DNA is made up of four main types of molecules called nucleotides. Each nucleotide has three parts:

1. A sugar molecule (deoxyribose)

2. A phosphate group

3. A nitrogenous base

There are four different nitrogenous bases in DNA:

* Adenine (A)

* Guanine (G)

* Cytosine (C)

* Thymine (T)

These nucleotides are linked together in a long chain, forming the backbone of DNA. The nitrogenous bases stick out from the chain and pair up with bases on another chain, forming the double helix structure of DNA.

The pairing rules for the nitrogenous bases are:

* Adenine (A) always pairs with Thymine (T)

* Guanine (G) always pairs with Cytosine (C)

This pairing is what gives DNA its unique structure and allows it to carry genetic information.