Final answer:
Chargaff's rules indicate that one purine pairs with one pyrimidine, meaning adenine pairs with thymine and guanine pairs with cytosine. Therefore, the number of purines on one DNA strand is not equal to the number of purines on the other strand, but rather, the purines pair with the pyrimidines on the complementary strand.
Step-by-step explanation:
According to Chargaff's rules, the statement that the number of purines on one strand of DNA equals the number of purines on the other strand is false. Chargaff's rules actually state that the number of adenine (A) units is approximately equal to the number of thymine (T) units, and the number of guanine (G) units is approximately equal to the number of cytosine (C) units. DNA is composed of two strands that form a double helix structure due to the specific base-pairing of purine and pyrimidine bases, where adenine pairs with thymine and guanine pairs with cytosine.
To expand on this further, since adenine and guanine are purines, and cytosine and thymine are pyrimidines, what Chargaff's rules suggest is that the amount of adenine always equals the amount of thymine and the amount of guanine always equals the amount of cytosine. This means that the number of purines on one strand (adenine and guanine combined) will be equal to the number of pyrimidines on the complementary strand (thymine and cytosine combined), and vice versa, maintaining a balanced proportion of purines and pyrimidines across both strands of the DNA molecule.
Therefore, while the total number of purines equals the total number of pyrimidines in the DNA molecule, it's not accurate to say that the number of purines in one strand equals the number of purines in the complementary strand. Instead, one purine pairs with one pyrimidine between the two strands, adhering to the base-pairing rules that are central to DNA's double helix structure.