Question

While analysing the DNA of an organism a total number of 5386 nucleotides were found, out of which the proportion of different bases were: Adenine = $29\%$, Guanine = $17\%$, Cytosine = $32\%$, Thymine = $17\%$. Considering the Chargaff’s rule it can be concluded that:

• A. No conclusion can be drawn
• B. It is single stranded DNA
• C. It is a double stranded linear DNA
• D. It is a double stranded circular DNA

Answer 1

The correct option is B It is single stranded DNA

Chargaff’s rule :

According to the Chargaff rule, the ratio of purines and pyrimidines remains constant and equal in DNA, that is $A+G=T+C$.


In addition to that, the percentage of adenine (A) is equal to the percentage of thymine (T) and the percentage of guanine (G) is equal to the percentage of cytosine (C) in double stranded DNA, that is A = T; G = C.



Double stranded, circular or linear DNA:

According to the Chargaff’s rule, $A+G=T+C$. In the above question it is given that, A = $29\%$, G = $17\%$, C = $32\%$, T = $17\%$.

$29\%+17\%$ is not equal to $32\%+17\%$, that is $46\%$ is not equal to $49\%$.

In addition to this Adenine is not equal to Thymine, similarly, Guanine is not equal to Cytosine. Here $A\ne T\text{and}G\ne C$.

Therefore, it is not a double stranded circular DNA and double stranded linear DNA.


Single stranded DNA :

The Chargaff’s rule is not applicable to single stranded DNA. It has a different proportion of purines and pyrimidines. Thus, the given is a single stranded DNA.

Final answer b. It is single stranded DNA