Ribosome/ribosomal

Ribosomes Ribosomes are organelles that process the cell s genetic instructions to create proteins. These organelles can float freely in the cytoplasm or be connected to the endoplasmic reticulum. 

Ribosomal RNA (rRNA), vtrhich is the most abundant type of RNA in the cell. It is used as a structural component of the ribosome. Ribosomal RNA associates with ribosomal proteins to form the complete, functional ribosome. 

The transcription of the DNA gives three kinds of RNA ribosomal, messenger, and transfer. The most abundant RNA is rRNA. Most rRNAs are large and are found in combination with proteins in the ribonucleoprotein complexes called ribosomes. Ribosomes are subcellular sites for protein synthesis. 

Table 5-4. There are many others.302 In addition, there are thousands of different mRNAs within a cell. The most abundant RNA molecules in cells are those of the ribosomes. Ribosomes consist of two elaborate RNA-protein subunits, a large subunit with sedimentation constant 30S in bacteria and 40S in eukaryotes.

Ogasawara, T., Sawasaki, T., Morishita, R., Ozawa, A., Madin, K., and Endo, Y. (1999) A new class of enzyme acting on damaged ribosomes ribosomal RNA apurinic site specific lyase found in wheat germ. EMBO J. 18, 6522-6531. 

The steps of initiation occur on the isolated small subunit (30S) of the prokaryotic ribosome. Ribosomes contain two subunits, a 30S and 50S subunit, which associate to form a 70S particle. (The S values refer to the rate at which each component sediments in the ultracentrifuge they don t always add up.) In general, the 30S subunit is mostly involved in the decoding and tRNA-mRNA interaction process, while the 50S subunit is involved in actual peptide bond synthesis. Ribosomal subunits are dissociated prior to the initiation reaction. 

The transcribed mRNA is bound by a particle called a ribosome. Ribosomes are huge complexes consisting of fifty-two separate proteins (of which several are present in multiple copies) and three pieces of RNA with lengths of 120,1,542, and 2,904 nucleotides. The ribosome can be readily broken down into two large pieces, called the 30S sub-unit and the 50S subunit.6 Incredibly, the ribosome is self-assembling. Experiments have shown that when ribosomes are separated into their components and then remixed, under the right conditions the components will spontaneously reform ribosomes. 

Ribosomes Ribosomes interpret cellular information from the nucleus and so synthesize appropriate proteins, as required. 

The mechanism of toxicity for aminoglycosides has not been fully explained and is therefore unclear. It is known that the drug attaches to a bacterial cell wall and is drawn into the cell via channels made up of a protein, porin. Once inside the cell, the aminoglycoside attaches to the 30S bacterial ribosomes. Ribosomes are the intracellular structures responsible for manufacturing proteins. This attachment either inhibits protein biosynthesis or causes the cell to produce abnormal, ineffective proteins. The bacterial cell cannot survive with this impediment. This explanation, however, does not account for the potent bactericidal properties of these agents, since other antibiotics that inhibit the synthesis of proteins (such as tetracycline) are not bactericidal. Recent experimental studies show that the initial site of action is the outer bacterial membrane. The cationic antibiotic molecules create fissures in the outer cell membrane, resulting in leakage of intracellular contents and enhanced antibiotic uptake. This rapid action at the outer membrane probably accounts for most of the bactericidal activity. 

Ribosomal RNA (rRNA) is the major component of ribosomes (Chapter 30). In prokaryotes there are three kinds of rRNA, called 23S, I6S, and 5S RNA because of their sedimentation behavior. One molecule of each of these species of rRNA is present in each ribosome. Ribosomal RNA was once believed to play only a structural role in ribosomes. We now know that rRNA is the actual catalyst for protein synthesis. 

Ribosomes. Ribosomes are the protein factories. They are particles consisting of proteins and another type of RNA called ribosomal RNA (rRNA). The fact that ribosomes are the loci of protein synthesis was only discovered in the early 1950s as a result of incorporation studies with radioactively tagged amino acids. Now, ribosomes are known to be large ribonucleoprotein particles on which the message carried by mRNA is translated into the protein s amino acid sequence. 

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