DNA is a chemical substance, located in the nucleus of your cells, that looks like a long thin twisted ladder. The rungs of the ladder are made up of things called base pairs.
There are four different substances that make up base pairs: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).
The base pairs always match up the same way: Adenine always connects to Thymine, and Cytosine always connects to Guanine. This pairing is the key to how DNA works.