The nucleus is the cell part responsible for controlling most activity inside a eukaryotic cell. It contains nearly all of the cell’s genetic material and regulates which genes are expressed, helping direct the production of proteins and other cellular functions.
The nucleus is enclosed by a double membrane called the nuclear envelope. Small openings called nuclear pores control the movement of large molecules, including messenger RNA and proteins, between the nucleus and cytoplasm. This separation lets DNA transcription occur in the nucleus while ribosomes translate messenger RNA in the cytoplasm.
The nucleus is not literally the controller of every cell process, and the statement applies mainly to eukaryotic cells. Prokaryotes, such as bacteria, do not have a membrane-bound nucleus. Some specialized eukaryotic cells also lack one: mature mammalian red blood cells, for example, discard their nuclei as they develop.