<Li> Smooth endoplasmic reticulum </Li> <Li> Cytosol (fluid that contains organelles, comprising the cytoplasm) </Li> <P> In cell biology, the nucleus (pl . nuclei; from Latin nucleus or nuculeus, meaning kernel or seed) is a membrane - enclosed organelle found in eukaryotic cells . Eukaryotes usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others have many . </P> <P> Cell nuclei contain most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes . The genes within these chromosomes are the cell's nuclear genome and are structured in such a way to promote cell function . The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression--the nucleus is, therefore, the control center of the cell . The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm, and the nuclear matrix (which includes the nuclear lamina), a network within the nucleus that adds mechanical support, much like the cytoskeleton, which supports the cell as a whole . </P>

A cell that has a nucleus is called