Definition · Plain-language
Nucleus
The nucleus is the control centre of a cell, holding the genetic material and directing the cell’s activities.
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The control centre of the cell
The nucleus is often called the control centre or "brain" of the cell because it directs everything the cell does. It contains the cell’s genetic material — the DNA — which carries the instructions for building the proteins that run the cell. By controlling which of these instructions are used, the nucleus controls the cell’s growth, activities and division. It is usually the largest and most prominent structure inside a eukaryotic cell, easy to spot under a microscope.
Holding the genetic material
Inside the nucleus, the DNA is organised into structures called chromosomes. The DNA carries genes, the coded instructions passed from parents to offspring. Keeping the DNA inside the nucleus protects it and keeps it separate from the busy reactions in the rest of the cell. When the cell needs to make a protein, a copy of the relevant instruction is made and sent out of the nucleus to the cell’s protein-building machinery, while the precious original DNA stays safely inside.
A defining feature of eukaryotic cells
Having a nucleus is what makes a cell eukaryotic. Animal cells, plant cells, fungi and protists all have a nucleus enclosed in a membrane. Prokaryotic cells, such as bacteria, have no nucleus — their DNA floats freely in the cytoplasm instead. The nucleus is surrounded by a double membrane, the nuclear envelope, dotted with pores that let materials pass in and out in a controlled way. This membrane is what keeps the genetic material organised in one protected compartment.
Key facts
At a glance
- Definition: the control centre of a eukaryotic cell
- Holds: the genetic material (DNA), organised into chromosomes
- Job: directs cell activities by controlling protein-making
- Surrounded by: a membrane (the nuclear envelope) with pores
- Found in: animal and plant cells (eukaryotes)
- Not in: prokaryotic cells such as bacteria
Common misconceptions
What people often get wrong
Often heard: Every living cell has a nucleus.
Actually: Prokaryotic cells, such as bacteria, have no nucleus — their DNA floats freely in the cytoplasm. Only eukaryotic cells (animals, plants, fungi and protists) have a nucleus. Having one is what defines a eukaryotic cell.
Often heard: The nucleus makes the cell’s energy.
Actually: Energy is released by the mitochondria, not the nucleus. The nucleus is the control centre that holds the DNA and directs the cell’s activities. Each organelle has its own job, and energy release belongs to the mitochondria.
Often heard: The DNA leaves the nucleus to make proteins.
Actually: The DNA stays inside the nucleus, where it is protected. When a protein is needed, a copy of the instruction is made and sent out to the protein-building machinery, while the original DNA remains safely in the nucleus.
Going deeper
