DNA structure (biology only): DNA contains four bases, A, C, G and T.

The idea

DNA contains four bases, represented by the letters A, C, G and T. These stand for adenine (A), cytosine (C), guanine (G) and thymine (T). The bases are the parts of the nucleotides that carry the genetic code, and their order along a DNA strand determines the sequence of amino acids in a protein. The bases pair in a specific way: A always pairs with T, and C always pairs with G. This complementary base pairing holds the two strands of the double helix together and ensures that DNA can be copied accurately during replication. When answering questions, use the full names as well as the letters, and remember that the letters are the initials of the base names, not the names themselves.

What earns the marks

• DNA contains four bases: adenine, cytosine, guanine and thymine.
• The letters A, C, G and T stand for adenine, cytosine, guanine and thymine.
• The sequence of bases along DNA carries the genetic code.
• Bases pair specifically: A with T, and C with G.
• Complementary base pairing allows DNA to be copied accurately.
• The order of bases determines the order of amino acids in a protein.

Common mistakes and how to put them right

• Mixing up the base names, for example calling G 'guanine' but C 'cytosine' incorrectly.

  Put it right: Learn A = adenine, C = cytosine, G = guanine, T = thymine.

• Pairing A with G or C with T.

  Put it right: A pairs with T, and C pairs with G.

• Thinking the letters are the names of the bases.

  Put it right: The letters are abbreviations for the full base names.

Exam tips

• Write the full name of each base at least once in an answer to show understanding.
• Use the pairing rule A–T and C–G when explaining DNA replication or protein synthesis.
• If asked for the number of bases, state four and name them all.