Question

In a DNA molecule cytosine is 18%. Percentage of adenine would be------
a. 32%
b. 64%
c. 36%
d. 18%

Answer 1

Hint: Chargaff’s rule of DNA base pairing is the technique used to determine the percentage of base pairs in and sequence in the DNA molecules, which was discovered by scientists of columbia University.

Complete answer:
The Chargaff’s rule of DNA base pairing states that if we can detect the DNA sequence on one strand of DNA, we can easily access the nucleotide base pairs of complementary strands.
The nucleotides of DNA molecules consist of purines and pyrimidines.
Purines are Adenine (A) and guanine (G)
Pyrimidines are thymine (T) and cytosine (C)
Adenine always pairs up with thymine, as well as cytosine always pairs up with guanine, where they form the hydrogen bonds in between the complementary nucleotides.
From the Chargaff’s rule, Number of purines should be equal to number of pyrimidines
Which means A + T = C + G
The amount of adenine should be equal to the amount of thymine And at the same time, the amount of cytosine should be equal to the amount of guanine.
In the above question they mentioned that the amount of cytosine is 18%, which means from the above mentioned statement, the guanine percentage is the same as cytosine, it means guanine percentage is also 18% .
If we consider the total percentage of A + T = C + G is 100
Then percentage of mentioned cytosine and equal amount of guanine percentage should be subtracted from 100
100- 18- 18 = 64%
64% is both the amount of adenine and thymine so it should be halved, which means 64/2 = 32
So from the above explanation, the percentage of Adenine is 32%

Note: one should always remember the Chargaff’s rule, and at the same time one should remember, the percentage of purines is equal to the percentage of pyrimidines, so if we know at least one nucleotides base percentage, one can easily calculate the other.