Erwin Chargaff observed that for double-stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equal to one . This fundamental discovery, known as Chargaff's rule, establishes that the molar amount of Adenine (A) is always equal to Thymine (T), and the molar amount of Guanine (G) is always equal to Cytosine (C) . This principle was critical in assisting Watson and Crick to propose the double-helix model of DNA based on base-pairing complementarity . Mastering such foundational molecular concepts is a high-priority strategy, as NCERT textbooks serve as the primary source for approximately 95% of NEET Biology questions .