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

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Which of the following are known as the four bases of DNA?

A, G, C, T
A, T, C, G
A, T, U, G
C, G, A, U

The correct answer is: A, T, C, G

The four bases of DNA are adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair specifically to form the rungs of the DNA ladder: adenine pairs with thymine, and cytosine pairs with guanine. This base pairing is crucial for DNA replication and the transmission of genetic information. The presence of thymine in DNA is a key feature that distinguishes it from RNA, which contains uracil (U) instead of thymine. Therefore, while uracil is relevant to RNA structures, it does not belong to DNA, making the other options invalid for DNA bases. In summary, the correct answer encapsulates the unique composition of DNA, which is essential for understanding its function in genetics.