OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

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What is the reason A pairs with T in DNA structure?

  1. They are both purines

  2. The amount of A equals the amount of T

  3. They are both pyrimidines

  4. A is always less than T

The correct answer is: The amount of A equals the amount of T

In the structure of DNA, adenine (A) pairs with thymine (T) due to the specific hydrogen bonding characteristics of these nitrogenous bases, which is explained by the complementary base pairing rule. The correct choice indicates that in a double-stranded DNA molecule, the amount of adenine is always equal to the amount of thymine. This is a fundamental property of DNA, known as Chargaff's rules, which states that in any double-stranded DNA, the concentration of adenine will match the concentration of thymine and the concentration of cytosine will match that of guanine. Adenine and thymine form two hydrogen bonds between them, which stabilizes the DNA structure. This pairing is not due to them both being purines or pyrimidines, as A is a purine and T is a pyrimidine. Therefore, other options do not reflect the fundamental pairing mechanism seen in DNA. In essence, the correct relationship between adenine and thymine underlines the importance of complementary base pairing in DNA's overall structure and function.

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