Which structure immediately identifies a cell as a eukaryote?

A eukaryotic cell's most identifiable trait is an enclosed nucleus. Eukaryotic cells are found in complex organisms, while prokaryotic cells are typically found as unicellular organisms and do not contain a nucleus.

Eukaryotic cells contain genetic information in DNA found in their nuclei. This genetic information is then utilized by organelles found within the cell to perform specific biological functions, as well as gene expression in new cells. The membrane in which the nucleus is stored is porous, allowing for the intake and release of vital nutrients and substances required to maintain the functions of the cell. In prokaryotic cells, genetic information is stored loosely within the cell membrane due to the lack of nuclei.