Here's why:

* Nucleic acids are polymers made up of repeating units called nucleotides.

* Each nucleotide consists of three components:

* A sugar (either ribose in RNA or deoxyribose in DNA)

* A phosphate group

* A nitrogenous base (adenine, guanine, cytosine, thymine/uracil)

* The sugar of one nucleotide forms a phosphodiester bond with the phosphate group of the next nucleotide.

* This repeated linkage creates a chain of alternating sugar and phosphate groups, forming the sugar-phosphate backbone of the nucleic acid.

Think of it like a beaded necklace where the beads are the nitrogenous bases and the string is the sugar-phosphate backbone.