What is the function of DNA polymerases during replication, and how do they differ between leading and lagging strands?

• A. Synthesize RNA primers for replication
• B. Proofread only, do not synthesize
• C. DNA polymerases synthesize DNA in the 5'→3' direction; the leading strand is synthesized continuously toward the fork by one polymerase, while the lagging strand is synthesized discontinuously away from the fork as Okazaki fragments
• D. Unwind the DNA helix

Answer: (C)

DNA polymerases are the enzymes that build new DNA strands by adding nucleotides in the 5' to 3' direction, using the original strands as templates. They also have proofreading activity to catch and correct errors as they synthesize.

At the replication fork, the two strands are antiparallel, so synthesis proceeds differently on each template. The leading strand is made continuously toward the fork by a single polymerase that stays attached to the replication machinery and follows the unwinding DNA. The lagging strand, on the other hand, is synthesized in short segments away from the fork as Okazaki fragments. Each fragment starts with an RNA primer laid down by primase, and another polymerase extends the fragment until it reaches the previous one. Afterward, the RNA primers are removed and the gaps are filled, and the fragments are joined to form a continuous strand.

This explains why the statement that polymerases synthesize DNA in the 5' to 3' direction and that the leading strand is synthesized continuously toward the fork while the lagging strand is synthesized discontinuously away from the fork (as Okazaki fragments) is the correct description. The other options misstate the primary role (RNA primer synthesis is done by primase, not DNA polymerase) or overlook the synthesis and proofreading functions, and they attribute unwinding to polymerase rather than helicase.