What describes semiconservative replication?

• A. Two daughter molecules each contain two new strands.
• B. Each daughter DNA molecule consists of one old strand and one new strand.
• C. Each daughter DNA molecule contains two old strands.
• D. No old strand is retained.

Answer: (B)

Semiconservative replication means that when a DNA molecule is copied, the two parental strands separate and each serves as a template for a new strand. As a result, the two daughter DNA molecules formed after replication each contain one strand from the original molecule and one newly synthesized strand. This preserves half of the parental information in every daughter and explains how genetic material is reliably passed on. The other possibilities don’t fit because they would involve either both strands being newly synthesized, or both strands being old, or no old strand being retained—none of which matches the observed pattern. Experimental evidence from the Meselson–Stahl experiment supports this one-old-one-new arrangement.