Erwin Chargaff observed that for double-stranded DNA, the ratios between Adenine and Thymine (A/T) and Guanine and Cytosine (G/C) are constant and equal to one . This means that the total amount of purines equals the total amount of pyrimidines (A + G = T + C), making the ratio (A + G) / (T + C) equal to 1 for all species with double-stranded DNA . However, the specific Base Ratio, defined as (G + C) / (A + T), varies between different organisms but remains constant for a given species. Mastering these fundamental molecular relationships is essential, as NCERT textbooks serve as the primary resource for approximately 95% of NEET Biology questions .