Ribosomes Are the Site of Protein Synthesis

We considered the structure of the ribosome in some detail in the previous chapter without referring to those sites that are functionally important in protein synthesis. Here we can localize some of the functional sites on the two ribosomal subunits (fig. 29.6). The mRNA binds to the smaller ribosomal subunit. The peptidyl transferase is an integral part of the 50S subunit, and the elongation factor EF-G binds to the 50S subunit. The nascent polypeptide chain exits through a channel in the 50S subunit. Two functional sites occur on 

Before we discuss the reactions that occur on the ribosome, it is useful to consider the crucial interaction between the anticodon on the tRNAs and the codons on the mRNAs. 

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Ribosomes Bacterial cells contain about 15,000 ribosomes. Each is composed of a small (SOS) subunit and a large (50S) subunit. The mass of a single ribosome is 2.3 X 10 daltons. It consists of 65% RNA and 35% protein. Ribosomes are the sites of protein synthesis. The mRNA binds to ribosomes, and the mRNA nucleotide sequence specifies the protein that is synthesized. 

NUCLEOPROTEINS. Nucleoprotein conjugates have many roles in the storage and transmission of genetic information. Ribosomes are the sites of protein synthesis. Virus particles and even chromosomes are protein-nucleic acid complexes. 

Ribosomes (79-87) are small organelles 17-23 nm in diameter. They can exist in clusters known as polysomes or be attached to the er where they bind to pores in the er membrane. A major constituent of the er pore is translocon, the heterotrimetric Sec 61 protein complex. Sec 61 binds to the 80s ribosomes (86). Ribosomes consist of subunits, a 30s subunit (16srRNA and 21 proteins), and a 50s subunit (23s and 5s RNAs, > proteins and the catalytic site of peptidyl transferase). Ribosomes are the sites of protein synthesis. 

Ribosomes Are the Site of Protein Synthesis The Genetic Code... 

Ribosomes are the site of protein synthesis and exist (1) in the cytoplasm as rosette-shaped groups called polysomes (in immature red blood cells there are usually five per group) (2) on the outer face of the RER or (3) in the mitochondrial matrix, although this last type is different in size and shape from ribosomes in the cytoplasm. Ribosomes are composed of RNA and protein and range in size from 15 to 20 nm. Their central role in protein synthesis is described in Chap. 16. 

Within the cytoplasm are specialized strnctnres called organelles that carry out specific functions in the cell. The ribosomes are the sites of protein synthesis. The mitochondria are the energy-prodncing factories of the cells. A mitochondrion has an outer and an inner membrane, with an intermembrane space between them. The fluid section surrounded by the inner membrane is called the matrix. Enzymes located in the matrix and along the inner membrane catalyze the oxidation of carbohydrates, fats, and amino acids. All these oxidation pathways eventnally produce CO2, H2O, and energy, which is used to form energy-rich componnds. Table 18.1 summarizes some of the functions of the components in animal cells. 

Ribosomes are ancient ribonucleoprotein complexes that are the sites of protein synthesis in living cells. Their core structures and fundamental functional mechanisms have been conserved throughout the three domains of life bacteria, archaea and eukaryotes. All ribosomes are organized into two subunits that are defined by their apparent sedimentation coefficient, measured in Svedberg units (S). There is a general... 

The eytoplasm is a viscous fluid and contains within it systems of paramount importance. These are the nucleus, responsible for the genehc make-up of the cell, and the ribosomes, whieh are the site of protein synthesis, hi addihon are found granules of reserve material suehas polylydioxybutyric add, an energy reserve, and polyphosphate or volutin granules, the exact funchon of which has not yet been elucidated. The prokaiyohc nueleus or bacterial chromosome exists in the cytoplasm in the form of a loop and is not surrounded by a nuclear membrane. Bacteria cany other chromosomal elements episomes, which are portions of the main chromosome that have become isolated firm it, and plasmids, whieh may be called miniature chromosomes. These are small annular pieees of DNA whieh carry a limited amount of genetic information. 

The internal volume bounded by the plasma membrane, the cytoplasm (Fig. 1-3), is composed of an aqueous solution, the cytosol, and a variety of suspended particles with specific functions. The cytosol is a highly concentrated solution containing enzymes and the RNA molecules that encode them the components (amino acids and nucleotides) from which these macromolecules are assembled hundreds of small organic molecules called metabolites, intermediates in biosynthetic and degradative pathways coenzymes, compounds essential to many enzyme-catalyzed reactions inorganic ions and ribosomes, small particles (composed of protein and RNA molecules) that are the sites of protein synthesis. 

Mitochondria can have different shapes, depending on the kind of cell they are in. The number of mitochondria present in a cell also varies with cell type and may range from a single large mitochondrion to thousands. The region inside the inner membrane is called the matrix. This is where the Krebs cycle that converts pyruvate into C02 and energy takes place, so it contains a lot of enzymes. Mitochondria contain ribosomes, small particles composed of RNA and protein that are the sites of protein synthesis. Mitochondria also contain their own special DNA. 

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. 

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.)... 

Protein biosynthesis is a complex process requiring ribosomes, messenger RNA (mRNA), transfer RNA (tRNA), and a number of protein factors. The ribosome is the site of protein synthesis. The mRNA and tRNA, which are bound to the ribosome in the course of protein synthesis, are responsible for the correct order of amino acids in the growing protein chain. 

The ribosomes, which are the site of protein synthesis, consist of particles of 100—200 A in diameter, built up from several kinds of proteins and RNA. Whereas the ribosomes of higher organisms form a sediment at 805, those of bacteria do so at 705. The latter are rich in magnesium, upon withdrawal of which (e.g. with EDT A), they split at once into two particles, one of 305and one of 505. Eukaryotic (805) ribosomes have much less magnesium, and are much harder to split. These properties argue for a structural difference between prokaryotic and eukaryotic ribosomes, even though the process of protein synthesis follows a similar course with both (Vazquez, 1964). 

Ribosomal RNA (rRNA) constitutes 80-85% of the total RNA of the cell. It is located in the cytoplasm in organelles called ribosomes, which are about 65% rRNA and 35% protein. The function of ribosomes as the sites of protein synthesis is discussed in Section 11.8. 

The hierarchy of protein structure is illustrated in hgure 11.4. Here too we have a wealth of structural information. The quaternary structures for many proteins are now known and generally available in databases. As complex as these are, this is not the end of the story. We have atom-by-atom structures for entities as complex as viruses and the ribosome, an intracellular RNA-protein complex and the site of protein synthesis. Modem structural biology continues to provide detailed insights into some of the most complex constracts of nature. We are better off for having these insights. 

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. 

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—... 

RNA also differs from DNA in that there are not the same regularities in the overall composition of its bases and it usually consists of a single polynucleotide chain. There are different types of RNA, which fulfill different functions. About 80% of the RNA in a cell is located in the cytoplasm in clusters closely associated with proteins. These ribonucleoprotein particles specifically are called ribosomes, and the ribosomes are the sites of most of the protein synthesis in the cell. In addition to the ribosomal RNA (rRNA),... 

RNA differs from DNA in three ways the sugar is D-ribose, the pyrimidine uracil replaces thymine (the other three bases are the same), and the molecules are mainly single-stranded. The three principal types of RNA are messenger RNA (involved in transcribing the genetic code), transfer RNA (which carries a specific amino acid to the site of protein synthesis), and ribosomal RNA. 

Deoxyribonucleic acid is the genetic material such that the information to make all the functional macromolecules of the cell is preserved in DNA (Sinden, 1994). Ribonucleic acids occur in three functionally different classes messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA) (Simons and Grun-berg-Manago, 1997). Messenger RNA serves to carry the information encoded from DNA to the sites of protein synthesis in the cell where this information is translated into a polypeptide sequence. Ribosomal RNA is the component of ribosome which serves as the site of protein synthesis. Transfer RNA (tRNA) serves as a carrier of amino acid residues for protein synthesis. Amino acids are attached as aminoacyl esters to the 3 -termini of the tRNA to form aminoacyl-tRNA, which is the substrate for protein biosynthesis. 

Electron microscopy reveals smaller features such as ribosomes (the site of protein synthesis) or centrioles (organizers of cell division), which are examples of macromolecular structures. Current research suggests that there may be many other macromolecular structures, often highly dynamic, such as assemblies of receptor protein kinases with substrates and adaptor molecules, or components of the cytoskeleton or nuclear matrix. 

The genetic information is contained within the chemical structure of DNA, deoxyribonucleic acid, namely the sequence of bases along the polynucleotide chain. RNA, ribonucleic acid, is used by an organism to act as a messenger between the genes and the site of protein synthesis, the ribosome. The genes of all cells and many viruses are made of DNA. 

Three types of RNA are fonnd in cells. Transfer RNA (tRNA) carries amino acids to the site of protein synthesis. Ribosomal RNA (rRNA) along with protein makes np ribosomes (the mechanism that synthesizes protein). Messenger RNA (mRNA) is the code or template for protein synthesis. Special enzymes synthesize the different forms of RNA. 

How does ribosomal RNA combine with proteins to form the site of protein synthesis Ribosomal RNA molecules tend to be quite large and are complexed with proteins to form ribosomal subunits. Ribosomal RNA also exhibits extensive internal hydrogen bonding. 

Today, the evolution of genes, programmed cell death (apoptosis), and the action of messenger RNA (mRNA) are three major targets of research. mRNA contains the blueprint for every protein in the body. It is transcribed from a DNA template, and carries information to ribosomes, the sites of protein synthesis. The sequences of nucleic acid polymers are translated by transfer RNA (tRNA) into amino acid polymers. tRNA recognizes the three-nucleotide sequences that encode each amino acid. Ribosomal RNA directs the ribosome s production of proteins. Codons carry the messages that terminate protein synthesis. 

Most of the RNA in cells is present in those parts known as ribosomes, which contain both protein and RNA, much of the latter being r-RNA. The ribosomes are the sites of synthesis of the proteins, and for this to take place, amino acids are supplied attached to t-RNA. The appropriate instructions for assembly of these are provided by the m-RNA, but the precise function of the r-RNA is not completely clear. The m-RNA carries coded instructions which have been transcribed from DNA in the nucleus by the process outlined above. 

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