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Prokaryotes include the bacteria and archaea domains. The underlining difference between prokaryotic and eukaryotic organisms is that prokaryotes do not have a nucleus. That is, they do not have a membrane-bound organelle that contains all the genetic information of the cell. In fact, prokaryotes do not have any membrane-bound organelle whatsoever, and this includes organelles such as the mitochondria and the endoplasmic reticulum that are commonly found in eukaryotes. All prokaryotes have a cell wall that surrounds the cell membrane and a region called the nucleiod (or nucleoid region) that usually contains one large double-stranded circular DNA molecule. The nucleoid, which does not have a membrane, also contains a smaller DNA fragment called the plasmid. Plasmids can be very important because they can give the cell resistance to drugs. Plasmids replicate independently of the large DNA molecule and this means that they can be easily passed down from one prokaryotic cell to another one via a hair-like appendage called pili. Prokaryotes, just like eukaryotes, contains ribosomes that are responsible for synthesizing proteins. However, the ribosomes in prokaryotes consists of slightly different subunits (30s and 50s in prokaryotes and 40s and 50s in eukaryotes). Prokaryotes also contain a structure called the flagellum that gives the cell its motility. Prokaryotic cells contain flagellum that consists of a different type of protein than compared to its eukaryotic counterpart. There are three common shapes that prokaryotic cells can exist in. They can be round (cocci), rod-shaped (bacilli) or helical (spirilla).
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