Final answer:
If two purines or two pyrimidines are paired together in DNA, the structure of DNA would be compromised and the stability of the DNA double helix would be compromised.
Step-by-step explanation:
If two purines or two pyrimidines were mistakenly paired together in DNA in a stable way, the structure of DNA would be compromised. The DNA double helix would be distorted, causing a bulge in the places where the incorrect pairing occurs. This would result in an unstable and potentially nonfunctional DNA molecule. Since purines, such as adenine and guanine, are larger than pyrimidines, such as cytosine and thymine, the incorrect pairing of two purines or two pyrimidines would disrupt the uniform gap between the two DNA strands. As a result, the stability of the DNA double helix would be compromised.