The membrane-bound organelle in a cell that contains the genetic material (DNA).The cell nucleus is the compartment inside a eukaryotic cell that holds the cell's main set of DNA. It is bounded by a double membrane called the nuclear envelope, which separates the genetic material from the rest of the cytoplasm. Nearly every cell in plants, animals, and fungi has one, and it is usually the largest structure in the cell. The nucleus is where DNA is stored, copied, and read out into RNA, so it functions as the cell's control center: instructions for building proteins and regulating cell behavior originate there. It also explains why eukaryotic cells can afford to keep their genomes large and organized, since the envelope protects DNA from the mechanical and chemical churn of the cytoplasm. Beyond storage, the nucleus organizes DNA into chromosomes and controls which genes are active in a given cell type, which is central to how a single fertilized egg can give rise to many different tissues.
The nucleus was first described by Antonie van Leeuwenhoek in the 17th century and named by Robert Brown in 1831, who observed it in orchid cells. Its role in heredity was established in the late 19th and early 20th centuries through the work of Walther Flemming, Eduard Strasburger, and others.
The nuclear envelope is a double membrane perforated by protein-lined channels called nuclear pore complexes. Small molecules diffuse through these pores freely, while larger cargo such as RNA and proteins are carried in and out by dedicated transport proteins. Inside, DNA is wrapped around histone proteins to form chromatin, which is further folded into chromosomes. A dense region called the nucleolus assembles ribosomes. During cell division, the envelope breaks down, chromosomes are segregated, and the envelope reassembles around the two daughter nuclei.
Nuclear organization and nuclear pore function are studied in cancer research and in gene therapy, where delivering DNA into the nucleus is a key step. Nucleus-targeting signals are also used in designing drugs and molecular probes.
If you stretched out all the DNA in one human cell nucleus, it would be about two meters long, yet it fits inside a space roughly six micrometers across.