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?

Verified step by step guidance

1. Understand the definitions: A missense mutation is a nucleotide change that results in a different amino acid being incorporated into the protein sequence, potentially altering the protein's function.
2. Recognize that a synonymous mutation (also called a silent mutation) is a nucleotide change that does not alter the amino acid sequence due to the redundancy of the genetic code.
3. Recall that the genetic code is degenerate, meaning multiple codons can code for the same amino acid, which is why some nucleotide changes do not affect the protein sequence.
4. Therefore, identify that missense mutations cause amino acid changes because they alter the codon to specify a different amino acid.
5. Conclude that synonymous mutations do not cause amino acid changes since the altered codon still codes for the same amino acid.

Key Concepts

Missense Mutations

Missense mutations are nucleotide changes in a gene that result in the substitution of one amino acid for another in the protein product. This can affect the protein's structure and function, potentially altering the organism's phenotype. They are important in studying genetic variation and disease.

Synonymous (Silent) Mutations

Synonymous mutations are nucleotide changes that do not alter the amino acid sequence of the protein due to the redundancy of the genetic code. Although they do not change the protein directly, they can sometimes affect gene expression or mRNA stability.

Genetic Code and Protein Translation

The genetic code translates nucleotide triplets (codons) into amino acids during protein synthesis. Because multiple codons can code for the same amino acid, some nucleotide changes do not affect the protein sequence (synonymous), while others do (missense). Understanding this code is essential to interpreting mutation effects.