Prokaryotic cells, distinguishing features

Micro-organisms, the major form of organisms for biotechnology, can be split into several groups based upon biochemical activity and structure. The basic classification of the protist is shown in Fig. 5.3. There are two major types, the Prokaryotes and the Eukaryotes and Table 5.4 illustrates the main distinguishing features between the two cell types. The major differences are reviewed in the following sections. 

Mycoplasmas differ from other prokaryotes by their lack of a cell wall. They are unable to produce even precursors of bacterial cell wall polymers, unlike L-forms of bacteria that can do so under the right environmental conditions. Their size is another distinguishing feature they are the smallest self-rephcating prokaryotes, with coccoid forms of only 0.3 pm diameter capable of reproduction. Their genome size is approximately one-sixth that of Escherichia coli. 

Organisms (excluding viruses) can be broadly separated into prokaryotes and eukaryotes on the basis of cellular structure. Prokaryotic cells contain no nuclear membranes and their DNA is not arranged in chromosomes. Eukaryotic cells always contain a nuclear membrane and their DNA is usually in chromosomal form. There are other differences between the two cell types but these are the most useful general distinguishing features. The prokaryotes comprise all bacteria (including cyanobacteria) and are therefore unicellular (also termed the Monera). All other organisms are eukaryotes, either unicellular (the Protista) or multicellular. 

Table 1.1 The main features distinguishing prokaryotic and eukaryotic cells... 

Table 5.4. Features Distinguishing Prokaryotic and Eukaryotic Cells...

Differences between bacteria and human cells. Different species of bacteria have some common structural features that distinguish them from animal cells. They are single-cell organisms that are prokaryotes ( before nucleus ). Their genetic material (DNA) is concentrated in the central region of the cell called a nucleoid, rather than a nucleus, because it is not separated from the rest of the cellular contents by a membrane. Likewise, bacteria contain no cytoplasmic organelles defined by membranes. They do have a plasma membrane that encloses the cytoplasm. External to this membrane is a peptidoglycan cell wall composed of extensively cross-linked polysaccharides that form a protective shield on the surface of the cell. 

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