Answer;
-Because the chemical groups that form hydrogen bonds are in the wrong positions.
Step-by-step explanation;
-A base pair is any of the pairs of nucleotides connecting the complementary strands of a molecule of DNA or RNA and consisting of a purine linked to a pyrimidine by hydrogen bonds.
-The base pairs are adenine-thymine and guanine-cytosine in DNA, and adenine-uracil and guanine-cytosine in RNA or in hybrid DNA-RNA pairing. A mutation in the sequence of DNA that makes an adenine to be paired with a cytosine results to an unstable interaction because the chemical groups that form hydrogen bonds are in the wrong positions.