# Explain how base substitution mutations can have varying effects on the phenotype of an organism.

> IB DP Biology Higher Level (2023 syllabus) — D1.3 Mutations and gene editing (HL only) · Explain · 4 marks

## Mark scheme (4 marks)

1. A base substitution changes one nucleotide base in the DNA sequence / changes one codon in the mRNA
2. 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
3. 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
4. A nonsense mutation produces a premature stop codon, resulting in a truncated / shorter non-functional polypeptide and a more severely altered phenotype

## Key terms

- [base substitution](https://www.gradenine.co.uk/glossary/base-substitution)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-base-substitution-mutations-can-17cef2b9) · Published by Druglandscape Ltd.