Sometimes the sequence of dna gets mutated and an adenine is paired to a cytosine. why is this interaction unstable?

Answer;

-Because the chemical groups that form hydrogen bonds are in the wrong positions.

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.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.