Which component of the cell is responsible for regulating cellular activities by controlling gene expression?

The nucleus is the component of the cell that regulates cellular activities by controlling gene expression. It serves as the information center of the cell, housing the cell's DNA, which contains the instructions needed for the development, functioning, and reproduction of the organism. Through processes such as transcription, the information in a gene is converted into messenger RNA (mRNA), which then guides the synthesis of proteins. This regulation of gene expression is crucial because it determines which proteins are produced, when they are produced, and in what quantities, thus directly influencing cellular activities and functions.

In contrast, ribosomes are responsible for protein synthesis but do not control gene expression. The Golgi apparatus is involved in modifying and packaging proteins for secretion or use within the cell, and the endoplasmic reticulum plays a role in protein and lipid synthesis, but neither has a direct role in regulating gene expression. Therefore, the nucleus is key for cellular regulation through its control over gene expression.