Prokaryote
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Prokaryotes are single-celled micro-organisms characterized by the lack of a membrane-bound nucleus and membrane-bound organelles. The word "prokaryote" is from Greek and means "before the nucleus". This definition can be misleading if approached from the evolutionary ladder view: eukaryotes (organisms with nuclei) are not evolved from prokaryotes, the two share a common ancestor, whose cells almost certainly looked nothing like modern prokaryotic cells.
There are two domains of prokaryote: the Eubacteria/Bacteria and the Archaebacteria/Archaea
Differences between Bacteria and Archaea
- Eubacteria have cell walls composed of peptidoglycan, Archaebacteria have cell walls composed of various different substances.
- Eubacteria have ester-linked straight-chain membrane lipids (fatty acids). Archaebacteria have ether-linked branched-chain member lipids.
- Eubacteria and Archaebacteria have differences in their DNA replication and transcription systems that suggest independent elaboration in these two groups 1.
- Eubacteria usually use N-formylmethionine as the initial amino acid of a protein, while Archaebacteria use plain methionine.
- The translation apparatus of Eubacteria is inhibited by such antibiotics as chloramphenicol, cycloheximide, tetracycline, streptomycin, and kanamycin, while the translation apparatus of Archaebacteria is not.
- The translational apparatus of Archaebactera is inhibited by diphtheria toxin, while the translational apparatus of Eubacteria is not.
All other living organisms are eukaryotes.
References
- Leipe DD, Aravind L, Koonin EV., "Did DNA replication evolve twice independently?", Nucleic Acids Res. 1999 Sep 1;27(17):3389-401. [1].
