A-LevelBiologyNucleic acids and protein synthesisOct/Nov 2011Paper 1 Q231 Mark
Bacteria were grown in a medium containing 15N. After several generations, all bacteria contained 15N. Some of these bacteria were transferred to a medium containing the isotope of nitrogen, 14N. The bacteria were allowed to divide once. The DNA of some bacteria was extracted and analysed. This DNA was all hybrid DNA containing equal amounts of 14N and 15N. Some bacteria from the medium with 15N were transferred into a medium of 14N. The bacteria were allowed to divide twice. The graph shows the percentages of 14N and 15N in the DNA of these bacteria. [Figure 23.1] Some bacteria from the medium with 15N were transferred into a medium of 14N. The bacteria were allowed to divide three times. What would be the percentages of 14N and 15N in the DNA extracted from these bacteria?
ABar chart showing 0% for 14N and 100% for 14N/15N
BBar chart showing 50% for 14N and 50% for 14N/15N
CBar chart showing 75% for 14N and 25% for 14N/15N
DBar chart showing 25% for 14N and 75% for 14N/15N
✓ Correct Answer
The correct answer is C: Bar chart showing 75% for 14N and 25% for 14N/15N
📋 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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