Question

A DNA fragment of $204\,nm$ was separated by a scientist from the nuclear genome of the plant. What will be the ratio between the number of two and three hydrogen bonds formed in the components of this fragment if it contains $40\%$ adenine?
(A) $2:3$
(B) $1:3$
(C) $4:1$
(D) $1:4$

Answer 1

Calculate the total number of base pairs from the known base pair relations. With the help of the percentage of adenine, calculate the number of base pairs of adenine and thymine. With the help of that, calculate the number of base pairs of guanines and the cytidine. Then take the ratio between the two and the three hydrogen bonds formed.

Complete answer:
The given data from the question are
The length of the DNA fragment, $L = 204\,nm$
The percentage of adenine present in the DNA fragment, $A = 40\%$
It is known that the single turn of the DNA double helix is $3.4\,nm$ and this single turn has$10\,bp$. By substituting this value for the $204\,nm$ length of the DNA fragment is calculated as follows.
Total base pairs= $\dfrac{{3.4}}{{10}} \times 204$
Total base pairs= $600\,bp$

The Chargaff rule states that in all the cellular DNA, the number of the adenine is equal to the number of the thymine and the number of the guanine is equal to the number of the cytidine. Thus A=T and G=C. Thus from the question, it is given that $A = 40\%$ , so $T = 40\%$. The number of adenine residues are calculated as follows.
Thus number of the adenine residues=$\dfrac{{40}}{{100}} \times 600$
Thus, number of the adenine residues= number of Thymine residues= $240$
In order to find the number of guanine and the cytidine base pairs, subtract the value of the adenine and the thymine numbers from the total base pairs.
Number of the base pairs of the both guanine and the cytidine base pairs= $600 - \left( {240 + 240} \right) = 120$
Number of base pairs of guanine=$\dfrac{{120}}{2} = 60\,bp$
Number of base pairs of cytidine= $60\,bp$
The number of double hydrogen bonds that the adenine forms with the thymine=$240$
The number of triple hydrogen bonds that the guanine forms with the cytidine= $60$
The ratio between the bonds is $240:60 = 4:1$.

Hence the correct answer is OPTION(C)

Note: Remember that the number of base pairs of the adenine is equal to that of the thymine and the number of base pairs of guanines is similar to that of cytidine. Then the double bond is formed between adenine and the thymine and the triple bond is formed between the quinine and cytidine.