nucleus, in biology, a specialized structure occurring in most cells (except bacteria and blue-green algae) and separated from the rest of the cell by a double layer, the nuclear membrane. This membrane seems to be continuous with the endoplasmic reticulum (a membranous network) of the cell and has pores, which probably permit the entrance of large molecules. The nucleus controls and regulates the activities of the cell (e.g., growth and metabolism) and carries the genes, structures that contain the hereditary information. Nucleoli are small bodies often seen within the nucleus. The gel-like matrix in which the nuclear components are suspended is the nucleoplasm.
Because the nucleus houses an organism’s genetic code, which determines the amino acid sequence of proteins critical for day-to-day function, it primarily serves as the information centre of the cell. Information in DNA is transcribed, or copied, into a range of messenger ribonucleic acid (mRNA) molecules, each of which encodes the information for one protein (in some instances more than one protein, such as in bacteria). The mRNA molecules are then transported through the nuclear envelope into the cytoplasm, where they are translated, serving as templates for the synthesis of specific proteins. For more information on these processes, see transcription; translation.
A cell normally contains only one nucleus. Under some conditions, however, the nucleus divides but the cytoplasm does not. This produces a multinucleate cell (syncytium) such as occurs in skeletal muscle fibres. Some cells—e.g., the human red blood cell—lose their nuclei upon maturation.
The cell nucleus is a membrane-bound structure that contains a cell’s hereditary information and controls its growth and reproduction. It is the command center of a eukaryotic cell and is usually the most notable cell organelle in both size and function.
Protein and Ribosome Synthesis
The nucleus regulates the synthesis of proteins in the cytoplasm through the use of messenger RNA (mRNA). Messenger RNA is a transcribed DNA segment that serves as a template for protein production. It is produced in the nucleus and travels to the cytoplasm through the nuclear pores of the nuclear envelope, which you’ll read about below. Once in the cytoplasm, ribosomes and another RNA molecule called transfer RNA work together to translate mRNA in order to produce proteins.
The shape of a nucleus varies from cell to cell but is often depicted as spherical. To understand more about the role of the nucleus, read about the structure and function of each of its parts.
Nuclear Envelope and Nuclear Pores
The cell nucleus is bound by a double membrane called the nuclear envelope. This membrane separates the contents of the nucleus from the cytoplasm, the gel-like substance containing all other organelles. The nuclear envelope consists of phospholipids that form a lipid bilayer much like that of the cell membrane. This lipid bilayer has nuclear pores that allow substances to enter and exit the nucleus, or transfer from the cytoplasm to the nucleoplasm.
The nuclear envelope helps to maintain the shape of the nucleus. It is connected to the endoplasmic reticulum (ER) in such a way that the internal chamber of the nuclear envelope is continuous with the lumen, or inside, of the ER. This also allows the transfer of materials as well.
The nucleus houses chromosomes containing DNA. DNA holds heredity information and instructions for cell growth, development, and reproduction. When a cell is “resting”, or not dividing, its chromosomes are organized into long entangled structures called chromatin.
Nucleoplasm is the gelatinous substance within the nuclear envelope. Also called karyoplasm, this semi-aqueous material is similar to cytoplasm in that it is composed mainly of water with dissolved salts, enzymes, and organic molecules suspended within. The nucleolus and chromosomes are surrounded by nucleoplasm, which cushions and protects nuclear contents.
Like the nuclear envelope, the nucleoplasm supports the nucleus to hold its shape. It also provides a medium by which materials, such as enzymes and nucleotides (DNA and RNA subunits), can be transported throughout the nucleus to its various parts.
Contained within the nucleus is a dense, membrane-less structure composed of RNA and proteins called the nucleolus. The nucleolus contains nucleolar organizers, the parts of chromosomes carrying the genes for ribosome synthesis. The nucleolus helps to synthesize ribosomes by transcribing and assembling ribosomal RNA subunits. These subunits join together to form ribosomes during protein synthesis.