Protein synthesis steps

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. 

The aminoglycosides exert their bactericidal effect by blocking a step in protein synthesis necessary for bacterial multiplication. They disrupt the functional... 

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. 

Figure 38-6. Diagrammatic representation of the initiation of protein synthesis on the mRNA template containing a 5 cap (G" TP-5 ) and 3 poiy(A) terminai [3 (A)J. This process proceeds in three steps (1) activation of mRNA ...

A variety of compounds, including several antibiotics, inhibit protein synthesis by affecting one or more of the steps involved in protein synthesis. 

Whereas DNA has a single role as the storehouse of genetic information, RNA plays many roles in the operation of a cell. There are several different types of RNA, each having its own function. The principal job of RNA is to provide the information needed to synthesize proteins. Protein synthesis requires several steps, each assisted by RNA. One type of RNA copies the genetic information from DNA and carries this blueprint out of the nucleus and into the cytoplasm, where construction of the protein takes place. The protein is assembled on the surface of a ribosome, a cell component that contains a second type of RNA. The protein is consfructed by sequential addition of amino acids in the order specified by the DNA. The individual amino acids are carried to the growing protein chain by yet a third type of RNA. The details of protein synthesis are well understood, but the process is much too complex to be described in an introductoiy course in chemistry. 

The second step of carotenogenesis, the period of protein synthesis, has clearly been separated from the sensory transduction by means of the inhibitors of protein synthesis cycloheximide12 78 142) and chloramphenical12 146 147). Regardless of their presence, light-induction was feasible, but carotenogenesis took place only after their removal. 

In protein synthesis, there are three classes of RNA to worry about ribosomal RNA, rRNA messenger RNA, mRNA and transfer RNA, tRNA. All three classes of RNA play key roles in the final stage of the process the biosynthesis of proteins. However, we are going to take this one step at a time. We turn attention first to the... 

The final step in protein biosynthesis is chain termination. Natural mRNA molecules contain termination codons UAA, UGA, or UAG There are no tRNAs that have anticodons which are complementary to these codons. When the growing peptide chain encounters one of these termination codons, the peptidyl-tRNAis transferred to water instead of another aminoacyl-tRNA. The peptidyl-tRNA is hydrolyzed to free the completed protein and the tRNA. Chain termination completes protein synthesis. 

---