What structure surrounds the nucleus of a cell?

The structure that surrounds the nucleus of a cell is known as the nuclear membrane, also referred to as the nuclear envelope. This double-layered membrane serves multiple essential functions. It provides a barrier that separates the contents of the nucleus from the cytoplasm, thus maintaining a distinct environment for the nucleic acids and the machinery necessary for processes such as transcription and replication.

The nuclear membrane is punctuated by nuclear pores, which are large protein complexes that control the exchange of substances—such as RNA and proteins—between the nucleus and the cytoplasm. This selective permeability is crucial for regulating gene expression and ensuring that the processes within the nucleus can operate independently yet communicate with the rest of the cell.

In contrast, the cell membrane surrounds the entire cell, regulating what enters and exits the cell. The cytoplasm is the jelly-like substance inside the cell that contains organelles but does not specifically surround the nucleus. The endoplasmic reticulum is an organelle involved in protein and lipid synthesis, but it is not a membrane that surrounds the nucleus. Thus, the nuclear membrane is indeed the correct answer for the structure that specifically surrounds the nucleus.