Prokaryotic cells lack a nucleus and store genetic material in a nucleoid and plasmids. They include Bacteria and Archaea and played a significant role in converting CO2 into oxygen. Eukaryotes emerged 600 million years ago, and the oxygen catastrophe caused mass extinction.
Prokaryotic cells are primitive cells that lack a nucleus. Instead of storing genetic material in well-organized chromosomes protected by a membrane-bound nucleus, prokaryotes store their genetic material in an irregularly shaped “nucleoid” made up of 60% DNA. Substantial DNA is also stored in small organelles outside the nucleoid called plasmids, which are small rings of genetic material (1-400 kilobases) that are copied along with the normal process of cell division and can also be exchanged between prokaryotes. Plasmid exchange is partially responsible for prokaryotes’ ability to quickly adapt to external threats such as antibiotics.
Prokaryotes include the domains Bacteria and Archaea, which are two of the three domains of all life, the other being Eukaryota, which contains all the multicellular organisms we are familiar with. Most prokaryotes are free-floating and independent, although some form various types of colonies and cyanobacteria even show some degree of cellular differentiation which can be interpreted as primitive plant characteristics such as stems, vines, etc.
The division between eukaryotes and prokaryotes is considered to be the most significant division in the entire realm of life. Simple prokaryotes of the Archaean domain have been around for 3.8 billion years or more, while eukaryotes have only been around for about 600 million years, a difference of more than a factor of six. Many astrobiologists and space enthusiasts hope to find eukaryotic life on other planets, where conditions are too extreme for the emergence of prokaryotic life to be very likely. One potential location often cited is the methane lakes of Saturn’s moon Titan.
Perhaps the greatest achievement of prokaryotes has been what made other life forms possible: the mass conversion of atmospheric CO2 into oxygen. This happened about 2.4 billion years ago and has been called the oxygen catastrophe, because it caused mass extinction among organisms not adapted to breathing the new air. We can pinpoint the date of the Oxygen Catastrophe because its occurrence led to the oxidation of most of the free iron on the earth’s surface, producing an abrupt transition from iron to rust in the strata.
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