Final answer:
In a double-stranded DNA molecule, if one strand has 25% purines and 75% pyrimidines, the other strand must have 25% pyrimidines (T and C) and 75% purines (A and G) due to base-pairing rules where adenine pairs with thymine and guanine with cytosine.
Step-by-step explanation:
In a double-stranded DNA molecule, the two strands are complementary because of the specific base-pairing rules. If one strand has 25% purines, consisting of adenine (A) and guanine (G), and 75% pyrimidines, consisting of cytosine (C) and thymine (T), then the other strand must have the opposite proportion due to the pairing rules: 25% pyrimidines and 75% purines.
This means that the complementary strand would contain 25% adenine and guanine (since these are the purines that pair with the pyrimidines on the opposite strand) and 75% cytosine and thymine (since these are the pyrimidines that would pair with the purines on the opposite strand).
Chargaff's rule is the basis of these ratios, whereby adenine always pairs with thymine, and guanine always pairs with cytosine, maintaining a specific pattern of base pairs across the helix structure of DNA.