Which of the following statements are incorrect with respect to nucleotides?
(A) Purines and pyrimidines are nitrogenous bases.
(B) Nucleotides are non-enzymatic molecules.
(C) Phosphate group is linked to –OH of 5′ C of a nucleoside through phosphoester linkage.
(D) In RNA, every nucleotide residue has an additional –OH group present at 2′ position in the ribose.
(E) Thymine is an example of Pyrimidine.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → None of these
(A) Purines and pyrimidines are nitrogenous bases. True.
(B) Nucleotides are non-enzymatic molecules. True. A nucleotide is a monomer, not an enzyme. It is a small unit that joins with others to form a nucleic acid. Some RNA chains act as enzymes (ribozymes), but a single nucleotide does not.
(C) Phosphate group is linked to –OH of 5′ C of a nucleoside through phosphoester linkage. True. The phosphodiester (3′–5′) linkage is a different bond: it joins two nucleotides into a dinucleotide.
(D) In RNA, every nucleotide residue has an additional –OH group present at 2′ position in the ribose. True. Ribose carries an OH at its 2′ carbon, which deoxyribose lacks. This 2′-OH makes RNA reactive and labile.
(E) Thymine is an example of Pyrimidine. True.
All five statements are correct, so none of them is incorrect.
Note: NTA dropped this question. The original NTA paper printed Option (3) as “(B) and (D) only”. All five statements are correct as per NCERT, so no printed option was right. Option (3) has been changed to “None of these” so that the question has one clear answer.
NCERT Extract:
Structure of Polynucleotide Chain – “Let us recapitulate the chemical structure of a polynucleotide chain (DNA or RNA). A nucleotide has three components – a nitrogenous base, a pentose sugar (ribose in case of RNA, and deoxyribose for DNA), and a phosphate group. There are two types of nitrogenous bases – Purines (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine). Cytosine is common for both DNA and RNA and Thymine is present in DNA. Uracil is present in RNA at the place of Thymine. A nitrogenous base is linked to the OH of 1' C pentose sugar through a N-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine and uridine or deoxythymidine. When a phosphate group is linked to OH of 5' C of a nucleoside through phosphoester linkage, a corresponding nucleotide (or deoxynucleotide depending upon the type of sugar present) is formed. Two nucleotides are linked through 3'-5' phosphodiester linkage to form a dinucleotide. … In RNA, every nucleotide residue has an additional –OH group present at 2′-position in the ribose.”