Question

A short length of a DNA molecule contains 120 adenine and 120 cytosine bases. The total number of nucleotides in this DNA fragment is
A.480
B.120
C.60
D.240

Answer 1

Chargaff in the year 1950 formulated some rules about DNA structure. These are called Chargaff's rules. According to this rule, the purines and pyrimidines have an equal amount and the amount of adenine is equal to that of thymine, and the amount of guanine is equal to cytosine.

Complete step by step answer: In 1950, Erwin Chargaff formulated an important generalization about DNA structure. These generalizations are called Chargaff's rule. These rules are:
The purines and pyrimidines are always in equal amount i.e. $A + G = T + C$.
The amount of adenine is always equal to that of thymine, and the amount of guanine is always equal to that of cytosine, i.e. $A = T$ and $G = C$.
On the basis of the above rule, in the question, it is given that the DNA molecule contains 120 adenine. Since $A = T$ therefore thymine is also 120. Similarly, it is given that cytosine is 120 therefore guanine is also 120 as $G = C$.
DNA is made up of sugar, nitrogenous bases, phosphate moiety. The total amount of nucleotides in the DNA chain is represented by the total number of nitrogenous bases.
The total number of nitrogenous bases is $A + T + G + C(120 + 120 + 120 + 120) = 480$.
Thus, the correct answer to the above question is option A i.e. 480.

Note: According to Chargaff $A + G = T + C$ and $A = T$ $G = C$. On the basis of these two rules, it is simply that in a DNA chain if the adenine number is 120 then the amount of thymine is also the same. Similarly, if the cytosine number is 120 then guanine is also equal. Adding all the nitrogenous bases the total becomes 480.