Cell nucleus

The cell nucleus (plural: cell nuclei) contains the cell's genes and controls the cell's growth and reproduction. It has a double layered nuclear membrane round it. The nucleus is usually the most prominent organelle in a cell. The nucleus is small and round, and works as the cell's control center. It contains chromosomes which house the DNA. The human body contains billions of cells, most of which have a nucleus.

Cells in tissue culture stained for DNA with the blue Hoechst dye. The middle and right cells are in interphase, so their entire nuclei are labeled. On the left, a cell is going through mitosis and its DNA has condensed, ready for division

Diagram of a typical animal cell 1.Nucleolus 2.Nucleus 3.Ribosomes 4.Vesicle 5. rough endoplasmic reticulum 6.Golgi apparatus 7.Cytoskeleton 8. smooth endoplasmic reticulum 9.Mitochondrion 10.Vacuole 11.Cytosol 12.Lysosome 13.Centriole 14.Cell membrane

The eukaryotic cell nucleus. Seen here are the ribosome-dotted double membranes of the nuclear envelope, the DNA complex, and the nucleolus

All eukaryote organisms have nuclei in their cells, even the many eukaryotes that are single-celled. Bacteria and Archaea, which are prokaryotes, are single-celled organisms of a different type and do not have nuclei. Cell nuclei were first found by Antonie van Leeuwenhoek in the 17th century.

The nucleus has a membrane around it but the things inside it do not. Inside it are many proteins, RNA molecules, chromosomes and the nucleolus. In the nucleolus ribosomes are put together. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA into proteins.

When a cell is dividing or preparing to divide, the chromosomes become visible with a light microscope. At other times when the chromosomes are not visible, the nucleolus will be visible.

Nuclear membrane

Large molecules cannot get through the double-layer nuclear membrane. However, nuclear pores exist. They control the movement of molecules across the membrane. The pores cross both nuclear membranes, providing a channel. The larger molecules are actively transported by carrier proteins, and there is 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.

Nucleolus

Within the nucleus is a structure called a nucleolus. It is made at a nucleolus organizer region (NOR). This is a chromosomal region around which the nucleolus forms. Inside the nucleolus ribosomes are made. These are exported through the nuclear pore complexes to the cytoplasm. There they work to build proteins. They become attached to the endoplasmic reticulum if they are making membrane proteins.