Which group of microorganisms is characterized by the absence of a true nucleus?

The group of microorganisms characterized by the absence of a true nucleus is prokaryotes. Prokaryotic cells are defined by their simple structure, lacking membrane-bound organelles and a defined nucleus. Instead of a nucleus, prokaryotes have a nucleoid region that contains their genetic material, which is typically a single circular strand of DNA. This distinct structural feature sets prokaryotes apart from eukaryotes, which have a well-defined nucleus and more complex cellular organization.

In addition to prokaryotes, it's important to note that viruses do not have a cellular structure at all; they are composed of genetic material encased in a protein coat and cannot perform metabolic processes independently. While fungi are classified as eukaryotes, they possess a true nucleus and are therefore not included in the group of microorganisms lacking one. Understanding these fundamental differences helps in identifying and categorizing different types of microorganisms effectively.