The largest organelle in a cell is the nucleus. The nucleus serves as the control center for cellular activities, containing the cell's genetic material, DNA. It is responsible for regulating gene expression, cell growth, and reproduction, thereby playing a crucial role in the overall function of the cell. The nucleus is surrounded by a double membrane called the nuclear envelope, which contains pores allowing for the exchange of materials between the nucleus and cytoplasm. Its size can vary among different cell types, but it is often the most prominent organelle visible under a microscope.