Problem

# Which of the following equalities is not true for double-stranded DNA?a. (G + T) = (A + C)...

Which of the following equalities is not true for double-stranded DNA?

a. (G + T) = (A + C)

b. (G + C) = (A + T)

c. (G + A) = (C + T)

#### Step-by-Step Solution

Solution 1
The Chargaff's rule of DNA base pairing states that- Adenine (A) pairs with Thymine (T). Adenine (purine) always pairs with the Thymine (Pyrimidine) - A with T Cytosine (C) pyrimidine always pairs with the guanine (G) purine. This base pairing is consistent with there not being enough space (20 A^{o}) for two purines to fit within the helix. Pyrimidine base pairs have much space to get close enough for forming the hydrogen bond between them. The hydrogen bond formation provides stability to the structure.

Chargaff’s rule states that the ratio of purines and pyrimidines is equal to one. According to these rules, % of A is equal to % of T and % of G is equal to % of C.

a) Based on the Chargaff’s rule into consideration the (G+T) = (A+C) is correct.

Hence, the option ‘a’ is incorrect

c) Based on the Chargaff’s rule into consideration the (G+A) = (C+T) is correct.

Hence, the option ‘c’ is incorrect.

b) Based on the Chargaff’s rule into consideration (G+C) = (A+T) is incorrect. According

to Chargaff’s rule the ratio is A=T;C= T for the stability of the DNA double helix. The

given ratio does not justify the equalities of the DNA double strand.

Hence, the option b is correct.