Assertion (A): In a DNA double helix, a G–C base pair is held together by more hydrogen bonds than an A–T base pair.
Reason (R): In DNA, adenine forms three hydrogen bonds with thymine, whereas guanine forms two hydrogen bonds with cytosine.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) is true but (R) is false.
In the DNA double helix the bases of the two strands are held together by hydrogen bonds. The number of these bonds differs between the two kinds of base pair.
Assertion (A): In a DNA double helix, a G–C base pair is held together by more hydrogen bonds than an A–T base pair. — True. Guanine pairs with cytosine through three hydrogen bonds, while adenine pairs with thymine through two.
Reason (R): In DNA, adenine forms three hydrogen bonds with thymine, whereas guanine forms two hydrogen bonds with cytosine. — False. The numbers are the wrong way round. Adenine forms two hydrogen bonds with thymine, and guanine forms three with cytosine.
So A is true but R is false.
NCERT Extract:
Structure of Polynucleotide Chain – “… (iii) The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds. As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix (Figure 5.2).”