Erwin Chargaff (1950) - Chargaff analyzed the DNA of different species and determined the composition of A, T, C, and G bases in them. He then formulated 'Chargaff's Rule' which states that:
"The ratios between Adenine (A) and Thymine (T) and Guanine (G) and Cytosine (C) are constant and equals to one in a double stranded DNA."
One of the salient features of the Double helix structure of DNA given by Watson and Crick was that the DNA is made of two polynucleotide chains. The backbone of the polynucleotide chain is made up of sugarphosphate, and the nitrogenous bases project inside.
Given below is a graph showing the percentage of Adenine in tow different double stranded DNA.

Study the graph given above and find out the percentage of Guanine in dsDNA1:

Options

A. 0.3
B. 0.2
C. 0.6
D. 0.45

Answer

B

Explanation

According to Chargaff. Purines (Adenine and Guanine) and Pyrimidines (Cytosine and Thýmine) are present in DNA in an equal ratio. Also, Purines and Pyrimidines are complementary in a polynucleotide chain. It means that the number of Adenine molecules should be equal to the number of Thymine molecules and the number of Guanine molecules should be equal to the number of Cytosine molecules.
% (A)=% (T)
% (G)=% (C)
Now, the dsDNA has 40% Adenine. It means that it would also have 40 % Thymine.
The percentage of (A)= percentage of % (T) = 30%
Hence, % of A+ T = 60%.
% of G+C (100-60)%= 40%
Now % (G) =% (C)= `(40)/(2)%`
% (G) =% (C)= 20%
GUANINE = 20%.