Q.35 In a double stranded DNA, which of the following ratios is/are always equal to 1?
A, T, G and C denote the number of bases.
(A) (A+T)/(G+C) (B) (A+G)/(T+C) (C) A/G (D) (G+T)/(A+C)

In double-stranded DNA, the ratios (A+G)/(T+C) and (G+T)/(A+C) are always equal to 1 due to Chargaff’s rules of base pairing. These rules ensure complementary pairing across strands. Option (B) and (D) are correct.

Chargaff’s Rules

Chargaff’s rules state that in double-stranded DNA, adenine (A) equals thymine (T), and guanine (G) equals cytosine (C), so A = T and G = C. Purines (A + G) also equal pyrimidines (T + C) across strands. These equalities hold universally in dsDNA.​

Option Analysis

• (A) (A+T)/(G+C): This AT/GC ratio varies by species (e.g., 0.3 in some organisms) and equals 1 only in AT-rich DNA.​

• (B) (A+G)/(T+C): Purines (A+G) pair with pyrimidines (T+C), so A + G = T + C = 1.​

• (C) A/G: Adenine-to-guanine ratio varies widely between species and is not fixed at 1.​

• (D) (G+T)/(A+C): Rearranges to G + T = A + C; since G = C and T = A, the ratio equals 1.​

In double stranded DNA, specific base ratios always equal 1 according to Chargaff’s rules, crucial for CSIR NET Life Sciences preparation. A, T, G, and C denote adenine, thymine, guanine, and cytosine counts, where complementary base pairing ensures A = T and G = C across strands.​

Key Ratios Explained

Chargaff’s first rule confirms A = T and G = C in dsDNA. The second rule adds purine (A+G) to pyrimidine (C+T) equality.​