Protein synthesis mRNA translation

The ultimate purpose of DNA expression is protein synthesis. mRNAs serve as the intermediate carrier of the DNA genetic information for protein synthesis. The DNA message is carried in the form of base sequences that are transferred to RNA, also in terms of base sequences, and finally these are transferred into amino acid sequences through a translation process based on the genetic code. This process of information from the RNA to the protein is called translation. 

Protein synthesis means translation into a peptide chain of a genetic message first transcribed into mRNA. Amino acid (AA) assembly occurs at the ribosome. Delivery of amino acids to mRNA involves different transfer RNA molecules (tRNA), each of which binds a specific AA. Each tRNA bears an anticodon nu-cleobase triplet that is complementary to a particular mRNA coding unit (codon, consisting of three nucleobases). 

The previous sections have introduced the major participants in protein synthesis—mRNA, aminoacylated tRNAs, and ribosomes containing large and small rRNAs. We now take a detailed look at how these components are brought together to carry out the biochemical events leading to formation of polypeptide chains on ribosomes. Similar to transcription, the complex process of translation can be divided into three stages—initiation, elongation, and termination—which we consider in order. We focus our description on translation in eukaryotic cells, but the mechanism of translation is fundamentally the same in all cells. 

The final reaction, namely protein synthesis or translation of mRNA information into amino acid sequences, then occurs via a specific anticodon of specific RNASs (Fig. 8.4.4). A triplet of nucleotides codes for a single amino acid, e.g., UUC or UUU for Phe. 

Only one RNA polymerase catalyzes the synthesis of all three classes of RNA in prokaryotes. mRNA is transcribed from DNA template carrying genetic codes necessary for protein synthesis. mRNA, which encodes for one protein is called monocitronic, whereas mRNA encoding for more than one protein are referred to as polycitronic. Prokaryotic mRNA is polycitronic and prokaryotic transcription occurs concomitantly with translation. 

Translation Second step in gene expression, the synthesis of proteins from mRNA... 

As described in the preceding sections protein synthesis involves transcription of the DNA to rtiRNA followed by translation of the mRNA as an amino acid sequence In addition to outlining the mechanics of transcription we have described the relationship among mRNA codons tRNA anticodons and ammo acids... 

Cellular protein biosynthesis involves the following steps. One strand of double-stranded DNA serves as a template strand for the synthesis of a complementary single-stranded messenger ribonucleic acid (mRNA) in a process called transcription. This mRNA in turn serves as a template to direct the synthesis of the protein in a process called translation. The codons of the mRNA are read sequentially by transfer RNA (tRNA) molecules, which bind specifically to the mRNA via triplets of nucleotides that are complementary to the particular codon, called an anticodon. Protein synthesis occurs on a ribosome, a complex consisting of more than 50 different proteins and several stmctural RNA molecules, which moves along the mRNA and mediates the binding of the tRNA molecules and the formation of the nascent peptide chain. The tRNA molecule carries an activated form of the specific amino acid to the ribosome where it is added to the end of the growing peptide chain. There is at least one tRNA for each amino acid. 

Messenger RNA (mRNA) serves to carry the information or message that is encoded in genes to the sites of protein synthesis in the cell, where this information is translated into a polypeptide sequence. Because mRNA molecules are transcribed copies of the protein-coding genetic units that comprise most of DNA, mRNA is said to be the DNA-like RNA. ... 

H Translation is the process by which mRNA directs protein synthesis. Each mRNA is divided into codons, ribonucleotide triplets that are recognized by small amino acid-carrying molecules of transfer RNA (tRNA), which deliver the appropriate amino acids needed for protein synthesis. 

Translation (Section 28.5) The process by which the genetic information transcribed from DNA onto mRNA is read by tRNA and used to direct protein synthesis. 

Increased protein synthesis Increased amino acid uptake/increased translation of mRNA Akt-mediated stimulation of system A amino acid transporter and stimulation of mRNA-translation through activation of p70S6kinase and elongation initiation factor 4 (elF4). Possible involvement of atypical PKCs... 

The rapid repression of pre-existing protein synthesis caused by anaerobic treatment is correlated with a near complete dissociation of polysomes in primary roots of soybeans (Lin Key, 1967) and maize (E.S. Dennis and A.J. Pryor, personal communication). This does not result from degradation of aerobic mRNAs, because the mRNAs encoding the pre-existing proteins remain translatable in an in vitro system at least five hours after anaerobic treatment is initiated (Sachs et al., 1980). This is in agreement... 

A ribosome is a cytoplasmic nucleoprotein stmcture that acts as the machinery for the synthesis of proteins from the mRNA templates. On the ribosomes, the mRNA and tRNA molecules interact to translate into a specific protein molecule information transcribed from the gene. In active protein synthesis, many ribosomes are associated with an mRNA molecule in an assembly called the polysome. 

Initiation of protein synthesis requires that an mRNA molecule be selected for translation by a ribosome. Once the mRNA binds to the ribosome, the latter finds the correct reading frame on the mRNA, and translation begins. This process involves tRNA, rRNA, mRNA, and at least ten eukaryotic initiation factors (elFs), some of which have multiple (three to eight) subunits. Also involved are GTP, ATP, and amino acids. Initiation can be divided into four steps (1) dissociation of the ribosome into its 40S and 60S subunits (2) binding of a ternary complex consisting of met-tRNAf GTP, and eIF-2 to the 40S ribosome to form a preinitiation complex (3) binding of mRNA to the 40S preinitiation complex to form a 43S initiation complex and (4) combination of the 43S initiation complex with the 60S ribosomal subunit to form the SOS initiation complex. 

Translation Synthesis of protein using mRNA as template. 

Synthesis of the transferrin receptor (TfR) and that of ferritin are reciprocally linked to cellular iron content. Specific untranslated sequences of the mRNAs for both proteins (named iron response elements) interact with a cytosolic protein sensitive to variations in levels of cellular iron (iron-responsive element-binding protein). When iron levels are high, cells use stored ferritin mRNA to synthesize ferritin, and the TfR mRNA is degraded. In contrast, when iron levels are low, the TfR mRNA is stabilized and increased synthesis of receptors occurs, while ferritin mRNA is apparently stored in an inactive form. This is an important example of control of expression of proteins at the translational level. 

With some RNA vimses, e.g. poliovims, the RNA strand fi cm the particle can act directly as mRNA and is translated into viral proteins on the host-cell ribosomes. In many other RNA vimses, however (e.g. the influenza vimses), the RNA strands are negative-sense RNAs (anhmessages) that have first to be transcribed to the complementary sequence by RNA-dependent RNA polymerases before they can function in protein synthesis. Sinee eukaryotie eells do not have these enzymes, the negative-sense RNA vimses must earry them in the virion. 

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