nucleus

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. The primary function of the nucleus is to store and protect the cell’s genetic material (DNA) and regulate gene expression, thereby directing key processes such as metabolism and cell division.

The nuclear membrane helps compartmentalize the cell’s genetic material and regulate interactions between the nucleus and cytoplasm. This membrane seems to be continuous with the endoplasmic reticulum (a membranous network) of the cell and has pores, which permit the selective transport of molecules such as RNA and proteins into and out of the nucleus. The nucleus controls and regulates the activities of the cell and houses the genes, structures that contain the hereditary information.

Within the nucleus are nucleoli, which are sites of synthesis for ribosomal RNA (rRNA) and ribosome assembly. The nucleus also contains chromatin (DNA associated with proteins), which exists in different forms, namely euchromatin and heterochromatin, depending on its level of activity. The gel-like matrix in which these components are suspended is the nucleoplasm. The nucleus is supported structurally by the nuclear lamina, a network of proteins lining the inner membrane.

Because the nucleus houses an organism’s genetic code, it serves as the control center of gene expression. In eukaryotic cells DNA is transcribed into messenger RNA (mRNA) within the nucleus. These mRNA molecules are then transported through the nuclear pores into the cytoplasm, where they are translated into proteins by ribosomes. Gene expression is tightly regulated, allowing cells to respond to internal and external signals.

A cell typically contains a single nucleus, but exceptions exist. Multinucleated cells may arise either through cell fusion (forming a syncytium, as in skeletal muscle fibers) or through repeated nuclear division without cytokinesis (coenocytic cells). Some cells—such as mature mammalian red blood cells—lose their nuclei during development to help maximize space for oxygen transport.