Here's why:

* Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and other internal organelles. Their DNA is located in a region called the nucleoid, which is not enclosed by a membrane.

* Eukaryotes are organisms whose cells have a membrane-bound nucleus containing their DNA, as well as other membrane-bound organelles.

So, the presence of a nucleus is a defining characteristic of eukaryotic cells, not prokaryotic cells.