Question

If a double-stranded DNA has 20% cytosine. What will be the percentage of adenine in it?
(A) 20%
(B) 40%
(C) 30%
(D) 60%

Answer 1

Hint: In Deoxyribose nucleic acid, the following base pairing is seen,
- Adenine binds with Thymine (double hydrogen bond)
- Cytosine binds with Guanine (triple hydrogen bond)

Complete Answer:
- In the 1950s, Watson and Crick discovered the structure of DNA (deoxyribose nucleic acid). The larger moiety DNA was made up of different components namely, ribose sugar, a phosphate group and nitrogenous bases together called as nucleotides. The nitrogenous bases are adenine (A), guanine (G), thymine (T) and cytosine (C).
- The percentage of the nitrogenous present in the DNA can easily be determined if a simple property of DNA is known. The property is called the complementarity of the nitrogenous bases. In DNA, A binds to its complementary nitrogenous base T and C binds with G.
The given data suggests that the DNA strand has 20% of cytosine.
- If the cytosine percentage is 20 the guanine percentage will be 20 too. This is because for DNA to be stable the same amount of complementary bases have to be present. This makes the total percentage as C+ G = 40%.
- To calculate the (A+T) percentage we have 60% left. So, as the remaining percentage has to be divided equally among two bases. We have 60 divided by 2. We get 30%.

Therefore, the correct answer is (C) 30%.

Note: The nitrogenous bases present in DNA are classified as purines and pyrimidines. Adenine and guanine are purine while cytosine and thymine are pyrimidines. Purines bind with pyrimidines with help of hydrogen bond, for example, adenine binds with thymine.