A-LevelBiologyNucleic acids and protein synthesis9700/12 · Oct/Nov 2017 · Q241 Mark
Two sets of bacteria were grown using different types of nitrogen-containing growth media. One set was grown in a medium containing the ‘heavy’ isotope of nitrogen, 15N, until all the DNA was labelled. The other set was grown in a medium containing the ‘light’ isotope of nitrogen, 14N, until all the DNA was labelled. The DNA from each set of bacteria was extracted and centrifuged. The diagram shows the position in the centrifuge tubes of this DNA. [Figure 24.1 - centrifuge tubes showing DNA positions for 14N and 15N] Bacteria with 15N labelled DNA were transferred to a medium containing 14N and allowed to reproduce once. The DNA of the new generation of bacteria was extracted and centrifuged. Which tube shows the position of DNA from this new generation of bacteria? [Figure 24.2 - four centrifuge tubes A, B, C, D]
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.
ACentrifuge tube with a single, heavy band at the bottom.
BCentrifuge tube with a single, light band at the top.
CCentrifuge tube with two bands, one heavy at the bottom and one light at the top.
DCentrifuge tube with a single, intermediate density band in the middle.
✓ Correct answer
D: Centrifuge tube with a single, intermediate density band in the middle.
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