Here's why:

* Prokaryotic cells (like bacteria and archaea) lack a true nucleus. Their genetic material (DNA) is located in a region called the nucleoid, which is not enclosed by a membrane.

* Eukaryotic cells (like plants, animals, fungi, and protists) have a well-defined nucleus where their DNA is stored and protected within a membrane.

Therefore, while a nucleus is a defining characteristic of eukaryotic cells, it is absent in prokaryotes.