The 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.

Chromosomes are located within the nucleus. 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.

The Nucleolus

Contained within the nucleus is a dense 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.

A ribosome is composed of ribosomal RNA and proteins.

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: