The backbone of a polynucleotide strand is made up of alternating sugar and phosphate groups.

Here's a breakdown:

* Sugar: In DNA, the sugar is deoxyribose, while in RNA, it's ribose.

* Phosphate: This is a negatively charged group that links the sugars together.

These sugar-phosphate units form a continuous chain, which is the backbone of the polynucleotide. The nitrogenous bases (adenine, guanine, cytosine, thymine, and uracil) attach to the sugars and project outward from the backbone.

Think of it like a ladder, where the sugar-phosphate backbone is the sides and the nitrogenous bases are the rungs.