Prokaryotic cells

Prokaryotic cells may have been among the earliest forms of life on Earth. They first appeared around 3.5 billion years ago when the surface of the Earth was a very hostile environment. Scientists believe that these early cells we re adapted to living in extremes of salinity, pH and temperature.
These organisms are known as extremophiles and they still exist today. They can be found in hydrothermal vents and salt lakes- similar environments to those believed to have made up the early Earth. They are usually of the domain Archaea and more recently they have been found in more hospitable environments such as soil and the human digestive system.
Prokaryotic organisms are always unicellular with a relatively simple structure. Their DNA is not contained within a nucleus, they have few organelles and the organelles they do have are not membrane-bound.

The structure of the DNA contained within prokaryotes is fundamentally the same as in eukaryotes but it is packaged differently. Prokaryotes generally only have one molecule of DNA, a chromosome, which is supercoiled to make it more compact. The genes on the chromosome are oft en grouped in to operons, meaning a number of genes are switched on or off at the same time.
The ribosomes in prokaryotic cells are smaller than those in eukaryotic cells. Their relative size is determined by the rate at which they settle, or form a sediment, in solution. The larger eukaryotic ribosomes are designated 80S and the smaller prokaryotic ribosomes, 70S. They are both necessary for protein synthesis, although the larger 80S ribosomes are involved in the formation of more complex proteins.

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