Final answer:
Chargaff's rule states that in DNA, adenine (A) equals thymine (T) and cytosine (C) equals guanine (G). These base-pairing rules are consistent and were fundamental in discovering DNA's double helix structure.
Step-by-step explanation:
Chargaff's rule explains that in a sample of DNA, the amount of adenine (A) is almost always equal to the amount of thymine (T), and the amount of cytosine (C) is almost always equal to the amount of guanine (G). Erwin Chargaff, an Austrian biochemist, discovered this pattern when examining the content of DNA across different species. He noted that these base-pairing rules, known as Chargaff's rules, are consistent: A=T and G=C. The amounts of these nucleotide bases are not found in equal quantities within DNA, varying from species to species, but they remain constant within any given species. Chargaff's observations were crucial in assisting Watson and Crick in their discovery of DNA's double helix structure, where these bases pair through hydrogen bonds — A with T forming two hydrogen bonds, and C with G forming three hydrogen bonds.