Which of the following make up the backbone of the DNA molecule?

The correct answer is Option (3) → Sugar and phosphates

A polynucleotide chain is built by joining nucleotides through 3′-5′ phosphodiester linkages. The sugar of one nucleotide is linked to the phosphate of the next, so the chain has a continuous sugar-phosphate backbone. The nitrogenous bases are attached to the sugar and project from this backbone; in the double helix they point inwards and pair with each other.

Option (1) Sugar and nitrogenous bases — Incorrect. The nitrogenous bases are not part of the backbone. They hang from the sugar and form the base pairs inside the helix.

Option (2) Nitrogenous bases and phosphates — Incorrect. Phosphate is part of the backbone, but the bases are not. The bases are linked to the sugar, never to each other along the chain.

Option (3) Sugar and phosphates — Correct. The backbone is formed by alternating sugar (deoxyribose in DNA) and phosphate groups. This is the first salient feature of the double helix.

NCERT Extract:
Structure of Polynucleotide Chain – “… Similarly, at the other end of the polymer the sugar has a free OH of 3′ C group which is referred to as 3′ - end of the polynucleotide chain. The backbone of a polynucleotide chain is formed due to sugar and phosphates. The nitrogenous bases linked to sugar moiety project from the backbone ….”
Structure of Polynucleotide Chain – “… The salient features of the Double-helix structure of DNA are as follows: (i) It is made of two polynucleotide chains, where the backbone is constituted by sugar-phosphate, and the bases project inside.”