Nucleus

The nucleus is the compartment of a eukaryotic cell that contains its DNA, packaged together with proteins as chromatin. It is usually the largest organelle, wrapped in a double membrane called the nuclear envelope, whose outer layer joins directly onto the endoplasmic reticulum. Nuclear pores in the envelope control traffic in both directions, letting RNA out to the cytoplasm and letting proteins in.

The nucleus is where DNA is copied before a cell divides and where genes are transcribed into RNA. A dense region inside it, the nucleolus, combines ribosomal RNA with proteins to build ribosome subunits, which leave through the pores to make proteins in the cytoplasm. When the cell divides, the chromatin condenses into compact, visible chromosomes and the envelope breaks down, to be rebuilt around each new set.

Most cells have one nucleus, but some have several, such as human skeletal muscle fibers and the protist Paramecium, while mature mammalian red blood cells have none. The same word is used in chemistry for the center of an atom and in neuroscience for a cluster of nerve cell bodies in the brain.