Which of the following bonds hold the bases of the two DNA strands together in a double helix?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Hydrogen bond
DNA is a double helix of two polynucleotide strands. Within each strand the nucleotides are joined by covalent phosphodiester bonds, which make the sugar-phosphate backbone. The bases of the two strands face each other and are paired by hydrogen bonds: adenine forms two hydrogen bonds with thymine, and guanine forms three with cytosine. Because a purine always pairs with a pyrimidine, the distance between the two strands stays nearly the same all along the helix. Hydrogen bonds are weak, so the two strands can be separated by heat, as in PCR, and can come together again.
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).”