State that the bases adenine and guanine are purines with a double ring structure, and that the bases cytosine, thymine and uracil are pyrimidines with a single ring structure (structural formulae for bases are not expected)

Adenine and guanine are classified as purines, which have a double-ring structure. In contrast, cytosine, thymine, and uracil are classified as pyrimidines, which have a single-ring structure.

• Purines (double-ring): Adenine (A) and Guanine (G)
• Pyrimidines (single-ring): Cytosine (C), Thymine (T), and Uracil (U)

These structural differences are crucial for base pairing in DNA and RNA, where purines pair with pyrimidines to maintain the structure of the nucleic acid molecules.