Here's why:

* Nucleic acids are large biopolymers, meaning they are made up of repeating smaller subunits called monomers.

* The monomers of nucleic acids are nucleotides.

* Each nucleotide consists of three parts:

* A sugar (ribose in RNA, deoxyribose in DNA)

* A phosphate group

* A nitrogenous base (adenine, guanine, cytosine, thymine in DNA; adenine, guanine, cytosine, uracil in RNA)

So, the chemical structure of RNA and DNA is built around the same fundamental building blocks: nucleotides. This commonality classifies them both as nucleic acids.