DNA Divergence Explained by Synonymous Substitution

The scenario describes a 5% DNA sequence divergence between two species, yet the resulting protein sequence remains identical. This observation is best explained by synonymous substitution.

Understanding Synonymous Substitution

• Genetic Code Degeneracy: The genetic code is degenerate, meaning that multiple codons (three-nucleotide sequences) can code for the same amino acid. For example, leucine is coded by codons CUU, CUC, CUA, CUG, UUA, and UUG.
• Mutation Effect: A synonymous substitution is a change in the DNA sequence at the nucleotide level that does not alter the resulting amino acid sequence of the protein. This occurs when a mutation changes a codon to another codon that specifies the same amino acid.
• Scenario Fit: In this case, the 5% DNA divergence likely consists of synonymous substitutions. These changes occurred in the DNA but did not change the protein's amino acid sequence, thus preserving its identical structure and function between the two species.

Analyzing Other Mutation Types

• Nonsense Mutation: Introduces a premature stop codon, leading to a truncated and usually non-functional protein. This contradicts the identical protein sequence.
• Non-synonymous Substitution: Changes a codon to one that codes for a *different* amino acid, altering the protein sequence. This is inconsistent with the observed identical protein.
• Frame-shift Mutation: Involves insertions or deletions (not in multiples of three) that shift the reading frame, altering all downstream amino acids and typically resulting in a non-functional protein. This also contradicts the identical protein sequence.

Therefore, synonymous substitution is the mutation type that allows for DNA sequence divergence without affecting the protein sequence.