AQA A-Level Chemistry Paper 3, 2021: Question 36
1 mark · Medium difficulty · Multiple Choice
This question checks your understanding of complementary base pairing in DNA. You are asked to identify the base pair that is held together by three hydrogen bonds—this is a key feature in distinguishing cytosine–guanine pairs from adenine–thymine pairs, which only form two hydrogen bonds.
Practise this questionQuestion
Question text
36 Which row shows a pair of bases that can link two strands of DNA with three
hydrogen bonds?
Use the Data Booklet to help you answer this question.
[1 mark]
Base 1 Base 2
A adenine guanine
B cytosine thymine
C cytosine guanine
D adenine thymine
Mark scheme
Show the mark scheme
36 C 1 Base 1 cytosine Base 2 guanine
How to answer it
Multiple Choice Explained: Which base pair forms three hydrogen bonds in DNA?
Correct Answer: C – cytosine and guanine
Why? In DNA, guanine (G) and cytosine (C) form a base pair connected by three hydrogen bonds. This is what gives G≡C pairs higher thermal stability than A=T pairs, which form only two hydrogen bonds.
Why A is incorrect:
Adenine and guanine are both purines, so they do not pair with each other. Purines always pair with pyrimidines (A with T/U, G with C).
Why B is incorrect:
Cytosine and thymine are both pyrimidines. Like option A, this pairing doesn’t occur in DNA. Pyrimidine-pyrimidine pairs are structurally incompatible with the DNA double helix.
Why D is incorrect:
Adenine and thymine do pair in DNA, but they are joined by only two hydrogen bonds, not three. This makes them incorrect for this question.
Topics
Organic Chemistry · 3.3.13 Amino Acids, Proteins and DNA
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.