nucleus
Meaning of nucleus: Keeping DNA behind a membrane is what separates human cells from bacteria, and it lets the cell control which genes are read.
Definition of nucleus
The nucleus is the compartment inside most human cells that holds the chromosomes, the packaged strands of DNA carrying the cell's genes. A double membrane called the nuclear envelope encloses it, pierced by pores that control what passes in and out. The word has a second, unrelated use in the nervous system, where it names a cluster of nerve cell bodies.
What happens inside it?
The nucleus is where genetic information is stored and read. A gene is copied into messenger RNA, that copy is trimmed and edited, and it then leaves through a nuclear pore to be translated into protein in the surrounding cytoplasm. Keeping the copying step behind a membrane separates it from the machinery that reads the message, which allows the cell to edit the copy before releasing it and to control how much of any protein gets made. A dense region inside, the nucleolus, is where the components of ribosomes are assembled. Nothing enters or leaves at random: the pores act as gates, and the proteins the nucleus needs carry tags that let them through.
Which cells break the rule?
Bacteria have no nucleus at all. Their DNA sits loose in the cell, and this is the single deepest division in biology, between prokaryotes without a nucleus and eukaryotes with one. Among human cells, mature red blood cells eject their nucleus as they develop, which frees space for hemoglobin but means they cannot repair themselves and have a limited lifespan. Platelets are cell fragments and have none either. Skeletal muscle cells go the other way and carry many nuclei in one long fiber. A small amount of DNA also lies outside the nucleus altogether, inside mitochondria, and is inherited from the mother alone. In the nervous system the word takes an entirely separate meaning: a nucleus there is a cluster of nerve cell bodies deep in the brain, as in the caudate nucleus, and has nothing to do with the cell compartment.
Why do pathologists study it?
| Feature | What it describes |
|---|---|
| Nuclear size | Enlargement relative to the rest of the cell |
| Nuclear-to-cytoplasmic ratio | How much of the cell the nucleus occupies; a high ratio is common in cancer |
| Contour | Smooth and round, or irregular and notched |
| Chromatin | How the genetic material is distributed: fine and even, or coarse and clumped |
| Nucleoli | Prominent nucleoli suggest a cell building protein rapidly |
| Mitotic figures | Cells caught dividing; more of them means faster growth |
These features are read together rather than one at a time, and none of them alone identifies a cancer. They form much of what a pathologist means by the grade of a tumor, and they are the reason a stained slide can distinguish cells that look otherwise similar. Each observation is made on the molecule level only indirectly, through the way stains bind the packaged DNA.
Used in a sentence
The pathologist noted enlarged nuclei within the sample of abnormal cells.
Good to know
The size and shape of the nucleus, and how much of the cell it occupies, are examined during pathology evaluation, since these features are among the notable differences between normal and abnormal cells.
Word origin
Where “nucleus” comes from
From Latin nucleus, meaning kernel or inner part, a diminutive of nux, meaning nut.