The Cell Nucleus

Definition, Structure, and Function

Cell nucleus
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The cell nucleus​ is a membrane bound structure that contains the cell's hereditary information and controls the cell's growth and reproduction. It is the command center of a eukaryotic cell and is commonly the most prominent organelle in a cell.

Distinguishing Characteristics

The cell nucleus is bound by a double membrane called the nuclear envelope. This membrane separates the contents of the nucleus from the cytoplasm.

Like the cell membrane, the nuclear envelope consists of phospholipids that form a lipid bilayer. The envelope helps to maintain the shape of the nucleus and assists in regulating the flow of molecules into and out of the nucleus through nuclear pores. The nuclear envelope is connected with the endoplasmic reticulum (ER) in such a way that the internal compartment of the nuclear envelope is continuous with the lumen of the ER.

The nucleus is the organelle which houses chromosomes. Chromosomes consist of DNA, which contains heredity information and instructions for cell growth, development, and reproduction. When a cell is "resting" i.e. not dividing, the chromosomes are organized into long entangled structures called chromatin and not into individual chromosomes as we typically think of them.


Nucleoplasm is the gelatinous substance within the nuclear envelope. Also called karyoplasm, this semi-aqueous material is similar to cytoplasm and is composed mainly of water with dissolved salts, enzymes, and organic molecules suspended within.

The nucleolus and chromosomes are surrounded by nucleoplasm, which functions to cushion and protect the contents of the nucleus. Nucleoplasm also supports the nucleus by helping to maintain its shape. Additionally, nucleoplasm provides a medium by which materials, such as enzymes and nucleotides (DNA and RNA subunits), can be transported throughout the nucleus.

Substances are exchanged between the cytoplasm and nucleoplasm through nuclear pores.

The Nucleolus

Contained within the nucleus is a dense, membrane-less structure composed of RNA and proteins called the nucleolus. The nucleolus contains nucleolar organizers, which are parts of chromosomes with the genes for ribosome synthesis on them. The nucleolus helps to synthesize ribosomes by transcribing and assembling ribosomal RNA subunits. These subunits join together to form a ribosome during protein synthesis.

Protein 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. Once in the cytoplasm, ribosomes and another RNA molecule called transfer RNA work together to translate mRNA to produce proteins.

Eukaryotic Cell Structures

The cell nucleus is only one type of cell organelle. The following cell structures can also be found in a typical animal eukaryotic cell: