Nucleus: Definition, Structure, Function


In biology, the nucleus is a special structure found in most cells (except bacteria and blue-green algae) and is separated from the rest of the cell by a nuclear membrane. This membrane is permanent with the cell’s endoplasmic reticulum (membrane network) and has holes in it, allowing entry into macromolecules. The nucleus controls and regulates cell activity (such as growth and metabolism) and picks up genes and structures that contain genetic material. Cores are small bodies often seen inside the nucleus. The gem-like matrix in which the atomic components are suspended is the nuclear plasma.

Since the nucleus contains the organism’s genetic code, which determines the amino acid sequence of proteins essential for daily function, it essentially functions as an information centre for the cell. The information in DNA is copied, or transcribed, into a group of messenger RNA (mRNA) molecules, each of which encodes information for a single protein (in some cases more than one protein, as in bacteria). The mRNA molecules are then transported through the nuclear envelope into the cytoplasm, where they are translated, to serve as templates for the synthesis of specific proteins. For more information on these processes, see Transcription; Translation.

A cell usually contains only one nucleus. However, under some conditions, the nucleus divides but the cytoplasm does not divide. This results in a multinucleate (hypothalamic) cell as it occurs in skeletal muscle fibers. Some cells – such as human red blood cells – lose their nucleus at maturity. See also Cell.

The Nuclear Membrane

The nuclear membrane is one of the factors that distinguishes eukaryotic cells from prokaryotic cells. While eukaryotic cells have a membrane attached to the nucleus, this is not the case with prokaryotes (such as bacteria) that lack membrane-bound organelles.

Like the other cellular organelles of eukaryotes, the nucleus is an organelle attached to a membrane. The nuclear membrane, like the cell membrane, is a double-layered structure composed of phospholipids (which form the lipid bilayer nucleus shell).

Nuclear membranes have nuclear holes (made up of proteins) through which substances enter or leave cells (RNA, proteins, etc.). As the two fatty layers separate between them (around the centre) in a thin space, studies have shown that they have merged into the pores.


Also known as karyoplasm, the nucleoplasm is a type of protoplasm made up of enzymes, dissolved salts, and many organic molecules. In addition, the nucleoplasm helps protect the lining and thus the nucleus and chromosomes while also helping to maintain the overall shape of the nucleus.


Just as the nucleus is the most prominent organelle in a cell, the nucleus is the most prominent structure of the nucleus. Unlike the nucleus, this dense structure lacks its own membrane.
During cell division (mitosis), the nucleus only disperses after mitosis to repair specific parts of the chromosome.

Although the nucleus is the most prominent (and thus visible) structure of the nucleus, its size largely depends on the level of production of the ribosome as well as the different types of molecular processes that take place in the nucleus. The nucleus is the site for the replication and processing of ribosomal genes. In some organisms, the nucleus has up to four centers.

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