Which description best defines the nucleus?

• A. The site of protein synthesis on ribosomes.
• B. A membrane-bound organelle containing RNA and DNA that serves as the control center of the cell.
• C. The energy powerhouse of the cell.
• D. The packaging center for lipids.

Answer: (B)

The nucleus acts as the cell’s control center. It is a membrane-bound organelle that houses DNA, the genetic instruction manual, and the RNA transcripts that are made from that DNA. By regulating which genes are expressed and when, the nucleus directs the cell’s activities and helps control growth, development, and responses to the environment. Its envelope, with nuclear pores, protects the genetic material while controlling what enters and leaves. The nucleus also contains the nucleolus, where ribosomal components are assembled, linking it to protein synthesis without being the site where proteins are made. This set of features—protective, membrane-bound housing of genetic material, and central role in gene expression—best matches the description.