Students should be able to evaluate the work of scientists in validating…

The idea

Meselson and Stahl tested DNA replication models by growing bacteria in a medium with the heavy isotope nitrogen-15, so all DNA bases contained it. They then transferred cells to a medium with ordinary nitrogen-14 and sampled after each replication round. DNA was centrifuged in a density gradient, settling at a level matching its density. After one replication, all DNA formed a single band of intermediate density. This rules out conservative replication, which predicts one heavy and one light band, but is consistent with both semi-conservative and dispersive models. After two replications, there were two bands in equal quantities: one intermediate and one light. This rules out dispersive replication, which predicts a single band becoming steadily lighter. Only the semi-conservative model fits both results.

What earns the marks

• Growing cells in nitrogen-15 so that all their DNA contains the heavy isotope, then transferring them to nitrogen-14.
• Separating DNA according to density, with heavier DNA settling lower in the tube.
• Stating the single band of intermediate density after one replication rules out the conservative model, as it shows the DNA contains both heavy and light isotopes.
• Using the second generation result, equal amounts of intermediate and light DNA, to rule out the dispersive model.
• Stating that repetition by other scientists and peer review are needed before a model is accepted.

Common mistakes and how to put them right

• Describing nitrogen-15 as radioactive, when it is a stable heavy isotope and the bands are separated by density.

  Put it right by referring to it as a heavy isotope.

• Treating the first generation result as proof of the semi-conservative model, when it only eliminates the conservative one.

  Put it right by stating it is consistent with both semi-conservative and dispersive models.

• Saying the experiment proves the model true, rather than that it supports it and fails to disprove it.

  Put it right by using terms like 'supports' or 'provides evidence for'.

• Forgetting that nitrogen is in the bases, and explaining the labelling as being in the sugar or the phosphate.

  Put it right by specifying the nitrogenous bases.

• Evaluating with vague statements such as 'they did lots of repeats' with no reference to the actual data.

  Put it right by referring to specific band positions and generations.

Exam tips

• Sketch the three predicted band patterns for generation one and generation two before you write; the answer then almost writes itself.
• An 'evaluate' command needs both what the evidence shows and a limitation or a reason for confidence, so include both.
• Refer to the data given in the question by value or by band position rather than describing it in general terms.