Procaryotic cells

The development of electron microscopy showed that there was a major dichotomy between the various groups relating to the internal structure and organisation of cells. Two very different types of cells were discovered, the small, relatively simple procaryotic cell, and the more complex eucaryotic cell which is usually considerably larger. 

The bacteria are procaryotic cells. This group also includes the organisms which used to be known as the Blue-green Algae or Cyanophyceae, and are now known as the Cyanobacteria. 

Procaryotic cells are surrounded by a cell membrane, a thin flexible sheet composed of protein and lipid. This has an important function in regulating the molecules which can pass into and out of the cell. 

In the great majority of cases, there is a rigid cell wall outside the membrane. 

In the procaryotes, there is no discrete nucleus surrounded by a nuclear membrane, and the deoxyribonucleic acid (DNA) is not associated with the basic proteins known as histones. The DNA consists of covalently closed circles. 

---

Girbes, L. Citores, and F. Soriano. Primary structure of omega-hordothionin, a member of a novel family of thionins from barley endosperm, and its inhibi- HV028 tion of protein synthesis in eucaryotic and procaryotic cell-free system. Eur J Biochem 1996 239(1) 67-73. 

Eucaryotic cells are generally more complex than procaryotic cells and possess a variety of membrane-bound compartments called organelles. These intracellular membranes allow diverse and more specialized functions. For detailed information, the reader is recommended to consult specialized textbooks [2, 8, 9]. 

The principle distinguishing features of the procaryotic cell are 1) absence of internal membranes which separate the resting nucleus from the cytoplasm, and isolate the enzymatic machinery of photosynthesis and of respiration in specific organelles 2) nuclear division by fission, not by mitosis, a character possibly related to the presence of a single structure which carries all the genetic information of the cell and 3) the presence of a cell wall which contains a specific mucopeptide as its strengthening element, (p. 21 in Stanier and van Neil 1962) . 

All these organisms share the distinctive structural properties associated with the procaryotic cell..., and we can therefore safely infer a common origin for the whole group in the remote evolutionary past... [40]... 

In eucaryotic cells, glycolysis, gluconeogenesis and fatty acid synthesis takes place in the cytosol, while the Krebs cycle is isolated within mitochondria glycogen is made in glycogen granules, lipid is synthesized in the endoplasmic reticulum and lysosomes carry on a variety of hydrolytic activities. As in procaryotic cells, ribosomes in the cytosol are the site of protein synthesis. 

For the complete structures of ferrocytochrome c (Fe- ). see Takano, T. Trus, B. L. Mandel. N. Mantlet, G. Kallai, O. B. Swan.son, R. Dickerson. R. E. J. Biol. Chem. 1977, 252. 776-785. Sec Stryer. L. Biochemistry, 3rd cd. Freeman New York, 1981 pp 328-329. Dickerson, R. E. Sci. Amer. 1972, 226(4) pp 58-72. Eucaryotic cells have their DNA in true nuclei, as opposed to procaryotic cells (bacteria and blue-green algae) which do not. u The prefix met- is used to signify that the iron atom, normally in the -I-2 oxidation state, has been oxidized to -t-3. 

Two systems have recently been described that result in methyl ester formation at the C-terminus of a protein. Both appear to be widely distributed in eucaryotic cells, but have not yet been observed in procaryotic cells. In one system, the C-terminal leucine residue of one or more 36 kDa cytosolic polypeptides is the target of methylation (Xie and Clarke, 1993). These methyl ester linkages are much more labile in cell extracts than would be predicted on the basis of their chemistry and this observation suggests the presence of a methylesterase activity (Xie and Clarke, 1994a). The protein substrate for this methyltransferase has recently been identified as the catalytic subunit of protein phosphatase 2A and its reversible methylation may modulate its activity (Xie and Clarke, 1994b). 

Fredrickson, A. G., Ramkrishna, D., and Tsuchiya, H. M., Statistics and Dynamics of Procaryotic Cell Populations, Mathematical Biosciences 1 (1967) 327-374. 

Sec Stryer. L. Biochemistry. 3rd cd. Freeman New York. 1981 pp 328-329. Dickerson. K. E. Sci. Amcr. 1972, 226(4) pp 38-72. Eucaryotic celts have thdr DNA in true nudei. as opposed to procaryotic cells (bacteria and blue-green algae) which do not. 

Nalidixic acid and all its analogs act by selectively inhibiting bacterial DNA synthesis. The actual biochemical target of the quinolones is the enzyme DNA-gyrase, a topoisomerase enzyme found only in procaryotic cells. This accounts for the selective toxicity of these drugs. 

Like conventional combinatorial biosynthesis, glycorandomization requires flexible glycosyltransferases. As recently pointed out in the case of novobiocin [79], a highly specific glycosyltransferase limits the library size. Despite such issues that need to be addressed in future work, glycorandomization is a promising approach to make use of the metabolic potential of procaryotic cells and should promote drug development in the future. 

Considering the Unity of Biochemistry perhaps it is more surprising that actin does not appear to play the same role in procaryotic cells. But actin is found in essentiaffy aff eucaryotes in a similar role, often paired with myosin as the contractife apparatus. 

The Cu2Zu2 enzymes can be obtained from many sources. Apart from two exceptionsthey are exclusively found in eucaryotic organisms in nearly all tissues Only the enzymes from higher species have a blocked N-terminal group. These enzymes are generally more stable. Whether this phenomenon can be ascribed to the acetylation or not is still open to discussion. Iron SOD s are exclusively prokaryotic, whereas manganese superoxide dismutases are present in procaryotic cells as well as in mitochrondria and the serum of vertebrates Their relative molecular... 

There is also a considerable difference in the ribosome structure of procaryotic and eucaryotic cells. Ribosomes are the structures upon which proteins are synthesised. Procaryotic cells possess ribosomes which are 70S in size, whereas eucaryotes have 80 S ribosomes. 

A cell membrane consisting of a protein and lipid mixture is present, but the eucaryotic cell membrane contains significant quantities of steroids, which are not found in most procaryotic cells. There may or may not be a cell wall outside the membrane depending on the type of cell. 

The Eubacteria are a vast heterogeneous group of procaryotic cells which can be classified in numerous ways. These include their morphology and staining, their nutritional, oxygen, and temperature requirements, and a variety of other biochemical factors. 

Figure 5.2 Two antibiotics which inhibit protein synthesis in procaryotic cells and which are widely usedfor suppressing bacterial growth in fungal media...

Pristinamycin inhibits the multiplication of some procaryotic cells, but has no effect on the growth of most eucaryotic cells. Pristinamycin is active against Gram positive bacteria (especially Staphylococcus, Streptococcus) some Gram negative bacteria (Haemophilus, Neisseria) Legionnella and Mycoplasma. 

There are two classes of microbes, procaryotes and eucaryotes. Procaryotic cells are usually characterized by simple unicellular structures. Exceptions are also known such as the cyanobacteria, which are often multicellular. The only procaryotes are bacteria which are also the most abundant form of life on earth. Eucaryotes encompass all other forms of life, including animal and plant. 

This conclusion is true not only for our cells and other eukaryotic cells (i.e., sophisticated cells which store their DNA in the special cell organelle called nucleus), but also for more primitive procaryotic cells (which do not... 

Whether some procaryotes contain sterols and others do not has, of course, an important bearing on evolutionary relationships. One may ask, for example, whether Methylococcus and related methanotrophs were ancestral to a line of aerobic bacteria which then abandoned the sterol pathway or whether in another independently evolving branch of aerobes sterol biosynthesis was never invented. These issues are of interest not only by themselves but also because they bear on the matter of how the wide evolutionary gap between procaryotes and eucaryotic cells was bridged. As pointed out earlier, the possession of the sterol molecule is a universal feature of eucaryotic forms of life. For eucaryotes, phylogenetic relationships are much more certain than they are for procaryotic cells so that one can speak with some confidence of eucaryotes that are primitive and those that are more advanced. In the present context it is therefore significant that the red algae, supposedly the most primitive extant eucaryotes, not only contain sterols in substantial amounts but also that in these cells the biosynthetic pathway is modern, i.e. fully developed. Cholesterol or related cholestane derivatives are the dominant sterols in all members of the order Rhodophytae so far examined. ... 

---