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Prokaryotes are smaller and simpler than eukaryotic cells. They have no cell nucleus. They can multiply faster than eukaryotic cells, mainly for two reasons: 1) They have shorter genetic instructions to be replicated; and 2) The replication process goes about ten times as fast. Prokaryotes don't combine and specialize to form multicelled creatures. Prokaryotes are also called bacteria. They come in a wide variety of types; their diversity is much greater than that of eukaryotes.

Prokaryotes were here first, appearing very soon after Earth had cooled enough for life to survive. The oldest rocks that could contain recognizable fossils contain evidence of domelike structures left by colonies of cyanobacteria and other bacteria. Even older rocks contain chemical evidence that the metabolism of these bacteria was under way (5).

Prokaryotes are divided into two major subkingdoms: eubacteria and archaebacteria. Eubacteria, or "true bacteria", are more familiar and ubiquitous, thriving in soil, water, our own mouths, etc. Archaebacteria differ from eubacteria in some basic ways. For example, their ribosomes (nanoscale protein factories) have a different shape. In fact, archaebacteria are in some ways more similar to eukaryotes than to eubacteria. Biologists now think, based on the reconstruction of genetic "trees," that archaebacteria are the oldest kind of cell. Today some biologists maintain that archaebacteria constitute a third domain of life which could be called simply archaea (6-8).

There are four types of archaea. Two are known for their ability to inhabit extremely hostile environments such as very salty brines, and boiling springs or ocean thermal vents. The third group of can metabolize some very unappetizing chemicals to make methane. A fourth type, the sulfate-reducers, were recently distinguished from the others (9).

eukaryote / The ESG Biology Hypertextbook

Returning to the computer analogy, the relationship between prokaryotes and eukaryotes is like the relationship between handheld calculators and desktop personal computers. Both kinds of cells come in a broad range of sizes. Prokaryotes are, on average, about an order of magnitude smaller, like handheld calculators. And they come in a wide variety, each with a narrow special purpose. Consider scientific calculators, inventory scanners, GPS units, cellphones, cordless phones, pagers, beepers, walkie-talkies, PDAs, TV remote controllers, keyless entry buttons, Gameboys, Walkmans, iPods, guitar tuners, electronic or medical diagnostic kits, digital cameras, smoke detectors, portable radios, digital thermometers and cordless shavers. Like eukaryotes, personal computers have greater memory capacity, have more complicated structure, and can be networked (eukaryotes form multicelled creatures).

The size of a cell's genome can be compared to the amount of programming stored in a computer, using the equation, 4 nucleotides = 8 bits = 1 byte. The simplest prokaryotic cell would correspond to a handheld calculator with about 200 kilobytes of stored programs; the E. coli bacterium would correspond to a handheld calculator with about 1.2 megabytes of stored programs. Among eukaryotic cells, counting the backup copy of the genome and the "silent" DNA, a yeast cell would correspond to a personal computer with 12 megabytes of program storage capacity; a human cell corresponds to a personal computer with 1.5 gigabytes of program storage capacity. And the human body would correspond to a computer network of a hundred trillion (10^14) or more such units.