A-LevelBiologyNucleic acids and protein synthesisOct/Nov 2016Paper 1 Q231 Mark
Bacteria were grown in a medium containing ¹⁵N. After several generations, all of the DNA contained ¹⁵N. Some of these bacteria were transferred to a medium containing the common isotope of nitrogen, ¹⁴N. The bacteria were allowed to divide once. The DNA of some of these bacteria was extracted and analysed. This DNA was all hybrid DNA containing equal amounts of ¹⁴N and ¹⁵N. In another experiment, some bacteria from the medium with ¹⁵N were transferred into a medium of ¹⁴N. The bacteria were allowed to divide three times. The DNA of some of these bacteria was extracted and analysed. What is the composition of this DNA? [Figure 23.1]
ABar for ¹⁴N is 0%, bar for ¹⁴N/¹⁵N is at 75%.
BBar for ¹⁴N is at 50%, bar for ¹⁴N/¹⁵N is at 50%.
CBar for ¹⁴N is at 75%, bar for ¹⁴N/¹⁵N is at 25%.
DBar for ¹⁴N is at 75%, bar for ¹⁴N/¹⁵N is at 25%.
✓ Correct Answer
The correct answer is C: Bar for ¹⁴N is at 75%, bar for ¹⁴N/¹⁵N is at 25%.
📋 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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