The study and classification of prokaryotic organisms, revealing their evolutionary history and relationships. Unlike eukaryotic classification, which often relies on morphology, prokaryotic taxonomy heavily utilizes molecular biology.
Prokaryotic and Eukaryotic Cells→
Phylogenetics is the study of evolutionary relatedness among various groups of organisms. It traces the origin and evolution (phylogeny) of a set of organisms. In modern taxonomy, the sequence of 16S rRNA is considered the most reliable method for determining the phylogenetic relationship between two different prokaryotic species because it is highly conserved.
The five-kingdom system was proposed by American biologist Robert H. Whittaker in 1969. This system was based on the fundamental distinction between prokaryotes and eukaryotes. All prokaryotic organisms were grouped into a single kingdom called Monera. This system was later revised by Lynn Margulis and Karlene V. Schwartz in 1988 but maintained the Monera kingdom for prokaryotes.
Advancements in molecular systematics revealed that prokaryotic life consists of two distinct domains that evolved independently from a common ancestor. In 1977, Carl Woese identified that archaea have a separate evolutionary lineage from bacteria, a discovery that fundamentally changed prokaryotic classification. This led to the widely accepted three-domain system, which divides all life into:
The standard reference for bacterial classification is Bergey's Manual. It classifies bacteria based on shared characteristics such as cell wall composition, morphology, and gram-staining reactions. For example, bacteria with a thin, Gram-negative type cell wall are placed in a specific category (Gracilicutes).
The ancestors of modern bacteria were single-celled microorganisms that first appeared on Earth approximately 4 billion years ago. For about 3 billion years, all life was microscopic, with bacteria and archaea being the dominant life forms. The most recent common ancestor of bacteria and archaea is believed to have lived 2.5 to 3.2 billion years ago and was likely a hyperthermophile (an organism that thrives in extremely hot environments).
While taxonomy focuses on classification, understanding the internal ultrastructure is vital for identifying prokaryotic groups.
The mesosome is an invagination of the plasma membrane in the form of vesicles, tubules, or lamellae. It plays a role in DNA replication, cell division, and export of exocellular enzymes.
In prokaryotes, several ribosomes can be seen attached to a single mRNA strand during protein synthesis. This complex is known as a polysome.