Prokaryotes are a group of organisms that are characterized by the absence of a true nucleus. In prokaryotic cells, the genetic material, which is typically in the form of a single circular DNA molecule, is not enclosed within a nuclear membrane. Instead, it resides in the cytoplasm in a region known as the nucleoid. This fundamental difference in cell structure is a defining feature that distinguishes prokaryotes from eukaryotes, which do have a nucleus that contains their DNA.
Bacteria, which fall under the broader category of prokaryotes, exemplify this characteristic, but they are not the only prokaryotic organisms. Other types of prokaryotes include archaea. Eukaryotes, which include organisms like fungi, plants, and animals, possess a nucleus, making them different from prokaryotes. Thus, the defining characteristic of lacking a nucleus is specifically associated with prokaryotes as a whole rather than just bacteria or fungi.