The cell nucleus is the command center and thus controls the activities of the eukaryotic cell. A double-walled cell nuclear envelope separates the nucleus from the cytoplasm and controls its shape while allowing chemical compounds to pass to and from the nucleus to control other activities in the cell. The nucleus contains the chromosomes, consisting of DNA, that regulate cell growth and reproduction.
The nucleus contains the nucleolus, a dense RNA and protein structure. Inside this structure are the organizers that form chromosomes with genes necessary for ribosome synthesis. The nucleus also regulates protein synthesis in the cytoplasm using messenger RNA, which travels to the cytoplasm through pores in the nuclear envelope. Once there, messenger RNA is translated using ribosomes and transfer RNA in the production of proteins. The nucleus regulates the molecules allowed through the pores of the membrane to prevent some larger molecules entering from the cytoplasm.
In animals, the nucleus is the largest cell organelle. Human cells have a nucleus that averages between 11 and 22 micrometers in diameter and occupies approximately 10 percent of the cell volume. It is large enough to observe and study using an electron microscope. Scientists believe mutations occurring in the nucleus are a key to developing diseases as well as the body's response to these diseases.