What are prokaryotes?

Prokaryotes are defined as microscopic single-celled organisms that lack a membrane-bounded nucleus. This characteristic is fundamental to their classification. Prokaryotic cells are simpler in structure compared to eukaryotic cells, which do have a nucleus and membrane-bound organelles. The absence of a defined nucleus and other organelles allows prokaryotes to have a more straightforward cellular organization, making them generally smaller and easier to replicate.

In addition, prokaryotes include two main domains: bacteria and archaea, both of which play critical roles in various ecosystems and possess unique biochemical processes. Understanding prokaryotes is essential in fields such as microbiology, genetics, and biotechnology due to their diverse functions and applications.

The other options describe characteristics that pertain to other types of organisms. For example, eukaryotes, which fit the description of having a complex cell structure with organelles, and multicellular organisms with specialized tissues refer to more advanced life forms that include plants, animals, and fungi. A type of virus that affects bacterial cells is known as a bacteriophage, which is not classified as a prokaryote. Therefore, the definition encapsulated in the correct choice accurately represents what prokaryotes are.