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Nucleic Acids and Protein Synthesis: Question 10

Syllabus 6.1

Structured AS 6 marks

A microbiologist grows a culture of bacteria for many generations in a growth medium in which the only available nitrogen source is a heavy isotope of nitrogen, so that all the nitrogen atoms in the bacterial DNA become heavy. The bacteria are then transferred to a growth medium containing only the normal, light isotope of nitrogen and allowed to divide once. The DNA is extracted and separated by density-gradient centrifugation, a technique that separates DNA molecules according to their density: denser (heavier) DNA forms a band further down the tube, and less dense (lighter) DNA forms a band further up the tube.

(a) Predict, with an explanation, the position of the band(s) of DNA that would be seen after this single round of replication in the light medium, if DNA replication is semi-conservative. [3]

(b) The bacteria are allowed to divide once more, still in the light medium, and the DNA is again separated by density-gradient centrifugation. Describe how the pattern of bands now seen would differ between the semi-conservative model and the (incorrect) conservative model of DNA replication. [3]

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Worked solution

Part (a): Prediction after one round of replication

Under the semi-conservative model, when a heavy DNA molecule replicates, its two strands separate, and each original (heavy) strand acts as a template for a new, complementary strand. Because the bacteria have been transferred to the light-nitrogen medium, every new strand made from this point on is built from light nucleotides.

The result is that every DNA molecule produced by this one round of replication is made of one original heavy strand and one newly made light strand, a hybrid molecule of intermediate density. Since all the molecules are identical hybrids, density-gradient centrifugation would show a single band, positioned at an intermediate density, roughly halfway between where a fully heavy molecule would band and where a fully light molecule would band. Crucially, no fully heavy band remains, because every original heavy strand has been separated from its heavy partner and paired with a new light strand.

Part (b): Comparing the two models after a second round

Semi-conservative model: Each of the hybrid molecules from round one has its two strands (one heavy, one light) separated again, and each strand templates a new light strand. This produces two types of molecule in equal numbers:

  • the original heavy strand, now paired with another new light strand, giving another hybrid (intermediate density) molecule;
  • the light strand made in round one, now paired with a further new light strand, giving a fully light molecule.

So after the second round, semi-conservative replication predicts two bands: the same intermediate (hybrid) band as before, and a new, fully light band, present in roughly equal (1:1) proportions.

Conservative model: If replication were conservative, the original double helix would remain completely intact in one daughter molecule at every round, while the other daughter molecule produced at each round would be made of two entirely new strands. This model predicts a persistent fully heavy band (from the single original molecule, unchanged generation after generation) together with a growing fully light band (from the new molecules), but, importantly, it predicts that an intermediate (hybrid) band would never appear at any generation, since the two original strands are never separated from one another.

The key distinguishing observation is therefore the hybrid band: its appearance after just one round of replication, and its persistence into the second round alongside a new fully light band, is consistent only with the semi-conservative model, not the conservative model.

Final answers

  • (a) A single, intermediate-density (hybrid) band, because every new molecule = one original heavy strand + one new light strand.
  • (b) Semi-conservative: hybrid band + new fully light band (roughly 1:1). Conservative: unchanging fully heavy band + growing fully light band, with no hybrid band ever forming.