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What are the properties of the genetic code, and how do they shape the relationship between DNA and proteins?

Describe the genetic code and explain its key properties: it is a triplet code, degenerate, non-overlapping and near universal

A focused answer to the H2 Biology Molecular Genetics outcome on the genetic code. Codons and the triplet code, degeneracy, the non-overlapping and near-universal properties, start and stop codons, and why these properties matter for mutation and gene transfer.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
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What this dot point is asking

SEAB wants you to describe how base sequences specify amino acids and to explain the defining properties of the genetic code: it is a triplet code, it is degenerate, it is non-overlapping and it is near universal. You should also know about start and stop codons. These properties explain why mutations have varied effects and why genetic engineering across species is possible.

The answer

Codons and the triplet code

The sequence of bases in a gene is read in groups of three called codons. Each codon specifies one amino acid (or a signal to start or stop). Three bases are needed because there are four bases; reading them singly gives only four meanings and in pairs only sixteen, neither enough for the twenty amino acids, whereas triplets give sixty-four combinations.

The properties of the code

  • Triplet. Each amino acid is coded by three bases.
  • Degenerate. Most amino acids are coded by more than one codon, with synonymous codons usually differing in the third base.
  • Non-overlapping. Each base belongs to only one codon; the code is read in consecutive, separate triplets.
  • Near universal. Almost every organism uses the same codons for the same amino acids.

Start and stop codons

Translation begins at a start codon (AUG, which also codes for methionine) and ends at one of three stop codons, which code for no amino acid and signal the ribosome to release the finished polypeptide.

Examples in context

Example 1. Silent mutations. Because the code is degenerate, a substitution in the third base of a codon frequently produces a synonymous codon, so the amino acid and the protein are unchanged. Degeneracy therefore buffers the organism against many point mutations.

Example 2. Frameshift from a base insertion. Because the code is read in fixed triplets from a start point, inserting or deleting one base shifts the reading frame for every codon downstream, usually scrambling the protein entirely. The non-overlapping, triplet nature of the code is what makes such frameshift mutations so damaging.

Try this

Q1. State what is meant by a triplet code. [1 mark]

  • Cue. Each amino acid is specified by a sequence of three bases (a codon).

Q2. Explain why the genetic code being near universal is useful in biotechnology. [2 marks]

  • Cue. The same codons mean the same amino acids in different species, so a gene from one organism can be inserted into and correctly translated by another, as when human genes are expressed in bacteria.

Q3. Explain why a frameshift mutation usually has a greater effect than a single base substitution. [2 marks]

  • Cue. A frameshift (insertion or deletion) shifts the reading frame so every codon after it is misread, whereas a substitution changes at most one codon.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksState four properties of the genetic code and briefly explain what each property means.
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Examiners want four named properties, each with a short explanation.

Triplet: each amino acid is coded by a sequence of three bases (a codon). Three bases are needed because four bases taken in pairs would give only sixteen combinations, too few for twenty amino acids.

Degenerate: most amino acids are coded by more than one codon. This means a change in the third base of a codon often still codes for the same amino acid.

Non-overlapping: each base is part of only one codon; the code is read in fixed, consecutive triplets without sharing bases between adjacent codons.

Near universal: the same codons specify the same amino acids in almost all organisms, which is evidence of a common ancestry and makes genetic engineering across species possible.

Markers reward four correctly named properties with an accurate one-line explanation of each.

Original3 marksExplain why the degeneracy of the genetic code can reduce the effect of some point mutations.
Show worked answer →

The answer links degeneracy to silent mutations.

Because the code is degenerate, several different codons code for the same amino acid, and these synonymous codons usually differ only in the third base.

If a point mutation (a base substitution) changes the third base of a codon, the new codon may still code for the same amino acid. The resulting protein is unchanged, so the mutation has no effect on the phenotype. This is called a silent mutation.

Markers reward the link between several codons coding for one amino acid, the typical position of the change (third base), and the conclusion that the amino acid sequence and therefore the protein can remain unchanged.

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