CAIE A Level Biology 9700 · Nucleic acids and protein synthesis
A scientist compares the DNA of two bacterial species. Species X lives in a hot spring at 70°C, while species Y lives in a temperate lake at 20°C. The scientist finds that the DNA of species X has a higher proportion of guanine and cytosine nucleotides compared to species Y. Explain why this higher proportion of C-G base pairs increases the thermal stability of the DNA.
5 marks, so the mark scheme is looking for about 5 separate creditworthy points. Making the same point twice in different words earns it once.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
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