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Prokaryotic domains

Topic: Classification And Structure Of Prokaryotic Cells

The kingdom of Prokaryotes is made up of the domains, Archaea and Bacteria.

The domain bacteria are prokaryotes, single-celled organisms that have no membrane-bound organelles and make up a large proportion of living organisms. The domain bacteria contains five major groups: proteobacteria, chlamydias, spirochetes, cyanobacteria, and gram-positive bacteria. Mostly, those from the domain bacteria are what we encounter every day. Some are symbiotic with plants, others live in hot vents deep under the sea, others are pathogens and cause human diseases, some are photosynthesizers, and several are both harmless bacteria and harmful ones.

Archaea are also classed as prokaryotes. These are single-celled organisms that are visually similar to bacteria but contain genes and several metabolic pathways that are more similar to eukaryotes than to bacteria. They are prokaryotes that inhabit extreme environments (high salt, temperature, or chemicals). So far, no archaea that are human pathogens have yet been discovered. Archaea do live in our bodies and seem to be neither harmless or beneficial.

This phylogeny (evolutionary tree) depicts the evolutionary relationships between the three domains of life: Eukarya, Archaea, and Bacteria. The two prokaryotic domains (Archaea and Bacteria) each comprise several smaller taxonomic groupings. Within the Archaea are the euryarchaeotes, crenarchaeotes, nanoarchaeotes, and korarchaeotes. Within the Bacteria are proteobacteria, chlamydias, spirochetes, cyanobacteria, and gram-positive bacteria.

Domains of life: Bacteria and Archaea are both prokaryotes but differ enough to be placed in separate domains. An ancestor of modern Archaea is believed to have given rise to Eukarya, the third domain of life.

Practice Questions


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Key Points

• The domain prokaryote contains two classes: Archaea and Bacteria

• Bacteria are the most common prokaryote organisms that we encounter every day, ranging from harmful to beneficial ones.

• Archaea are prokaryotes that inhabit extreme environments, such as inside of volcanoes; Their ancestor is believed to have given rise to Eukarya, the third domain of life.

Key Terms

Prokaryote: an organism whose cell (or cells) are characterized by the absence of a nucleus or any other membrane-bound organelles

Domain: in the three-domain system, the highest rank in the classification of organisms, above kingdom: Bacteria, Archaea, and Eukarya

Archaea: a taxonomic domain of single-celled organisms lacking nuclei, formerly called archaebacteria, but now known to differ fundamentally from bacteria

Symbiotic: a mutually beneficial relationship between two organisms

Pathogens: a microorganism that causes disease

Photosynthesizer: Any organism that uses photosynthesis to generate carbohydrates

Bacteria: single-celled organisms

Eukaryote: an organism with genetic material within a distinct nucleus

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