There are two main types of cells:

* Eukaryotic cells: These cells have a true nucleus enclosed within a membrane. This is where their DNA is stored and protected. Most organisms, including plants, animals, fungi, and protists, are made up of eukaryotic cells.

* Prokaryotic cells: These cells are simpler and lack a nucleus. Their DNA is located in a region called the nucleoid, which is not enclosed by a membrane. Bacteria and archaea are examples of organisms composed of prokaryotic cells.

So, while the nucleus is a defining feature of eukaryotic cells, it is absent in prokaryotic cells.