Describe the process of DNA replication, including helicase, primase, DNA polymerase, and ligase.

• A. Helicase unwinds; primase lays RNA primers; DNA polymerase adds nucleotides; ligase seals Okazaki fragments on lagging strand.
• B. DNA ligase seals nicks; primase unwinds.
• C. Topoisomerase unwinds and translates DNA; RNA polymerase replicates; ligase degrades RNA.
• D. Helicase seals Okazaki fragments; DNA polymerase unwinds.

Answer: (A)

DNA replication relies on a coordinated sequence where the double helix is opened, primers are laid down, new DNA strands are synthesized, and fragments are joined. Helicase unwinds the DNA to form the replication fork, separating the two strands so each can serve as a template. Since DNA polymerase can only start adding nucleotides after a primer is present, primase lays down short RNA primers to provide starting points for synthesis. Then DNA polymerase extends from those primers, adding nucleotides in the 5′ to 3′ direction to build the new strands. On the leading strand, this happens continuously toward the fork, while on the lagging strand synthesis is discontinuous, creating short segments called Okazaki fragments. Ligase seals the nicks between these fragments, joining the backbone of the new strand. This sequence explains why these four players are essential: helicase opens the template, primase provides primers, DNA polymerase builds the new DNA, and ligase links the fragments together.