Question

A gene coding for a particular protein exhibits 2% DNA sequence divergence between humans and chimpanzees. However, protein sequences encoded by them are identical. Which one of the following processes explains this?

• A. Nonsynonymous changes in the gene sequences
• B. Synonymous changes in the gene sequences
• C. Nonsense mutations in the gene sequences
• D. Frameshift mutations in the gene sequences

Hint:

Synonymous mutations do not affect the protein product because they code for the same amino acid, even though the DNA sequence may change.

Answer 1

The Correct Option is B

Step 1: Understanding synonymous mutations.
Synonymous mutations are mutations in the DNA sequence that do not lead to a change in the amino acid sequence of the resulting protein. This happens because of the redundancy of the genetic code — multiple codons can code for the same amino acid. For example, both the codons UUU and UUC code for phenylalanine (Phe), so a mutation that changes one codon to another codon for the same amino acid will not change the protein sequence.

Step 2: Exploring the relationship between DNA and protein sequences.
In this case, the 2% divergence in the DNA sequence between humans and chimpanzees means that there are small differences in the gene sequences. However, since the protein sequences are identical, this suggests that the changes in the DNA sequence are synonymous. These mutations do not affect the final protein product because they still encode the same amino acids.

Step 3: Evaluating the options.
- (A) Nonsynonymous changes: Nonsynonymous mutations lead to a change in the amino acid sequence, which would result in a different protein. This is not the case here because the protein sequences are identical.
- (B) Synonymous changes: Correct, these mutations do not affect the protein sequence despite changes in the DNA sequence.
- (C) Nonsense mutations: These mutations introduce a stop codon in the sequence, which would truncate the protein. This does not apply because the protein sequences are the same.
- (D) Frameshift mutations: Frameshift mutations result in a change in the reading frame of the gene and would lead to a different protein sequence. This is not the case here because the protein sequences are identical.

Step 4: Conclusion.
The correct explanation for why the protein sequences are identical despite 2% DNA sequence divergence is that the mutations are synonymous, meaning they do not alter the protein sequence. Therefore, the correct answer is (B).