How does DNA replication ensure copying from the correct template strand?

• A. DNA polymerases synthesize new strands complementary to each template; replication is semiconservative with antiparallel templates and proofreading.
• B. Polymerases copy randomly both strands; no proofreading.
• C. Replication uses only one template strand.
• D. There is no proofreading.

Answer: (A)

The main idea is that replication is template-directed and directional, with high fidelity. DNA polymerases can only add nucleotides in the 5' to 3' direction and they read the template strand in the opposite direction, 3' to 5'. Because the two parental strands run antiparallel, each strand serves as a template for a complementary new strand, so both sides of the double helix are copied in a coordinated, semi-conservative manner: each daughter DNA contains one old strand and one new strand. The polymerase’s proofreading activity (3' to 5' exonuclease) checks each newly added base andCorrects mismatches, greatly increasing accuracy and ensuring the correct template information is preserved in the new strand. If copying were random, or if proofreading were absent, or if replication used only one template strand, the fidelity and structure of the resulting DNA would not match what is observed.