A point mutation where one nitrogenous base is changed to another

• A. Substitution mutation
• B. Point mutation
• C. Insertion mutation
• D. Deletion mutation

Answer: (A)

Changing one nitrogenous base to a different one is a substitution mutation. This is a single-nucleotide change, a specific type of point mutation where the existing base is replaced with another without adding or removing any nucleotides. Insertion would add a base, and deletion would remove a base, so they don’t fit the description. Because only one base is altered, the rest of the sequence stays the same, and the effect on the organism can vary from no change (a silent mutation) to a different amino acid in a protein, depending on where it occurs and how the codons read.