Question

Which one of the following bases is not present in DNA?
A) Cytosine
B) Thymine
C) Quinoline
D) Adenine

Answer 1

Hint: Deoxyribonucleic acid, DNA is composed of two polypeptide chains that coil around each other. Each polypeptide chain is composed of nucleotides and each nucleotide is composed of four nitrogen-containing nucleobases. To answer this question, recall these four nitrogen-containing nucleobases.

Complete step by step answer:
In the question, we are asked which of the bases from the given options is not present in DNA. Let us discuss the bases which are present in DNA.
DNA stands for Deoxyribonucleic acid and is a type of nucleic acid. Nucleic acids are long chain polymers of nucleotides. Nucleotides consist of phosphate groups, nitrogenous bases, and pentose sugar. Thus, DNA is basically composed of nucleotides and each nucleotide in DNA is further composed of nitrogenous bases or nitrogen-containing nucleobases.
The four nitrogenous bases present in DNA with their structure are as follows:

Thus, among the given options, only option C i.e., quinoline is not a nitrogenous base and consequently, not present in DNA.
So, the correct answer is “Option C”.

Additional Information:
The most important clue of the structure of DNA came from the work of Erwin Chargaff. According to Chargaff’s Rule, the number of adenine bases is equal to the number of thymine bases in DNA. But later, a double helical model of DNA came which was postulated by Watson and Crick in 1983. According to Watson and Crick, adenine always pairs with thymine with a double bond and guanine always pairs with cytosine with a triple bond.

Note: A key point to note is that the fifth nitrogenous base which is not present in DNA is uracil (U). But uracil is present in RNA (Ribonucleic acid). One can remember the four bases found in DNA as A, G, C, and T and the bases found in RNA as A, G, C, and U.