Here's why:

* Nucleic acids are long chains of monomers called nucleotides.

* Nucleotides are composed of three parts:

* A sugar (either ribose or deoxyribose)

* A phosphate group

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

So, a single nucleotide is the smallest, fundamental building block of a nucleic acid.