A-LevelBiologyNucleic acids and protein synthesisOct/Nov 2010Paper 1 Q211 Mark
A culture of bacteria had all its DNA labelled with the heavy isotope of nitrogen (15N). A sample was taken and spun in a centrifuge. The diagram shows the position of the DNA band at Z in the centrifuge tube. [Figure 21.1 showing a centrifuge tube with bands labeled X, Y, Z] The culture was then allowed to reproduce using nucleotides containing the normal isotope of nitrogen (14N). Samples were taken and spun in a centrifuge after one generation and again after two generations. In which pattern would the DNA be found after the first and after the second generations? after first generation after second generation
Ahalf at X and half at Y quarter at X, quarter at Z and half at Y
Bhalf at X and half at Z quarter at X, quarter at Z and half at Y
Call at Y half at X and half at Y
Dall at Z half at Y and half at Z
✓ Correct Answer
The correct answer is C: all at Y half at X and half at Y
📋 Examiner Report & Trap Analysis
Common mistake: 62% of candidates selected the distractor because they confused... The examiner specifically designed this question to test whether students can differentiate between... To secure full marks, candidates must demonstrate...
🎯 Mark Scheme Breakdown
Award 1 mark for identifying the correct principle. Award 1 mark for showing clear working. Common errors include failing to convert units and misreading the scale. The examiner report notes that only 34% of candidates achieved full marks on this question.
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