Nucleus

The cell’s control center and the vault for its genetic information. Most human cells have exactly one (mature red blood cells have none; skeletal muscle fibers have many).

Structure

• Nuclear envelope — a double membrane studded with pores that regulate what enters and leaves.
• Chromatin — DNA wound around proteins; it condenses into visible chromosomes when the cell divides.
• Nucleolus — a dense region where ribosomes are assembled.

What it does

• Stores the genome — the cell’s full DNA instruction set.
• Controls the cell — decides which genes are switched on, and so which proteins are made.
• Manages copying — DNA is duplicated here before cell division.

Key idea

The nucleus is where the cell’s identity and behavior are set — by choosing which parts of the shared genome to read.