Ribosome eukaryotic, composition

FIGURE 11.25 The organization and composition of prokaryotic and eukaryotic ribosomes. 

The many thousands of proteins present in each cell are made within the ribosomes, which are able to read each specific mRNA that comes their way. While ribosomes appear as little more than blurred dots in most electron micrographs, the 15,000 ribosomes of one cell of E. coli represent one-fourth of the total mass of the cell. Eukaryotic cells contain many times more of these little molecular machines. When ribosomes were first observed in the early 1950s,1 3 nobody could imagine either their composition or their function. 

Whereas a prokaryotic ribosome has a sedimentation coefficient (see Topic G9) of 70S and subunits of 30S and 50S, a eukaryotic ribosome has a sedimentation coefficient of 80S with subunits of 40S and 60S (see Topic G9). The composition of eukaryotic ribosomal subunits is also more complex than prokaryotic subunits (see Topic G9) but the function of each subunit is essentially the same as in prokaryotes. 

One major difference between the prokaryotic and eukaryotic translational machinery is the composition of the two ribosomal subunits. Unlike the small... 

Evolutionary relationship of recombinases and topoisomerases Section 27.5.2 Similarities in transcriptional machinery between archaea and eukaryotes Section 28.2.4 Evolution of spliceosome-catalyzed splicing Section 28.2.4 Classes of aminoacyl-tRNA synthetases Section 29.2.5 Composition of the primordal ribosome Section 29,3.1... 

Reflecting the bacterial ancestry of mitochondria, mitochondrial ribosomes resemble bacterial ribosomes and differ from eukaryotic cytosolic ribosomes in their RNA and protein compositions, their size, and their sensitivity to certain antibiotics (see Figure 4-24). For instance, chloramphenicol blocks protein synthesis by bacterial and mitochondrial ribosomes from most organisms, but cycloheximide does not. This sensitivity of mitochondrial ribosomes to the important aminoglycoside class of antibiotics is the main cause of the toxicity that these antibiotics can cause. Conversely, cytosolic ribosomes are sensitive to cycloheximide and resistant to chloramphenicol. In cultured mammalian cells the only proteins synthesized in the presence of cycloheximide are encoded by mtDNA and produced by mitochondrial ribosomes. I... 

Contrast eukaryotic and prokaryotic ribosomes with respect to composition and size. 

The molecular mechanism of translation in eukaryotes is very similar to that in bacteria. The activation of amino acids and their attachment to tRNA molecules, and the steps of initiation, elongation, and termination of polypeptide chains, are essentially the same in overall terms. The small and large ribosomal subunits of bacteria and eukaryotes, although different is size and composition, are equivalent with respect to their roles in initiation and elongation of polypeptide chains. Differences lie mainly in the details of some steps, particularly initiation, and in the greater munher of accessory proteins involved in each step. 

Both prokaryotic and eukaryotic RNA polymerases are composed of several subunits. Although only one form of RNA polymerase predominates in bacteria and blue-green algae, in eukaroytes there are multiple forms of RNA polymerase which have different nuclear and organelle compartmentalization, function, and subunit composition. Polymerase I (or A) is located in the nucleolus and transcribes 18 S and 28 S ribosomal RNA genes. By contrast, polymerase II (or B) and III (or... 

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