Each human cell packs about two metres of DNA into a six-micrometre nucleus
Stretched out, the DNA inside one human cell would reach roughly two metres. It fits inside a nucleus only about six micrometres across, wound around proteins called histones and folded into chromatin. Thousands of guarded pores in the nucleus's double wall decide which molecules may pass in or out.
The nucleus, from the Latin for kernel or seed, is the membrane-wrapped control centre of eukaryotic cells. It holds nearly the entire genome as long, linear chromosomes and governs the cell by regulating which genes are switched on. Most cells have one, but mammalian red blood cells have none and bone-dissolving osteoclasts have many. A small share of genes lives separately in mitochondria. The nucleus is the biggest organelle in animal cells and often fills about a tenth of a cell's volume.
Its envelope has two membranes. The inner one defines the nucleus and anchors the fibrous nuclear matrix that gives it shape. The outer one is continuous with the endoplasmic reticulum and studded with ribosomes making proteins. A mammalian envelope carries 3,000 to 4,000 nuclear pore complexes, each built from about thirty kinds of proteins called nucleoporins, though small glial cells may have only a few hundred and large Purkinje neurons around 20,000. Each pore is 100 nanometres wide, but the free channel through its centre is only about 9, so ions and small molecules drift through while RNA and large proteins must be actively escorted.
Inside, DNA is organised by how busy it is. Loosely packed euchromatin holds frequently used genes, while dense heterochromatin holds rarely read DNA and structural elements such as telomeres and centromeres. Between divisions each chromosome occupies its own patch, a chromosome territory, with active genes tending to sit near its edge. There are no internal membranes, but distinct nuclear bodies form, most famously the nucleolus, where ribosomes are assembled.
The nucleus also matters in medicine. Antibodies against components of chromatin, especially nucleosomes, are called anti-nuclear antibodies and are associated with autoimmune diseases such as systemic lupus erythematosus; they have also been observed alongside multiple sclerosis as part of broader immune dysfunction.
Source: Cell nucleus
