What is a hallmark feature of semi-conservative replication?

• A. One parental and one daughter strand per new double helix
• B. Two parental strands per double helix
• C. Two daughter strands per double helix
• D. RNA replaces one strand in the new double helix

Answer: (A)

The hallmark of semi-conservative replication is that each newly formed DNA double helix contains one parental (old) strand and one newly synthesized strand. During replication, the two strands of the original DNA separate, and each serves as a template for building a complementary new strand. The result is two daughter molecules, each with one old strand from the parent and one new strand, preserving one half of the original molecule in each new duplex.

The other ideas don’t fit because both strands being parental would mean no new synthesis, which isn’t how replication works. Both strands being newly synthesized would imply a fully conservative model, which isn’t the case. RNA’s role is limited to providing short primers to start synthesis; it is not used to replace an entire strand in the final DNA double helix.