How would the deoxyribose sugar phosphate back bone of nucleotide chains look if purines paired only with and pyrimidines pyrimidines?

The deoxyribose sugar phosphate backbone of nucleotide chains would look quite unusual if purines paired only with pyrimidines, and pyrimidines only with pyrimidines. This is because purines and pyrimidines are different in size. Purines have a double-ring structure, while pyrimidines have a single-ring structure. This difference in size would cause the nucleotide chains to be uneven, with the purines sticking out further from the backbone than the pyrimidines. This would make it difficult for the nucleotide chains to form the regular A-form or B-form double helix structures.

In addition, the base-pairing rules would be different if purines paired only with pyrimidines. Instead of the Watson-Crick base pairs, which are A-T and G-C, we would have A-C and G-T pairs. These pairs would not be as stable as the Watson-Crick pairs, and they would be more likely to break apart. This would make the DNA less stable and more prone to mutations.

Overall, the deoxyribose sugar phosphate backbone of nucleotide chains would look quite different if purines paired only with pyrimidines, and pyrimidines only with pyrimidines. The nucleotide chains would be uneven, the base-pairing rules would be different, and the DNA would be less stable.