers to initiate DNA synthesis.
• D. It unwinds the DNA double helix by breaking hydrogen bonds between complementary bases at the replication fork.

Answer: (D)

Verified step by step guidance

1. Understand the role of helicase in DNA replication: helicase is an enzyme that acts at the replication fork, where the double-stranded DNA is separated into two single strands.
2. Recall that the DNA double helix is stabilized by hydrogen bonds between complementary base pairs (A-T and G-C). Helicase breaks these hydrogen bonds to separate the strands.
3. Recognize that by unwinding the DNA, helicase creates single-stranded templates necessary for DNA polymerase to synthesize new complementary strands.
4. Differentiate helicase's function from other enzymes involved in replication: DNA polymerase adds nucleotides, ligase seals nicks, and primase synthesizes RNA primers.
5. Conclude that the primary function of helicase is to unwind the DNA double helix by breaking hydrogen bonds between complementary bases at the replication fork, enabling replication to proceed.