Chargaff's Rule describes the regularity observed in the ratios of nucleotide bases in DNA.

Chargaff's Rule is based on the observation that in a DNA molecule, the amount of adenine (A) equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C). Mathematically, this can be expressed as:

A=T

G=C

In other words, the base pairs adenine-thymine and guanine-cytosine are complementary and occur in equal proportions in a DNA molecule.

This insight was crucial for the development of the double helix model of DNA proposed by James Watson and Francis Crick, with assistance from Rosalind Franklin's X-ray diffraction images.

Chargaff's Rule provides a fundamental understanding of the structure and complementarity of DNA base pairs, which is essential to the stability and replication of the DNA molecule.