Answer:
T and G
Explanation:
The rules of nucleotide pairing, also known as Watson-Crick rules, are A with T and C with G (as you can see in the image I added), a pyrimidine with a purine. Two purines or two pyrimidines affect the DNA structure because the first ones would occupy a larger space and the second ones won't get close enough to bond. T and G can't bond because the do not form hydrogen bond.
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