Nucleus - the brain
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The nucleus is the largest organelle in a cell. The nucleus directs all activity in the cell. It also controls the growth and reproduction of the cell. The nucleus could be compared to a manager of a company or mayor of a city, they both control activity in their facility. The nucleus is surrounded by a nuclear membrane that protects it from other parts of the cell. It also contains a nucleolus, which is a secondary organelle.
It also contains the blueprints of the cell in the chromatin inside the nucleus. The chromatin is like long strands in the nucleus that carry DNA and proteins.

The nuclear envelope keeps the contents of the nucleus, separate from the cytoplasm of the cell.