Understanding DNA Divergence vs. Protein Identity

The question highlights a scenario where the DNA sequences of a specific gene differ by 2% between humans and chimpanzees, yet the resulting proteins are identical. This implies that the DNA changes occurred but did not alter the final amino acid sequence.

Analyzing Mutation Types

Let's examine how different types of genetic mutations affect protein sequences:

• Nonsynonymous changes: These mutations alter the DNA sequence in a way that changes one or more amino acids in the protein. This contradicts the observation of identical protein sequences.
• Synonymous changes: These mutations alter the DNA sequence (specifically, the codon) but do not change the resulting amino acid. This is possible due to the degeneracy of the genetic code, where multiple codons can code for the same amino acid. This perfectly explains the observed phenomenon.
• Nonsense mutations: These introduce a premature stop codon, leading to a truncated and usually non-functional protein. This is inconsistent with identical protein sequences.
• Frameshift mutations: These insertions or deletions shift the reading frame of the genetic code, altering every amino acid downstream of the mutation and typically resulting in a non-functional protein. This also contradicts the observation.

Explaining Identical Proteins Despite DNA Differences

The genetic code's degeneracy is key. For example, the amino acid Leucine can be coded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). A 2% DNA divergence might involve changes in these codons (e.g., from CUU to CUC), which are synonymous changes. These changes affect the DNA sequence but do not change the amino acid translated, thus preserving the protein sequence's integrity.

Therefore, synonymous changes in the gene sequences between humans and chimpanzees explain why the DNA differs, but the encoded protein remains identical.