What is the defining characteristic of prokaryotic cells?

Prokaryotic cells are characterized by their lack of a nucleus. This fundamental distinction sets them apart from eukaryotic cells, which contain a membrane-bound nucleus where their genetic material is enclosed. In prokaryotes, such as bacteria and archaea, the genetic material is found in a nucleoid region, which is not surrounded by a membrane. This absence of a true nucleus is a key feature that influences many other aspects of prokaryotic cell structure and function, including their typically simpler organization and smaller size compared to eukaryotic cells.

Other choices outline features associated with eukaryotic cells. The presence of mitochondria is a characteristic of eukaryotic cells, involved in energy production through aerobic respiration. A nucleus is another hallmark of eukaryotic cells, housing the cell's DNA. Lastly, the multicellular structure is also not typical of prokaryotes, as they are primarily unicellular organisms, while many eukaryotes can exist as multicellular organisms. Therefore, the defining characteristic of prokaryotic cells effectively points to their lack of a nucleus.