What is the defining characteristic of prokaryotic cells?

Prokaryotic cells are characterized primarily by the absence of a membrane-bound nucleus. Instead of having their genetic material enclosed within a nucleus, prokaryotic cells have a nucleoid region where the DNA is located. This is a fundamental distinction between prokaryotic and eukaryotic cells, the latter of which do have membrane-bound organelles and a defined nucleus.

The absence of a membrane-bound nucleus is significant because it influences various cellular functions and organization. Prokaryotic cells tend to be simpler and smaller compared to eukaryotic cells, allowing them to reproduce quickly and adapt efficiently to various environments. This characteristic aids in understanding the evolutionary differences and biological functions of prokaryotes, such as bacteria and archaea.

The other options describe features that are either characteristics of eukaryotic cells, which are more complex, or describe multicellularity, which is not a feature of prokaryotes. Thus, the defining characteristic of prokaryotic cells is specifically their lack of a membrane-bound nucleus.