semiconservative mechanism

Meaning: During replication the parental double helix unwinds, and each strand serves as a template for a new complementary strand made by DNA polymerase. Each daughter helix therefore has one old strand and one new strand. The rejected alternatives were the conservative model (one all-old helix and one all-new helix) and the dispersive model (each strand a mixture of old and new segments).

On the MCAT: The classic passage is the Meselson–Stahl experiment. Bacteria grown in heavy ¹⁵N are moved to light ¹⁴N medium, and DNA is separated by density gradient centrifugation. After one generation there is a single intermediate-density band, which rules out the conservative model. After two generations there are an intermediate band and a light band, which rules out the dispersive model. In later generations the intermediate band persists, but light DNA makes up a growing share of the total.

Common confusion: One generation of data does not prove the semiconservative model. It only excludes the conservative one, because the dispersive model also predicts a single intermediate band. Also, the term does not mean that half the genome is copied. The whole genome is copied, and each new molecule is half old and half new.