A cellular structure containing genetic material in the form of DNA.

Genetic material in the form of DNA is found in the nucleus. The nucleus is perhaps the most important structure within animal and plant cells. It is the main control center of the cell and acts like the brain of the cell. Only eukaryotic cells have a nucleus. In fact, the definition of a eukaryotic cell is that it contains a nucleus, whereas a prokaryotic cell is defined as having no nucleus. Most cells have only one nucleus. However, there are some cells that develop with more than one nucleus.