Nucleus (biology)

Nucleus (biology), membrane-bound structure of a cell that plays two crucial roles. The nucleus carries the cell’s genetic information that determines if the organism will develop, for instance, into a tree or a human; and it directs most cell activities including growth, metabolism, and reproduction by regulating protein synthesis (the manufacture of long chains of amino acids). The presence of a nucleus distinguishes the more complex eukaryotic cells of plants and animals from the simpler prokaryotic cells of bacteria and cyanobacteria that lack a nucleus.

The nucleus is the most prominent structure in the cell. It is typically round and occupies about 10 percent of the cell’s total volume. The nucleus is wrapped in a double-layered membrane called the nuclear envelope. The space between the nuclear envelope layers is called perinuclear space. The nuclear envelope is attached to a network of membrane-enclosed tubules that extends throughout the cell called the endoplasmic reticulum. The nuclear envelope is perforated by many holes, called nuclear pores, that permit the movement of selected molecules between the nucleus and the rest of the cell, while blocking the passage of other molecules.

The nucleus contains the nucleolus, which manufactures protein-producing structures called ribosomes. Genetic information in the form of deoxyribonucleic acid (DNA) is stored in threadlike, tangled structures called chromatin within the nucleus. During the process of cell division known as mitosis, in which the nucleus divides, the chromatin condense into several distinct structures called chromosomes. Each time the cell divides, the heredity information carried in the chromosomes is passed to the two newly formed cells.

The DNA in the nucleus also contains the instructions for regulating the amount and types of proteins made by the cell. These instructions are copied, or transcribed, into a type of ribonucleic acid (RNA) called messenger RNA (mRNA). The mRNA is transported from the nucleus to ribosomes, where proteins are assembled.