What structure is unique to prokaryotic cells?

The structure that is unique to prokaryotic cells is the nucleoid. Prokaryotic cells, such as bacteria, lack a defined nucleus that is characteristic of eukaryotic cells. Instead, they possess a nucleoid region, which is an irregularly-shaped area in the cell where the genetic material (DNA) is concentrated. This DNA is typically a single, circular chromosome that is not enclosed by a membrane, distinguishing the nucleoid from the eukaryotic nucleus, which is membrane-bound.

In contrast, eukaryotic cells contain a nucleus, as well as organelles such as the endoplasmic reticulum (ER), Golgi apparatus, and mitochondria, all of which are not present in prokaryotic cells. Therefore, the presence of a nucleoid is a defining feature of prokaryotes, highlighting the differences in cellular organization between these two broad categories of life.