(HT only) In the complementary strands a C is always linked to a G on the…

The idea

DNA is a double-stranded molecule held together by hydrogen bonds between bases on opposite strands. The strands are complementary, meaning the base on one strand determines the base on the other. Cytosine (C) always pairs with guanine (G), and thymine (T) always pairs with adenine (A). This is called complementary base pairing. Because of this rule, if you know the sequence of one strand, you can work out the sequence of the other.

For example: If one strand reads A T G C, the opposite strand reads T A C G. The pairing also explains Chargaff's observation that the amount of A equals the amount of T, and the amount of C equals the amount of G in double-stranded DNA.

What earns the marks

• DNA consists of two complementary strands held together by hydrogen bonds between bases.
• Cytosine pairs with guanine, and thymine pairs with adenine, on opposite strands.
• The base sequence of one strand determines the base sequence of the complementary strand.
• Complementary base pairing means the proportion of adenine equals the proportion of thymine, and cytosine equals guanine, in double-stranded DNA.
• Base pairing allows accurate copying of DNA because each strand can act as a template.

Common mistakes and how to put them right

• Pairing C with T or A with G.

  Put it right by stating that C pairs with G and T pairs with A.

• Writing the complementary sequence in the same direction without reversing it.

  Put it right by writing the partner bases opposite their partners.

• Forgetting that the pairs are held by hydrogen bonds, not covalent bonds.

  Put it right by naming hydrogen bonds between the bases.

Exam tips

• When asked for a complementary sequence, write each base directly opposite its partner to avoid errors.
• Use the full names and letters together, for example cytosine (C), to show precise knowledge.
• Check that every C has a G and every T has an A before finishing your answer.