A sequence of three bases is the code for a particular amino acid.

The idea

The genetic code is a triplet code: each group of three consecutive bases along a DNA strand specifies one amino acid. During protein synthesis, the DNA template strand is transcribed into a complementary mRNA sequence. For example, the DNA template triplet TAC is transcribed into the mRNA codon AUG, which codes for the amino acid methionine. The ribosome reads each codon in order to assemble the protein. Because there are four bases, there are 4³ = 64 possible triplets, which is more than the 20 amino acids used in proteins, so the code is degenerate: several triplets can code for the same amino acid. Understanding this explains how a gene's base sequence determines the primary structure of a protein, and why a mutation that changes one base can alter a single amino acid and potentially affect protein function.

What earns the marks

• A triplet is three consecutive bases along a DNA strand that together code for one amino acid.
• The sequence of triplets in a gene determines the sequence of amino acids in the polypeptide.
• During protein synthesis, the DNA template triplet is transcribed into a complementary mRNA codon.
• There are 64 possible triplets (4³) but only about 20 amino acids, so the code is degenerate.
• A change in a single base in a triplet can change the amino acid coded for, potentially altering protein structure and function.

Common mistakes and how to put them right

• Thinking that one base codes for one amino acid.

  Put it right: The code is read in groups of three bases, so a single base change may or may not alter the amino acid.

• Confusing the DNA triplet with the mRNA codon.

  Put it right: The triplet is on DNA, while the complementary codon is on mRNA and contains uracil instead of thymine.

• Assuming each amino acid has only one triplet.

  Put it right: Because the code is degenerate, most amino acids are coded for by more than one triplet.

Exam tips

• When asked to explain the genetic code, state clearly that three bases form a triplet and that each triplet codes for one amino acid.
• Use the terms triplet and codon correctly: triplet for DNA, codon for mRNA.
• If given a DNA base sequence, read it in groups of three from the start and use a codon table if provided to identify the amino acids.