Which step happens first in DNA replication?

• A. Helicase unwinds the DNA double helix at the origin of replication, separating the strands
• B. Primase removes RNA primers and replaces them with DNA nucleotides
• C. DNA polymerase III adds nucleotides to elongate the new strand in the $5\prime$ to $3\prime$ direction
• D. DNA ligase seals nicks between Okazaki fragments on the lagging strand

Answer: (A)

Verified step by step guidance

1. Identify the initial event in DNA replication, which involves preparing the DNA strands for copying.
2. Understand that the enzyme helicase is responsible for unwinding the DNA double helix at the origin of replication, separating the two strands to create a replication fork.
3. Recognize that after helicase action, primase synthesizes RNA primers to provide starting points for DNA synthesis.
4. Know that DNA polymerase III then adds nucleotides to the new strand in the 5\' to 3\' direction, extending from the RNA primers.
5. Finally, DNA ligase seals the nicks between Okazaki fragments on the lagging strand to complete the replication process.