Nucleus
The nucleus stores the cell’s DNA and decides which genes are read at any moment. Its double membrane, the nuclear envelope, is pierced by pores that let RNA and proteins pass between nucleus and cytoplasm.
- 1NucleusOrganelle · double membrane
- 2MitochondrionOrganelle · double membrane
- 3RibosomeMolecular machine · no membrane
- 4Endoplasmic reticulumOrganelle · single membrane network
- 5Golgi apparatusOrganelle · single membrane stacks
- 6LysosomeOrganelle · single membrane
- 7Cell membraneBoundary · phospholipid bilayer
- 8CentriolesMicrotubule pair · organizes the spindle
Animal cell: what you are looking at
A flexible, roughly rounded cell 10–30 µm across, bounded only by its membrane. It has a pair of centrioles near the nucleus and many lysosomes, but no wall, no chloroplasts, and no large central vacuole.
Centrioles. A pair of short microtubule cylinders at right angles; they organize the spindle during cell division.
The nucleus is outlined in coral; it occurs in animal cells, plant cells, fungi, protists. Hover or tab through the key to light up the other structures, and click any of them to open its page.
Compare all eight organelles ↗Know its structure
| Found in | Animal cells, Plant cells, Fungi, Protists |
| Membranes | Two: the nuclear envelope, whose outer membrane is continuous with the rough ER |
| Typical size | About 5–10 µm across, usually the largest organelle in an animal cell |
| Number per cell | One in most cells; skeletal muscle fibers carry hundreds, mature red blood cells have none |
| Contains | Chromatin (DNA wound around histone proteins), one or more nucleoli, and nucleoplasm |
| Discovered | Named by Robert Brown in 1831, who noticed it in orchid epidermal cells |
What it does
- Stores the genome. A human nucleus holds 46 chromosomes, about 2 m of DNA folded into a space a few micrometers wide
- Runs transcription. RNA polymerase copies genes into messenger RNA here; the message is spliced and capped before it leaves
- Builds ribosome parts. The nucleolus assembles ribosomal subunits from rRNA and imported proteins
- Separates copying from reading. Keeping DNA behind a membrane lets the cell edit mRNA before ribosomes ever see it
Nucleus: common questions
Wondering about the why? Start here.
What does the nucleus do?
The nucleus stores the cell’s genome, 46 chromosomes and about 2 m of DNA in a human cell, and controls which genes are transcribed into messenger RNA at any moment. Its nucleolus assembles ribosomal subunits, and its double membrane keeps DNA and transcription separate from the ribosomes in the cytoplasm, so mRNA can be capped and spliced before it is translated.
Read: DNA structureDo all cells have a nucleus?
No. Bacteria and archaea are prokaryotes: their circular DNA sits in a region called the nucleoid with no membrane around it. Among eukaryotic cells, mature mammalian red blood cells eject their nucleus to make room for hemoglobin, while skeletal muscle fibers fuse from many cells and carry hundreds of nuclei.
Read: prokaryotes vs. eukaryotesHow does mRNA get out of the nucleus?
Through nuclear pores, protein-lined channels about 120 nm wide that pierce the envelope; a typical nucleus has roughly 2,000 to 5,000 of them. A new transcript is capped, spliced, and given a poly-A tail first, then escorted through a pore into the cytoplasm, where free ribosomes or ribosomes on the rough ER translate it, often within minutes of the gene being switched on.
Meet the ribosome