At the end of replication, what is the composition of each DNA molecule?

• A. Each molecule contains two original strands.
• B. Each molecule contains one original and one new strand.
• C. Each molecule contains two new strands.
• D. Each molecule contains one mutated and one normal strand.

Answer: (B)

DNA replication follows a semiconservative pattern: each parental strand serves as a template for a new complementary strand. DNA polymerases add nucleotides in the 5' to 3' direction, so the two new strands are built using the old strands as guides. After replication, each daughter DNA molecule contains one original (parental) strand and one newly synthesized strand. This contrasts with models where a molecule would be entirely original or entirely new. Experiments, such as those demonstrating semiconservative replication, support this one-old-one-new composition for every daughter molecule.