A-LevelBiologyNucleic acids and protein synthesis9700/12 · Oct/Nov 2016 · 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]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
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
C: Bar for ¹⁴N is at 75%, bar for ¹⁴N/¹⁵N is at 25%.
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