Eukaryotic cells are characterized by the presence of which cellular structure?

Eukaryotic cells are distinguished by the presence of a nucleus, which is a membrane-bound organelle that houses the cell's genetic material—DNA. The nucleus plays a crucial role in controlling cellular activities such as gene expression and replication. This compartmentalization allows for more complex regulation of genetic processes in eukaryotic cells, facilitating a higher level of organization and specialization compared to prokaryotic cells, which do not have a nucleus.

While ribosomes are also present in eukaryotic cells and are essential for protein synthesis, they are not exclusive to eukaryotes, as they are also found in prokaryotic cells. A nucleoid refers to the region in prokaryotic cells where the genetic material is located, and a cell wall is present in some eukaryotic cells (like plant cells and fungi), but not all. Thus, the defining feature of eukaryotic cells is the nucleus.