Prokaryotic cells ribosomes

Although the interior of a prokaryotic cell is not subdivided into compartments by internal membranes, the cell still shows some segregation of metabolism. For example, certain metabolic pathways, such as phospholipid synthesis and oxidative phosphorylation, are localized in the plasma membrane. Also, protein biosynthesis is carried out on ribosomes. 

Bacterial protein biosynthesis is a cascade of events which manufacture chains of amino acids before they are folded into specific structures to carry out various biological functions. Protein biosynthesis is absolutely essential for the survival of prokaryotic and eukaryotic cells. Ribosomes, macromolecular complexes made up of proteins and RNA, participate in decoding the genetic message to synthesize both essential and nonessential proteins to carry out cellular functions. 

In eukaryotic cells transcription and translation occur in two distinct temporal and spacial events, whereas in prokaryotic cells they occur in one step. As humans have eukaryotic cells, we will look at this process. Transcription occurs on DNA in the nucleus and translation occurs on ribosomes in the cytoplasm. 

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. 

There are three major types of RNA that participate in the process of protein synthesis ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA). They are unbranched polymers of nucleotides, but differ from DNA by containing ribose instead of deoxyribose and uracil instead of thymine. rRNA is a component of the ribosomes. tRNA serves as an adaptor molecule that carries a spe dfic amino acid to the site of protein synthesis. mRNA carries genetic information from the nuclear DNA to the cytosol, where it is used as the template for protein synthesis. The process of RNA synthesis is called transcription, and its substrates are ribonucleoside triphosphates. The enzyme that synthesizes RNA is RNA polymerase, which is a multisub-irit enzyme. In prokaryotic cells, the core enzyme has four subunits—... 

Eventually, one of three termination codons (also called Stop codons) becomes positioned in the A site (Fig. 7). These are UAG, UAA and UGA. Unlike other codons, prokaryotic cells do not contain aminoacyl-tRNAs complementary to Stop codons. Instead, one of two release factors (RF1 and RF2) binds instead. RF1 recognizes UAA and UAG whereas RF2 recognizes UGA. A third release factor, RF3, is also needed to assist RF1 or RF2. Thus either RF1 + RF3 or RF2 + RF3 bind depending on the exact termination codon in the A site. RF1 (or RF2) binds at or near the A site whereas RF3/GTP binds elsewhere on the ribosome. The release factors cause the peptidyl transferase to transfer the polypeptide to a water molecule instead of to aminoacyl-tRNA, effectively cleaving the bond between the polypeptide and tRNA in the P site. The polypeptide, now leaves the ribosome, followed by the mRNA and free tRNA, and the ribosome dissociates into 30S and 50S subunits ready to start translation afresh. 

Because mitochondria contain 70S-type ribosomes that function similarly to those in prokaryotic cells, these compounds also inhibit mitochondrial protein synthesis. 

The cytoplasm is densely packed with ribosomes. Unlike eukaryotic cells these are not associated with a membranous structure the endoplasmic reticulum is not a component of prokaryotic cells. Bacterial ribosomes are 70S in size, comprising two subunits of 30S and 50S. This is smaller than eukaryotic ribosomes, which are 80S in size (40S and 60S subunits). Differences will therefore exist in the size and geometry of RNA binding sites. 

Microbial cell structure is varied with a tremendous diversity in size and shape. Prokaryotic cells typically contain a cell wall, 70s ribosomes, a chromosome that is not membrane bound, various inclusions and vacuoles, and extrachromosomal DNA or plasmids. Eukaryotic microorganisms are equally varied with a variety of forms many are photosynthetic or harbor photosynthetic symbionts. Many eukaryotic cells contain prokaryotic endosymbionts, some of which contain their own set of plasmids. Given the variety of eukaryotic microorganisms, they have been labeled protists, since they are often a mixing of algal and protozoan characteristics within apparently related groups. 

The answer is e. (Murray, pp 452-467. Scriver, pp 3-45. Sack, pp 1-40. Wilson, pp 101-120.) Puromycin is virtually identical in structure to the 3 -terminal end of tyrosinyl-tRNA. In both eukaryotic and prokaryotic cells, it is accepted as a tyrosinyl-tRNA analogue. As such, it is incorporated into the carboxy-terminal position ol a peptide at the aminoacyl (A) site on ribosomes, causing premature release of the nascent polypeptide. Thus, puromycin inhibits protein synthesis in both human and bacterial cells. Streptomycin, like tetracycline and chloramphenicol, inhibits ribosomal activity. Mitomycin covalently cross-links DNA, which prevents cell replication. Rifampicin is an inhibitor of bacterial DNA-dependent RNA polymerase. 

RTA and related RIPs are toxic to most eukaryotes, but not to prokaryotes. As a result, recombinant RTA is difficult to express in yeast cells, but it can be readily overexpressed and properly folded by E. coli. The inability of RTA to disrupt protein synthesis effectively in prokaryotic cells may be due to significant divergence in the sequences of ribosomal proteins or the precise three-dimensional strucmre of rRNA near the RTA cleavage site (Gale et al., 1981 Endo et al., 1991 Vater et al., 1995). 

Prokaryotic cells are small and structurally simple. They are bounded by a cell wall and a plasma membrane. They lack a nucleus and other organelles. Their DNA molecules, which are circular, are located in an irregularly shaped region called the nucleoid. At low magnification ribosomes appear to be present in an otherwise featureless cytoplasm. 

Protein synthesis takes place on the ribosomes (Figure 27.17). The smaller subunit of the ribosome (30S in prokaryotic cells) has three binding sites for RNA molecules. It binds the mRNA whose base sequence is to be read, the tRNA carrying the growing peptide chain, and the tRNA carrying the next amino acid to be incorporated into the protein. The larger subunit of the ribosome (508 in prokaryotic cells) catalyzes peptide bond formation. 

In a prokaryotic cell, the cytosol (the fluid portion of the cell outside the nuclear region) frequently has a slightly granular appearance because of the presence of ribosomes. Because these consist of RNA and protein, they are also called ribonucleoprotein particles they are the sites of protein synthesis in aU organisms. The presence of ribosomes is the main visible feature of prokaryotic cytosol. (Membrane-bound organelles, characteristic of eukaryotes, are not found in prokaryotes.)... 

The Endosymbiotic Theory states that mitochondria and chloroplasts were once free living and possibly evolved from prokaryotic cells. At some point in our evolutionary history, they entered the eukaryotic cell and maintained a symbiotic relationship with the cell, with both the cell and organelle benefiting from the relationship. The fact that they both have their own DNA, RNA, ribosomes, and are capable of reproduction helps to confirm this theory. 

In bacterial cells, nucleic acids are found in the form of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA carries the genetic blueprint for the cell and RNA acts as an intermediary molecule to convert the blueprint into proteins [8]. RNA has three forms namely, ribosomal, messenger and transfer RNAs. All the three types of RNA are essential for protein synthesis. Ribosomal RNA is the most abundant macromolecule, next to proteins, in an actively growing prokaryotic cell. It is a major component of the ribosome, the cellular machinery used to synthesize new proteins. There are three ribosomal RNA molecules in prokaryotes namely 5S (ca. 120 nucleotides), 16S (1500 nucleotides), and 23S (2900 nucleotides). 

Functional mRNA is single-stranded. In prokaryotic cells, transcription and translation are usually coupled mRNA becomes bound to ribosomes and translation begins before transcription is complete. The messenger is usually translated by several ribosomes at once, and thus several to many protein molecules are made from it. In prokaryotes, however, mRNA lifetimes are short, with half-times of several minutes Eukaryotic mRNA is normally stable for hours or days... 

Unlike prokaryotic cells, eukaryotes contain three DNA-dependent RNA polymerase activities. Polymerase I (or A) catalyses the synthesis of ribosomal RNA precursors. Polymerase II (or B) transcribes the structural genes for proteins and polymerase III (or C) transcribes the genes for tRNA and 5S RNA. Consequendy, it is expected that the activity of these enzymes will be controlled in different ways and modem techniques of genetic manipulation have enabled considerable strides in the study of putative control regions for each of them. It cannot be said, however, that a clear consensus has yet emerged for the binding to the DNA of any of the three enzymes. 

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