Read the passage given below and answer the following 5 questions.
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two types of nucleic acids found in living systems. DNA acts as the genetic material in most of the organisms. RNA though it also acts as a genetic material in some viruses, mostly functions as a messenger. RNA has additional roles as well. It functions as adapter, structural, and in some cases as a catalytic molecule. 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 and Pyrimidines.
Which of the following pairs of nitrogenous bases is present in DNA?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Adenine and Thymine
The nitrogenous bases are of two types. Purines are adenine (A) and guanine (G). Pyrimidines are cytosine (C), thymine (T) and uracil (U). Adenine, guanine and cytosine occur in both DNA and RNA. Thymine is found only in DNA, and uracil takes its place in RNA.
Option (1) Adenine and Thymine — Correct. Adenine (a purine) and thymine (a pyrimidine) are both present in DNA. In the double helix, adenine pairs with thymine through two hydrogen bonds.
Option (2) Guanine and Uracil — Incorrect. Guanine is present in DNA, but uracil is not; uracil is found only in RNA.
Option (3) Adenine and Uracil — Incorrect. Adenine is present in DNA, but uracil is an RNA base. DNA has thymine in its place.
Option (4) Cytosine and Uracil — Incorrect. Cytosine is present in DNA, but uracil is not. Both of these bases are pyrimidines, and uracil belongs to RNA.
NCERT Extract:
Structure of Polynucleotide Chain – “… 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.”