Paper 2 · DNA replication
In Meselson and Stahl's experiment, bacterial cells were first grown in medium containing the heavy nitrogen isotope ¹⁵N, then transferred to medium containing the lighter isotope ¹⁴N. After each generation the DNA was centrifuged and separated into bands of differing density, showing that DNA molecules become progressively lighter as replication proceeds in ¹⁴N medium, with each new DNA molecule retaining exactly one original strand. What process of DNA replication does this experiment demonstrate?
Model answer
semi-conservative replication
Also accepted: semiconservative replication, semi-conservative DNA replication
Explanation: The experiment showed that after one round of replication in ¹⁴N medium, all the DNA was of intermediate (hybrid) density, meaning each new double helix contained one original ¹⁵N strand and one newly synthesised ¹⁴N strand. Each daughter molecule conserving one parental strand demonstrates semi-conservative replication, ruling out the conservative and dispersive models.
Derived from ZIMSEC Biology Paper 2, November 2005, Q3
