Nucleotides and nucleic acids: (f) the nature of the genetic code

The idea

The genetic code is the set of rules by which the base sequence of DNA is translated into the amino acid sequence of a polypeptide. Teach learners that the code is a triplet code: three bases (a codon) specify one amino acid, and because there are four bases, 4³ = 64 possible codons arise. Of these, 61 code for amino acids while three are stop codons that terminate translation; a start codon also signals initiation. The code is degenerate, meaning most amino acids are encoded by more than one codon, yet it is unambiguous because each codon specifies only one amino acid. It is non-overlapping, so each base is read once, and it is universal, since the same codons carry the same meaning in almost all organisms. A concrete method: read a short mRNA sequence in groups of three from the start codon and assign each codon to its amino acid.

What earns the marks

• States that the code is a triplet code in which three bases (a codon) code for one amino acid.
• Explains that 4³ = 64 codons exist, of which 61 code for amino acids and three are stop codons.
• Describes the code as degenerate because most amino acids are coded for by more than one codon.
• Describes the code as unambiguous because each codon specifies only one amino acid.
• States that the code is non-overlapping, with each base read only once.
• States that the code is universal, as the same codons carry the same meaning in almost all organisms.

Common mistakes and how to put them right

• Thinking one base codes for one amino acid.

  Put it right by stating that three bases form a codon coding for one amino acid.

• Confusing degenerate with ambiguous.

  Put it right by explaining that degeneracy means several codons share an amino acid, while each codon still specifies only one amino acid.

• Assuming all 64 codons code for amino acids.

  Put it right by noting that three are stop codons.

• Believing the code overlaps so bases are shared between codons.

  Put it right by stating that each base is read once in a non-overlapping triplet code.

Exam tips

• Learn the named features of the code (triplet, degenerate, unambiguous, non-overlapping, universal) and be ready to match each to its definition.
• Use the calculation 4³ = 64 to justify the number of codons rather than recalling it blindly.
• When a sequence is given, group the bases into threes from the start codon before assigning amino acids.