Paper 2 · November 2005 · DNA and Molecular Genetics
In Meselson and Stahl's experiment, cells were grown in medium containing the heavy isotope ¹⁵N, then transferred to medium containing the light isotope ¹⁴N and allowed to undergo semi-conservative DNA replication. After the DNA had replicated twice in ¹⁴N medium (the 2nd generation), what proportion of the DNA molecules were hybrid (¹⁵N/¹⁴N) compared with fully light (¹⁴N/¹⁴N)?
AAll of the DNA was hybrid, with none fully light.
BHalf of the DNA was hybrid and half was fully light.
CA quarter of the DNA was hybrid and three-quarters was fully light.
DAll of the DNA was fully light, with no hybrid DNA remaining.
Explanation
After the first replication in ¹⁴N medium, every DNA molecule is hybrid (one ¹⁵N strand, one ¹⁴N strand). At the second replication, each hybrid molecule separates into its ¹⁵N and ¹⁴N template strands, and each template is copied using ¹⁴N nucleotides; each hybrid molecule therefore gives rise to one hybrid daughter molecule and one fully light daughter molecule, giving a 1:1 ratio of hybrid to light DNA across the population.
Derived from ZIMSEC Biology Paper 2, November 2005, Q3