Answer;
Formation of an adenine-uracil base pair
Step-by-step explanation;
-Mutations are sudden spontaneous changes that occur randomly in the genome or genetic make up of an organism. These changes occurs in our DNA sequence, either due to mistakes when the DNA is copied or as the result of environmental factors such as UV light and cigarette smoke.
-Mutations range in size; they can affect anywhere from a single DNA building block (base pair) to a large segment of a chromosome that includes multiple genes.
-Mutations happen for several reasons which includes; failure of DNA to copy accurately, external influences or factors such as specific chemicals and radiations.