[Campbell Biology P.370] An experiment is conducted where parental DNA is labeled with a hea... | Practice Question

An experiment is conducted where parental DNA is labeled with a heavy isotope. After one round of replication in a medium containing only light isotopes, all resulting DNA molecules are found to have an intermediate density containing both heavy and light components . After a second round of replication in the same light isotope medium, half of the DNA molecules have intermediate density, and the other half have light density. Which model of DNA replication do these results support?

  • A: Conservative model
  • B: Semiconservative model
  • C: Dispersive model
  • D: Both conservative and semiconservative models
  • E: All three models equally

Explanation

The results described are the classic findings of the Meselson-Stahl experiment, which elegantly supported the semiconservative model. After one round of replication, all DNA molecules are hybrids intermediate density because each contains one old heavy strand and one new light strand. After a second round, the hybrid molecules serve as templates, leading to some new hybrid molecules and some entirely new light-light molecules, resulting in half intermediate density and half light density.