Explain how base substitution mutations can have varying effects on the phenotype of an organism.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A base substitution changes one nucleotide in the DNA sequence, altering one codon in the mRNA.
If the new codon still codes for the same amino acid (degeneracy of the genetic code), the mutation is silent/synonymous and the protein remains unchanged, so the phenotype is unaffected.
If the codon now codes for a different amino acid, the mutation is missense. The altered residue can disturb the protein’s tertiary structure or active site, potentially reducing or abolishing its function and changing the phenotype.
If the substitution creates a premature stop codon, it is a nonsense mutation. Translation terminates early, producing a truncated, usually non‑functional polypeptide, leading to a more severe phenotypic change.
If the new codon still codes for the same amino acid (degeneracy of the genetic code), the mutation is silent/synonymous and the protein remains unchanged, so the phenotype is unaffected.
If the codon now codes for a different amino acid, the mutation is missense. The altered residue can disturb the protein’s tertiary structure or active site, potentially reducing or abolishing its function and changing the phenotype.
If the substitution creates a premature stop codon, it is a nonsense mutation. Translation terminates early, producing a truncated, usually non‑functional polypeptide, leading to a more severe phenotypic change.
Examiner tips
- Show the four steps in order: change in DNA → codon change → silent/missense/nonsense → phenotypic outcome.
- Use the exact terms ‘silent’, ‘missense’, ‘nonsense’ and explain the effect on protein structure or function.
- Mention the genetic code’s degeneracy for silent mutations.
- Link the protein change to the phenotype clearly.
Common mistakes
- Confusing silent with missense mutations; forgetting to mention the genetic code’s redundancy.
- Not explaining how a missense mutation can alter tertiary structure or active sites.
- Failing to state that nonsense mutations produce truncated proteins and usually abolish function.
Mark scheme (4 marks)
- A base substitution changes one nucleotide base in the DNA sequence / changes one codon in the mRNA
- A silent/synonymous mutation occurs when the altered codon still codes for the same amino acid due to the degeneracy/redundancy of the genetic code, so the protein is unchanged and phenotype is unaffected
- A missense mutation changes one amino acid in the polypeptide, which may alter the tertiary structure / active site of the protein, potentially changing or abolishing its function, affecting phenotype
- A nonsense mutation produces a premature stop codon, resulting in a truncated / shorter non-functional polypeptide and a more severely altered phenotype
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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