Prokaryotes cell structure

Prokaryotic cells have only a single membrane, the plasma membrane or cell membrane. Because they have no other membranes, prokaryotic cells contain no nucleus or organelles. Nevertheless, they possess a distinct nuclear area where a single circular chromosome is localized, and some have an internal membranous structure called a mesosome that is derived from and continuous with the cell membrane. Reactions of cellular respiration are localized on these membranes. In photosynthetic prokaryotes such as the cyanobacteria,... 

Fig. 3-7 A thin section through a prokaryotic cell. Note that the nuclear material (N) is not bound by a membrane, but is free in the cytoplasm. Mitochondria and other intracytoplasmic structures are absent. (Reprinted with permission from J. J. Cardamone, Jr., Univ. of Pittsburgh/Biological Photo Service.)...

Busby S, Ebright RH Promoter structure, promoter recognition, and transcription activation in prokaryotes. Cell 1994 79 ... 

Physically, all these prokaryotes are small, diameter about 1.0 pm and are of rigid, simple shape. They usually have little or no internal structure so that chemical diffusion is relatively rapid. Secondary compartments are rare but vesicles and vacuoles (even nuclei) are found in a very few large bacteria. We shall see that all the prokaryote cells have controlled, autocatalytic, internal metabolism, but are relatively little affected by external circumstances, except by shortage of nutrients. 

Prokaryote Cell or organism lacking a membrane-bound, structurally discrete nucleus and other subcellular compartments. Bacteria are prokaryotes (see also eukaryote). 

Complex multicellular cells, such as those of plants and humans, are termed eukaryotes. The cell structure is considerably more complex than that of the prokaryote cells (see Fig. A2.3 for a human eukaryote cell plant cells are not shown they have a well-defined cell wall and different structure). 

Lipid transfer peptides and proteins occur in eukaryotic and prokaryotic cells. In vitro they possess the ability to transfer phospholipids between lipid membranes. Plant lipid transfer peptides are unspecific in their substrate selectivity. They bind phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and glycolipids. Some of these peptides have shown antifungal activity in vitro The sequences of lipid transfer proteins and peptides contain 91-95 amino acids, are basic, and have eight cysteine residues forming four disulfide bonds. They do not contain tryptophan residues. About 40% of the sequence adopts a helical structure with helices linked via disulfide bonds. The tertiary structure comprises four a-helices. The three-dimensional structure of a lipid transfer peptide from H. vulgare in complex with palmitate has been solved by NMR. In this structure the fatty acid is caged in a hydrophobic cavity formed by the helices. 

Although all tetracyclines have a similar mechanism of action, they have different chemical structures and are produced by different species of Streptomyces. In addition, structural analogues of these compounds have been synthesized to improve pharmacokinetic properties and antimicrobial activity. While several biological processes in the bacterial cells are modified by the tetracyclines, their primary mode of action is inhibition of protein synthesis. Tetracyclines bind to the SOS ribosome and thereby prevent the binding of aminoacyl transfer RNA (tRNA) to the A site (acceptor site) on the 50S ri-bosomal unit. The tetracyclines affect both eukaryotic and prokaryotic cells but are selectively toxic for bacteria, because they readily penetrate microbial membranes and accumulate in the cytoplasm through an energy-dependent tetracycline transport system that is absent from mammalian cells. 

Ribosomal RNAs (rRNAs) are found in association with several proteins as components of the ribosomes—the complex structures that serve as the sites for protein synthesis (see p. 433). There are three distinct size species of rRNA (23S, 16S, and 5S) in prokaryotic cells (Figure 30.2). In the eukaryotic cytosol, there are four rRNA size species (28S, 18S, 5.8S, and 5S). [Note "S" is the Svedberg unit, which is related to the jnolecular weight and shape of the compound.] Together, rRNAs make up eighty percent of the total RNA in the cell. 

Microbes are relatively simple in structure in that, when viewed under the light and electron microscope, few complex structures are observed. The general structure of prokaryotic cells is shown in Fig. 5.4<5). 

Large group of organisms that do not have organelles enclosed in cell membranes and have DNA in both a chromosome and circular plasmids. They have a protein and complex carbohydrate cell wall over a plasma membrane. Although eukaryotic and prokaryotic cells are structurally different, their basic biochemical processes are similar. Volume 1(1, 2), Volume 2(3). 

Cells are broadly classified as either prokaryotes or eukaryotes. Prokaryotic cells are found in simpler organisms, such as bacteria. They do not have a membrane enclosed nucleus, but their DNA is dispersed in the cytoplasm of the cell. Eukaryotic cells are found in all members of the animal kingdom. Their structures contain distinct membrane encapsulated compartments, such as the nucleus, which contains their DNA, mitochondria and lysomes. These separate compartments are known as organelles. 

INTRON A region of a gene (i.e., ENA) that is transcribed in the synthesis of RNA, but enzymatically removed (by "splicing") from the final mRNA before its translation into an amino acid sequence in protein introns are characteristic of gene structure in eukaryotic, but not prokaryotic, cells. (See also EXON and CODING SEQUENCE)... 

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