A single base substitution is LEAST likely to be deleterious (dangerous) when the change results in _____.

• A. Replacement of a codon specifying a hydrophilic amino acid with a codon that specifies a hydrophobic amino acid.
• B. Replacement of a codon which codes for an amino acid with a stop codon.
• C. The change of a codon specifying a specific amino acid important for the active site of the protein.
• D. Replacement of a codon specifying an amino acid with a redundant codon specifying the same amino acid.

Answer: (D)

Verified step by step guidance

1. Understand the concept of a single base substitution, which is a type of point mutation where one nucleotide base is replaced by another in the DNA sequence.
2. Recognize that the genetic code is redundant, meaning that multiple codons can code for the same amino acid. This is known as codon redundancy or degeneracy.
3. Consider the impact of a base substitution on protein function. If the substitution results in a codon that still codes for the same amino acid, the protein's structure and function are likely to remain unchanged. This is called a synonymous or silent mutation.
4. Evaluate the other options: Changing a codon to one that specifies a different amino acid (especially if it alters the protein's active site) or a stop codon can significantly affect protein function, potentially leading to deleterious effects.
5. Conclude that a base substitution is least likely to be deleterious when it results in a synonymous mutation, where the new codon specifies the same amino acid as the original codon, thus maintaining the protein's normal function.