If the total amount of adenine and thymine in a double-stranded DNA is 45%, the amount of guanine in this DNA will be
A. 22.5%
B. 27.5%
C. 45%
D. 55%

Hint: DNA is a polymer of nucleotides which consists of Sugar, phosphate, and nitrogenous bases. These bases are present inside the DNA in a regularly arranged fashion where Adenine and thymine form two hydrogen bonds while cytosine and guanine form 3 hydrogen bonds.

Complete answer: DNA is a double helical structure in which the two strands are arranged in an antiparallel fashion. The nitrogenous bases form the ladder of the DNA while the backbone is formed by the sugar and phosphate group.
These bases are joined together using hydrogen bonding. Adenine forms 2 hydrogen bonds with
Thymine and Cytosine form three hydrogen bonds with Guanine.
Adenine and guanine are together known as purines while cytosine and thymine are together known as pyrimidines. According to Erwin Chargaff’s rule, the percentage of two purine and two pyrimidine in a DNA is equal to one another or in the ratio of 1:1.
This means that- % A = % T and % G = % C
It is given that the total amount of thymine and adenine is = 45%, then the total amount of Guanine and cytosine will be equal to 100% - 45% = 55%.
Also, % G = % C, Therefore- %G =% C = 55 %/2 = 27.5 %.
So, the percentage of Guanine in DNA will be equal to 27.5 %.

Hence, the correct option is (B), 27.5%.

Note: Ergin Chargaff gave the above-mentioned rule at that time he was not aware of the reason for the same, but as we now know that this rule is followed so that the structure of DNA molecules is stable and regularly arranged nitrogen bases may be present.