Final answer:
It is false that purines can bind to purines; in DNA, a purine (adenine or guanine) always pairs with a pyrimidine (thymine or cytosine) through hydrogen bonds to maintain the structural integrity of the DNA double helix.
Step-by-step explanation:
Purines binding to purines is false. In DNA, the complementary base pairs rule dictates that each purine pairs with a specific pyrimidine; adenine (A) with thymine (T), and guanine (G) with cytosine (C). This base pairing occurs due to hydrogen bonding and is crucial for maintaining the structure of the DNA double helix. The molecule's stability is ensured by a purine pairing with a pyrimidine, which maintains a uniform distance between the DNA strands.
The purines, adenine and guanine, have a double ring structure which is larger compared to the single-ring structure of the pyrimidines, cytosine and thymine. Due to the size and structure of the bases, a purine-purine pairing would disrupt the uniform width necessary for the helical structure of DNA. Therefore, it is critical for the integrity of the DNA molecule that purines only bind to pyrimidines.