In semiconservative replication, each new DNA molecule contains...

• A. One old and one new strand per molecule
• B. Both strands are old
• C. Both strands are new
• D. One strand is RNA and one is DNA

Answer: (A)

Semiconservative replication means each new DNA molecule keeps one strand from the original double helix and builds a new complementary strand alongside it. The old strand serves as a template, so DNA polymerase adds matching nucleotides to form the new strand, yielding two DNA molecules that each contain one old and one new strand. This pattern was demonstrated by the Meselson-Stahl experiment, which showed daughter molecules with intermediate density consistent with one old and one new strand per molecule. The other scenarios would require both strands to be old (conservative), or both to be new, or include RNA in the final DNA molecule—none of which describes the actual final DNA structure, since RNA primers are replaced with DNA and the finished molecule is fully DNA.