Question

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. \( (G + T)/(A + C) \)

Hint:

Chargaff's rule states that in double-stranded DNA, the number of adenines equals the number of thymines, and the number of guanines equals the number of cytosines.

Answer 1

The Correct Option is B, D

Step 1: Understanding base pairing in DNA. 
In double-stranded DNA, the base pairing rule (Chargaff's rule) states that the amount of adenine (A) is equal to thymine (T), and the amount of guanine (G) is equal to cytosine (C). This means: \[ A = T \,\, \text{and} \,\, G = C \]

Step 2: Analyzing the options. 
(A) \( (A + T)/(G + C) \): This ratio does not necessarily equal 1.
(B) \( (A + G)/(T + C) \): This ratio is always equal to 1, as \( A = T \) and \( G = C \), thus \( A + G = T + C \).
(C) A/G: This ratio does not necessarily equal 1, as the ratio of A to G can vary in different DNA sequences. 
(D) \( (G + T)/(A + C) \): This ratio is always equal to 1, as \( A = T \) and \( C = G \), thus \( A + C= T + G \).
 

Step 3: Conclusion. 
The correct answer is 

(B) \( (A + G)/(T + C) \)
(D) \( (G + T)/(A + C) \)