Question

In a DNA segment having six coils, there are 22 nitrogen base pairs linked by two hydrogen bonds. How many cytosine bases are found in that segment?

• A. 22
• B. 38
• C. 44
• D. 76

Answer 1

Answer: (B)

38

• Since, one complete turn of the double helix has 10 base pairs, 6 coils of DNA will have 10x6= 60 base pairs.

DNA has four bases; two purines and two pyrimidines; purines are namely adenine and guanine and the pyrimidines are cytosine, thymine. Adenine in one DNA strand makes two hydrogen bonds with thymine in other DNA strand and guanine makes three hydrogen bonds with cytosine.

• According to Chargaff’s rule, in all cellular DNAs, regardless of the species, the number of adenosine residues is equal to the number of thymidine residues which means that A = T; and the number of guanosine residues is equal to the number of cytidine residues; G = C.

• According to the question, there are 22 base pairs with two hydrogen bonds which mean that out of total 60 base pairs in that DNA segment, the number of A-T pairs is 22; the number of adenine = 22 and the number of thymine = 22; A+T = 44, which makes options A and C wrong.

Thus, number of G-C pairs = 60-22 = 38; number of guanine = 38 and number of cytosine = 38; option B is correct.