Price et al. [(1999). J. Bacteriol. 181:2358–2362] conducted a genetic study of the toxin transport protein (PA) of Bacillus anthracis, the bacterium that causes anthrax in humans. Within the 2294-nucleotide gene in 26 strains they identified five point mutations—two missense and three synonyms—among different isolates. Necropsy samples from an anthrax outbreak in 1979 revealed a novel missense mutation and five unique nucleotide changes among ten victims. The authors concluded that these data indicate little or no horizontal transfer between different B. anthracis strains.

Which types of nucleotide changes (missense or synonyms) cause amino acid changes?

Key Concepts

Nucleotide Changes

Nucleotide changes refer to alterations in the DNA sequence of a gene. These changes can be classified into different types, including point mutations, which involve the substitution of a single nucleotide. The two main types of point mutations are missense mutations, which result in a different amino acid being incorporated into a protein, and synonymous mutations, which do not change the amino acid sequence despite a change in the nucleotide sequence.

Missense Mutations

Missense mutations are a specific type of nucleotide change where a single nucleotide substitution results in the coding of a different amino acid in the protein. This can affect the protein's structure and function, potentially leading to altered biological activity or disease. For example, a missense mutation in the toxin transport protein of Bacillus anthracis could impact its ability to transport toxins effectively.

Synonymous Mutations

Synonymous mutations are nucleotide changes that do not alter the amino acid sequence of a protein. Although the nucleotide sequence is modified, the resulting codon still codes for the same amino acid due to the redundancy in the genetic code. These mutations are often considered neutral, as they typically do not affect the protein's function, but they can still play a role in gene regulation and expression.