Under normal circumstances, it is not possible for adenine to pair up with guanine or cytosine, nor for any other mismatches to occur. describe the two factors that prevent a mismatch from occurring

The chemical structure of adenine only allows it to form a hydrogen bond with thymine. While the structure of guanine allows it to form a hydrogen bond with cytosine. DNA Polymerase will then proofread a strand of DNA to ensure there is no mismatching bases.