Prokaryotes
Prokaryotes encompass two entire domains of life, the Bacteria and Archaea. All of these organisms are single-celled[25] and lack a nuclear membrane, as well as membranes around other cellular organelles. Due to the lack of a membrane bound nucleus, their DNA is located directly in the cytoplasm. The genome tends to be made up of one or two circular chromosomes, but may also be organized as linear strands bound to the cytoplasmic membrane[1][2]. Prokaryotic cells tend to be about 1 to 100 μm3 in volume. This is opposed to Eukaryotes, whose cells are much larger (1 to 106 μm3), genomes linear, and nuclei and organelles membrane-bound[5].
Fossil evidence[4, p515-516] records only prokaryotes from 3.5 billion years ago to 2.1 billion years ago, making prokaryotes the oldest known organisms and root of the tree of life. |
Bacteria & Archaea
As stated above, both domains share a number of features that distinguish them from eukaryotes. However there are also differences between the two, and genetic similarities between archaea and eukarya (such as enzymes involved in transcription and translation[26]) that might suggest the hypothesis that archaea form a clade with eukarya, rather than bacteria. Archaea are often adapted to thrive in extremely harsh environments, such as extreme temperatures, pHs, pressures[7, first paragraph], or salinity.
Viruses
Viruses lack many characteristics usually possessed by living organisms, such as having cellular structure or metabolic processes. However they have their own genome and evolve by natural selection, so viruses are often considered at the edge of life[3]. Viruses replicate by parasitizing living cells, and each strain of virus is able to infect only a limited range of organisms[4]. Infection and replication can follow two patterns. In the lytic cycle, the virus injects its genome into the host cell, where it uses the host cells organelles to express it's genes into the materials for new viruses, which then assemble and lyse the cell. The other option, the lysogenic cycle, has the viral genome combine with the host cells genome, which will continue operating normally, reproducing the viral DNA. Occasionally the viral genome will separate from the hosts' and initiate the lytic cycle, creating more viruses[4, p386].
|
Prions
Another infectious agent, prions, are misfolded, protease-resistant[19] proteins that have the ability to propagate their misfolded state throughout the other normal versions of that protein. Unlike viruses, there is no debate on whether prions are living[16], however they are hypothesized to cause a number of diseases in the organisms they infect[4, p393-394].
References
[1]http://euarch.blogspot.com/
[2]http://en.wikipedia.org/wiki/Prokaryote
[3]http://en.wikipedia.org/wiki/Virus#Life_properties
[4]Campbell & Reece, Biology 8th Edition
[5]https://microbewiki.kenyon.edu/index.php/Three_Domains_of_Life
[7]http://web.mst.edu/~microbio/bio221_1999/s_sulfataricus.html
[16]http://scienceline.ucsb.edu/getkey.php?key=2648
[19]http://www.stanford.edu/group/virus/prion/prion2.html
[25]http://www.uic.edu/classes/bios/bios104/mike/bacteria01.htm
[26]http://microbewiki.kenyon.edu/index.php/Three_Domains_of_Life
[2]http://en.wikipedia.org/wiki/Prokaryote
[3]http://en.wikipedia.org/wiki/Virus#Life_properties
[4]Campbell & Reece, Biology 8th Edition
[5]https://microbewiki.kenyon.edu/index.php/Three_Domains_of_Life
[7]http://web.mst.edu/~microbio/bio221_1999/s_sulfataricus.html
[16]http://scienceline.ucsb.edu/getkey.php?key=2648
[19]http://www.stanford.edu/group/virus/prion/prion2.html
[25]http://www.uic.edu/classes/bios/bios104/mike/bacteria01.htm
[26]http://microbewiki.kenyon.edu/index.php/Three_Domains_of_Life