The DNA
Read the following passage carefully and answer the given questions.
DNA is a long polymer of deoxyribonucleotides. The length of DNA is usually defined as number of nucleotides present in it. A nucleotide is the combination of three different components, such as a nitrogenous base, a pentose sugar (ribose in the case of RNA, and deoxyribose for DNA), and a phosphate group. There are two types of nitrogenous bases – purines and pyrimidines. 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. James Watson and Francis Crick, in 1953, proposed a very simple but famous Double Helix model for the structure of DNA based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin. One of the hallmarks of their proposition was base pairing between the two strands of polynucleotide chains. However, this proposition was also based on the observation of Erwin Chargaff that for a double stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equals one.
How many hydrogen bonds are present between adenine and thymine in the DNA?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 2
The passage says that the double helix model is based on base pairing between the two strands. The two strands are held together by hydrogen bonds between the bases. A purine always pairs with a pyrimidine: adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three.
(1) 1 — Incorrect. A single hydrogen bond does not join any base pair in DNA.
(2) 3 — Incorrect. Three hydrogen bonds join guanine and cytosine, not adenine and thymine.
(3) 2 — Correct. A=T is held by two hydrogen bonds.
(4) 0 — Incorrect. Without hydrogen bonds the two strands would not stay paired.
Note 1: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
Note 2: A similar question was also asked in CUET (UG) 2026 on 22-05-2026 (Slot 1).
NCERT Extract:
Structure of Polynucleotide Chain – …
