Choose the most appropriate option about the presence of nitrogenous bases; Cytosine, 5-methyl uracil and Uracil.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Cytosine in both; DNA and RNA, 5-Methyl Uracil only in DNA and Uracil only in RNA
5-methyl uracil is another chemical name for thymine. So the question asks where cytosine, thymine and uracil occur. Cytosine is found in both nucleic acids; thymine only in DNA; uracil only in RNA, at the place of thymine.
Option (1) Cytosine only in DNA, 5-Methyl Uracil in both; DNA and RNA, Uracil only in RNA — Incorrect. Cytosine is in both DNA and RNA, and thymine (5-methyl uracil) is only in DNA.
Option (2) Cytosine only in RNA, 5-Methyl Uracil only in DNA, Uracil in both DNA and RNA — Incorrect. Only the thymine part is right. Cytosine is in both, and uracil is found only in RNA.
Option (3) Cytosine in both; DNA and RNA, 5-Methyl Uracil only in DNA and Uracil only in RNA — Correct. Cytosine is found in both DNA and RNA. 5-methyl uracil is another name for thymine, found only in DNA, and uracil is found only in RNA.
Option (4) Cytosine only in RNA 5-Methyl Uracil in both; DNA and RNA, Uracil only in DNA — Incorrect. Every part is wrong: uracil is never in DNA, thymine is not in RNA, and cytosine is in both.
NCERT Extract:
Structure of Polynucleotide Chain – …
