Nucleic acids are made up of smaller repeating subunits called nucleotides. Each nucleotide consists of three components:

1. A sugar molecule: This is either ribose (in RNA) or deoxyribose (in DNA).

2. A phosphate group: This is a negatively charged group that gives nucleic acids their acidic properties.

3. A nitrogenous base: This is a nitrogen-containing molecule that comes in five main types: adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U).

Here's how the bases pair up in nucleic acids:

* DNA: Adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).

* RNA: Adenine (A) pairs with uracil (U), and guanine (G) pairs with cytosine (C).

These base pairings are crucial for the structure and function of nucleic acids. They determine the genetic code and allow for the replication and transcription of DNA and RNA.