<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 including osteoclasts have many . </P> <P> Cell nuclei contain most of the cell's genetic material, organized as multiple long linear DNA molecules in a 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> <P> Because the nuclear envelope is impermeable to large molecules, nuclear pores are required to regulate nuclear transport of molecules across the envelope . The pores cross both nuclear membranes, providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and ions . Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes . Although the interior of the nucleus does not contain any membrane - bound subcompartments, its contents are not uniform, and a number of sub-nuclear bodies exist, made up of unique proteins, RNA molecules, and particular parts of the chromosomes . The best - known of these is the nucleolus, which is mainly involved in the assembly of ribosomes . After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA . </P>

What is the control centre of a cell