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The process of translation, or protein synthesis, ..

decoding site for initiation of protein synthesis

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Translation is the central process of protein synthesis by ..

“This is the first insight into how the initiation mechanisms of protein synthesis work specifically for humans, and a step towards understanding at the molecular level what happens when a viral infection occurs,” said Doudna, a member of Berkeley Lab’s Physical Biosciences Division. “A better understanding of these mechanisms could open the door to new and improved therapies for viral infections.”

Translation is the synthesis of a protein from an mRNA template where the code in the mRNA is ..

The most active role in initiation seems to belong to IF2, which has binding sites for fMet-tRNAfMet, GTP, and both ribosomal subunits. Initiation Factor 2 promotes the association of fMet-tRNAfMet with the small subunit and, in particular, recognizes the blocked amino group of this tRNA. The activities of IF2 are partitioned between two domains of the protein—the C-terminal domain is thought to be responsible for initiator tRNA binding, while the central domain contains a GTPase function. Which portion of IF2 binds the 30S subunit remains to be determined. Hydrolysis of GTP to GDP occurs only upon 50S binding to the ternary complex; IF2 has no GTPase activity in the absence of the ribosome. As GTP hydrolysis is the final step in initiation, a conformational change in the ribosome is thought to eliminate IF2 from the initiation complex, further orient the initiator tRNA in the P-site, or otherwise contribute to a kinetic proofreading mechanism.

in mRNA during the translation phase of protein synthesis

It is the second and final step of protein synthesis

Assembly of the ribosomal subunits, mRNA, and initiator tRNA into a complex ready for protein synthesis requires several proteins called initiation factors. In prokary-otes, three initiation factors (IFs) transiently associate with the components of the translational machinery: IF1, IF2, and IF3. (In eukaryotes, more factors are required but the overall initiation process is similar with a few exceptions described below.) Table II summarizes the properties of E. coli initiation factors as well as protein factors involved in elongation and termination.

A combination of nucleotide signals identifies the beginning of an mRNA sequence to be translated into its protein product. The nucleotide triple AUG is the start codon that directs the ribosome to begin reading an mRNA and orients the message in the right frame (for example,… CUA GUG CAC C… rather than … C UAG UGC ACC…, which would be a different protein). However, AUG is also the codon for insertion of the amino acid methionine into the body of the polypeptide chain. What distinguishes the start AUG from other identical codons elsewhere in the message? A stretch of 3-10 nucleotides located about 10 nucleotides upstream (in the 5′-direction) from the start codon is called the Shine-Dalgarno sequence, after the researchers who identified it. This sequence is rich in A and G nucleotides, and is partially complementary to a short region of U and C nucleotides near the 3′-end of an RNA molecule embedded within the ribosomal small sub-unit. Such complementarity positions the incoming message properly on the ribosome, so that the start codon is in the ribosomal decoding site for initiation of protein synthesis. Once translation begins, the rest of the codons in the message need to be read, so the base-pairing interaction between mRNA and rRNA at the Shine-Dalgarno sequence must be transient.

in the first phase of translation for protein synthesis, ..

The process of polymerization of amino acids to form a polypeptide is called as Translation. It is the second and final step of protein synthesis. The order into which the amino acids are arranged is defined by the bases in mRNA (messenger). Ribosome is the cellular factory responsible for the protein synthesis. The ribosome consists of structural RNAs and about 80 different . It is in inactive stage and exists as two subunits, one large and other small. The synthesis of begins when the small subunit encounters an mRNA. The ribosome also acts as a catalyst for the formation of bonds.

Initiation of protein synthesis requires assembly of the ribosomal subunits, messenger RNA, and initiator tRNA at the start codon. This organization of translational components is facilitated by protein initiation factors (Fig. 5).

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Where there was one cell there are two, then four, then eight,..

At a resolution of 30 angstroms, the cryo-EM reconstructions of Doudna and Nogales and their collaborators show eIF3 to be a particle consisting of five lobes - analogous to a head, and a pair of arms and legs. The study shows that the left arm of the eIF3 complex binds to the eukaryotic protein complex that recognizes the methylated guanosine cap at the 5’-end of the eukaryotic mRNAs (mRNA consists of a coding region sandwiched between a 5’-end and a 3’-end). By drawing the mRNA’s 5’-end cap through the ribosome entry site and towards the exit, eIF3 ensures the mRNA is properly positioned for its genetic code to be translated.

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Protein synthesis in mammalian cells begins with the loading of mRNA onto the small ribosome subunit, 40S, which is, in part, one of the responsibilities of the eIF3 complex. The eIF3 complex also interacts with other translation elements that bind at the start of the mRNA, prevents premature joining of the 40S and 60S ribosomal subunits, and helps assemble active ribosomes. Until now, the structural basis for eIF3’s multiple activities has been unknown.

Animated overview of DNA translation ..

The results of this study are in the December 2, 2005 issue of the journal Science, in a paper entitled
Structural Roles for Human Translation Factor eIF3 in Initiation of Protein Synthesis. Co-authoring the paper with Doudna and Nogales were Bunpote Siridechadilok and Christopher Fraser of UC Berkeley, and Richard Hall of Berkeley Lab.

SparkNotes: SAT Subject Test: Biology: Protein Synthesis

The amino acids are joined to form proteins by peptide bonds. The formation of peptide bonds requires a good amount of energy. Therefore, in the first phase of translation for protein synthesis, the amino acids are activated in the presence of ATP and linked to their cognate tRNA (transfer RNA). This process is called as charging of the tRNA or aminoacylation of tRNA. If two such charged tRNA are brought close enough, the formation of peptide bond between them is favored energetically. This function occurs inside the , as it contains two sites for subsequent amino acids to bind to and thus be close enough for bonding.

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