What does semiconservative replication mean?

• A. Each new DNA molecule consists of one parental strand and one daughter strand
• B. Both strands are new
• C. The entire molecule is newly synthesized
• D. Replication occurs only at the ends of chromosomes

Answer: (A)

Semiconservative replication means that each new DNA molecule contains one strand from the original molecule and one newly made strand. This happens because DNA polymerases can only add nucleotides onto an existing strand, using it as a template, while the double helix is unwound to expose the templates. As replication proceeds, the two parental strands each guide the synthesis of a complementary new strand, so two daughter molecules are produced, each with one old strand and one new strand. Historic experiments by Meselson and Stahl showed that after one round, the DNA was hybrid—supporting the idea that one old and one new strand are present in each molecule. Descriptions where both strands are new or where replication occurs only at chromosome ends don’t fit how replication starts at origins and proceeds along the molecule, generating the new complementary strands.