What are Chargaff's rules?

• A. Amount Of Adenine Equals Thymine, And Amount Of Cytosine Equals Guanine; Base Composition Varies Between Species
• B. Adenine Always Pairs With Cytosine
• C. All Four Bases Are Present In Equal Proportions
• D. Purines Pair With Purines and Pyrimidines Pair With Pyrimidines

Answer: (A)

Chargaff's rules describe how DNA base composition works. In any given organism, the amount of adenine equals thymine, and the amount of cytosine equals guanine. This happens because A pairs with T and C pairs with G in the DNA double helix, so their quantities balance within the molecule. At the same time, different species can have different overall proportions of these bases, so the total A+T can differ from C+G across species.

The other statements are not correct for these reasons: adenine does not pair with cytosine, so that pairing would not yield equal A and T or C and G; all four bases are not necessarily present in equal amounts in a given organism; and purines (A and G) do not pair with purines—purines pair with pyrimidines (A pairs with T, G with C).