The genetic code and mutations

Genes and the code

A gene is a sequence of bases on a DNA molecule that codes for the sequence of amino acids in a polypeptide.

The genetic code is:

• Triplet: three bases (a codon) code for one amino acid. 4 bases give 4³ = 64 possible triplets.
• Non-overlapping: each base is part of only one triplet.
• Degenerate: most amino acids are coded for by more than one triplet (64 triplets, 20 amino acids).

There are start and stop codons that mark where translation begins and ends.

Types of mutation

Errors in DNA replication change the base sequence:

• Substitution: one base replaced by another. Because the code is degenerate, it may code for the same amino acid (no effect), a different amino acid, or a stop codon.
• Insertion or deletion of a base: causes a frameshift, changing every triplet after the mutation, so many amino acids change.

Effects

A changed amino acid sequence may change the protein's tertiary structure and stop it working. Mutations may have no effect, cause a genetic disorder (such as cystic fibrosis) or, in genes controlling cell division, contribute to cancer. Occasionally a mutation is beneficial.

Worked example

A coding sequence of 450 bases (no stop codon inside it) is read. How many amino acids does it code for?

1. The code is triplet and non-overlapping.
2. 450 / 3 = 150

Answer: 150 amino acids.

Check yourself

1. A section of gene with 360 bases codes for amino acids only (no stop codon). How many amino acids does it code for?
   Show answer

   120. 360 / 3 = 120.

2. How many different triplet codes are possible with four bases?
   Show answer

   64. 4 x 4 x 4 = 64.

3. What does it mean that the genetic code is degenerate?
   • Each base is read in more than one triplet
   • The code differs between organisms
   • Each triplet codes for several amino acids
   • Most amino acids are coded for by more than one triplet
   Show answer

   Most amino acids are coded for by more than one triplet. 64 triplets code for only 20 amino acids.

4. What does non-overlapping mean?
   • Each base is part of only one triplet
   • Each amino acid has one triplet
   • Genes never share chromosomes
   • Triplets are separated by gaps
   Show answer

   Each base is part of only one triplet. Triplets are read one after another without sharing bases.

5. Why does a deletion usually have a bigger effect than a substitution?
   • It changes only one amino acid
   • It never changes the protein
   • It shifts the reading frame, changing every triplet after it
   • It always produces a start codon
   Show answer

   It shifts the reading frame, changing every triplet after it. A frameshift alters all downstream codons.

6. A substitution changes a codon but the same amino acid is coded. Why?
   • The genetic code is degenerate
   • The code is non-overlapping
   • The gene is deleted
   • The codon is a stop codon
   Show answer

   The genetic code is degenerate. Several triplets code for the same amino acid.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.