A-LevelBiologyNucleic acids and protein synthesisMay/June 2010Paper 1 Q201 Mark
DNA is said to replicate in a semi-conservative way. Results of Meselson and Stahl's experiments gave overwhelming support to this theory. They used E. coli which has a generation time of 50 minutes. Here are the steps in their experiment but they are in the wrong order. P All bacteria contain ¹⁵N DNA. Q All bacteria contain hybrid DNA (¹⁵N DNA and ¹⁴N DNA). R Bacteria contain either all ¹⁴N DNA or hybrid DNA. S Bacteria grown in a ¹⁵N medium for many generations. T Bacteria transferred to a ¹⁴N medium and sampled every 50 minutes. Which sequence of letters shows the correct order of the steps in the experiment?
AP→Q → R → S → T
BP→S→T→R→Q
CS→P→T→Q→R
DS→R→Q→P→T
✓ Correct Answer
The correct answer is C: S→P→T→Q→R
📋 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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