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You are watching: Which is the most accurate statement concerning protein synthesis in cells

Griffiths AJF, fearbut JH, Suzuki DT, et al. An introduction to genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.


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We deserve to regard protein synthesis together a chemical reaction, and we shalltake this approach at first. Then us shall take a three-dimensional look at thephysical interactions of the major components.

In protein synthesis together a chemistry reaction:

1.

Each amino acid is attached to a tRNA molecule certain to the amino acid bya high-energy bond obtained from ATP. The process is catalytic analysis by a specificenzyme called a synthetase (the tRNA is claimed to it is in “charged”when the amino acid is attached):

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There is aseparate synthetase because that each amino acid.

2.

The power of the fee tRNA is converted right into a peptide bond linking theamino mountain to another one ~ above the ribosome:

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4.

This process continues until aan (the last amino acid) is added. The totality thing works only in thepresence that mRNA, ribosomes, several added protein factors, enzymes,and not natural ions.


Ribosomes

Ribosomes consist of two subunits that, in prokaryotes, sediment as 50S and 30Sparticles and associate to type a 70S particle, as checked out in number 10-30a. The eukaryotic counterparts are 60S and40S for the large and small subunits, and also 80S for the complete ribosome (Figure 10-30b). Ribosomes save on computer specificsites that allow them to bind to the mRNA, the tRNAs, and particular proteinfactors compelled for protein synthesis. Let’s look in ~ a general picture ofprotein synthetic on the ribosome and then examine each of the measures in development in much more detail.


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Figure 10-30

A ribosome includes a big and a small subunit. Each subunitcontains both rRNA of varying lengths and also a collection of proteins(designated by various shapes and also shading). There space twoprincipal rRNA molecule in all ribosomes. (a) ribosom fromprokaryotes (more...)


Figure 10-31 reflects a polypeptide beingsynthesized ~ above the ribosome. The mRNA binding to the 30S subunit. The tRNAs bindto two sites top top the ribosome. This sites overlap the subunits. The Asite is the entry site for an aminoacyl-tRNA (a tRNA delivering asingle amino acid). The peptidyl-tRNA moving the cultivation polypeptide chainbinds at the P site. Each new amino acid is added by the transferof the cultivation chain come the brand-new aminoacyl-tRNA, creating a brand-new peptide bond. Thedeacylated tRNA is climate released indigenous the ns site, and the ribosome move onecodon farther along the message, delivering the brand-new peptidyl-tRNA come the Psite and also leaving the A website vacant because that the following incomingaminoacyl-tRNA.


Figure 10-31

The enhancement of a single amino mountain to the cultivation polypeptide chain in the course of translate in of mRNA.


We deserve to separate the procedure of protein synthesis into three unique steps.Initiation, elongation, and termination. Let’sexamine every of these actions in detail, by using prokaryotes together an example.


Three actions of initiation.  

In enhancement to mRNA, ribosomes, and certain tRNA molecules, initiationrequires the joining of number of factors, termed initiationfactors IF1, IF2, and IF3. In E.coli and in most other prokaryotes organisms, the an initial amino mountain in any type of newly synthesized polypeptide isN-formylmethionine. That is put not bytRNAMet, however, however by one initiator tRNAcalled tRNAfMET. This initiator tRNA has the normal methionineanticodon however inserts N-formylmethionine rather thanmethionine (Figure 10-32). InE. Coli, AUG and also GUG, and on rarely occasions UUG, serveas initiation codons. When one of these triplets is present in theinitiation position, that is well-known by N-formylMet-tRNA,and methionine appears as the first amino mountain in the chain. Let’s examinethe actions in initiation in detail.


Figure 10-32

The structures of methionine (Met) andN-formylmethionine (fMet). A tRNA bearing fMetcan start a polypeptide chain in prokaryotes however cannot beinserted in a growing chain; a tRNA bearing Met deserve to be insertedin a farming chain yet will no initiate (more...)


1.

The first step in initiation is the binding the the mRNA come the 30Ssubunit (Figure 10-33). Thebinding is created by the initiation factor IF3. As soon as not engagedin protein synthesis, the ribosom subunits exist in the free form;they rally into finish ribosomes as a result of the initiationprocess.

2.

The initiation element IF2 binding to GTP and to the initiator fMet-tRNAand stimulates the binding that fMet-tRNA to the initiation complex,leading the fMet-tRNA right into the ns site, as shown in the center ofFigure 10-33.

3.

A ribosom protein splits the GTP bound come IF2, help to drive theassembly that the two ribosomal subunits (Figure 10-33, bottom). At this stage, thefactors IF2 and also IF3 space released. (The exact duty of IF1 is notcompletely clear, back it seems to take component in the recycling ofthe ribosome.)


Ribosome-binding sites.  

How are the exactly initiation codons selected from the numerous AUG and GUGcodons in an mRNA molecule? man Shine and Lynn Dalgarno very first noticed thattrue initiation codons were preceded by sequences the paired well with the3′ finish of 16S rRNA. Number 10-34shows several of these sequences. There is a short but change separationbetween the Shine-Dalgarno sequence and the initiation codon. Number 10-35 depicts the base pairingbetween idealized mRNA and the 16S rRNA that outcomes in ribosome-mRNAcomplexes bring about protein initiation in the existence of fMet-tRNA.


Figure 10-34

Ribosomal binding-site sequences in E. Coli andits bacteriophages have details features in common, i beg your pardon areshown in the fancy regions. The initiation codon (color) isseparated by several bases indigenous a quick sequence (color) the iscomplementary (more...)


Figure 10-35

Elongation

Figure 10-36 details the measures inelongation, which are aided by 3 protein factors, EF-Tu, EF-Ts,and EF-G. The steps are as follows:


1.

Elongation aspect EF-Tu mediates the entrance of amino-acyl-tRNAs into the Asite. To carry out so, EF-Tu very first binds to GTP. This triggered EF-Tu–GTPcomplex binds to the tRNA. Next, hydrolysis that the GTP that the facility toGDP helps drive the binding that the aminoacyl-tRNA to the A site, atwhich allude the EF-Tu is exit (Figure 10-36a), leave the brand-new tRNA in the A website (Figure 10-36b).

2.

Elongation aspect EF-Ts mediates the release of EF-Tu–GDP indigenous theribosome and the rejuvenation of EF-Tu–GTP.

3.

In the translocation step, the polypeptide chain on the peptidyl-tRNA istransferred come the aminoacyl-tRNA on the A website in a reaction catalyzedby the enzyme peptidyltransferase (Figure 10-36c). The ribosome then translocates by relocating onecodon farther follow me the mRNA, walk in the 5′ → 3′ direction. This stepis mediated by the elongation element EF-G (Figure 10-36d) and is driven by separating a GTPto GDP. This action releases the uncharged tRNA native the P site andtransfers the newly developed peptidyl-tRNA from the A site to the ns site(Figure 10-36e).


Release factors.  

In the earlier discussion of the genetic code, we defined the threechain-termination codons UAG, UAA, and UGA. Interestingly, this threetriplets space not known by a tRNA, but instead by protein factors,termed release factors, which space abbreviated RF1and RF2. RF1 establish the triplets UAA and also UAG, and RF2recognizes UAA and also UGA. A 3rd factor, RF3, likewise helps tocatalyze chain termination. When the peptidyl-tRNA is in the p site, therelease factors, in an answer to the chainterminating codons, bind to the Asite. The polypeptide is then released indigenous the ns site, and the ribosomesdissociate into two subunits in a reaction thrust by the hydrolysis that a GTPmolecule. Figure 10-37 gives aschematic watch of this process.


Nonsense suppressor mutations

It is interesting to think about the suppressors of the rash mutations thatBrenner and also co-workers defined. Many of this nonsense suppressormutations are well-known to alter the anticodon loop of specific tRNAsin together a way as to permit recognition of a rash codon in mRNA. Thus, anamino acid is put in solution to the nonsense codon, and translationcontinues past that triplet. In Figure10-38, the amber mutation replaces a wild-type codon v thechain-terminating nonsense codon UAG. By itself, the UAG would result inprematurely cutting turn off the protein at the corresponding position. Thesuppressor mutation in this instance produces a tRNATyr with ananticodon the recognizes the mutant UAG avoid codon. The suppressed mutantthus has tyrosine in ~ that position in the protein.


Figure 10-38

(a) discontinuation of translation. Below the translate into apparatus cannot go past a nonsense codon (UAG in this case), due to the fact that there is no tRNA that have the right to recognize the UAG triplet. This leads to the discontinuation of protein synthesis and also to the subsequent release (more...)


What wake up to normal termination signals at the end of proteins in thepresesnce of a suppressor? countless of the organic termination signal consistof two chain-termination signal in a row. Rash suppressors aresufficiently inefficient in translating v chain-terminating triplets,because the competition with release factors, that the probability ofsuppression at 2 codons in a row is small. Consequently, very couple of proteincopies that bring many extraneous amino acids resulting from translationbeyond the organic stop codon room produced.


Overview that protein synthesis

Figure 10-39 summarizes the steps inprotein synthesis extended in this section. A straight visualization of proteinsynthesis can be viewed in the electron micrograph displayed in number 10-40, which mirrors the simultaneously transcriptionand translate into of a gene in E. Coli.


Figure 10-39

The transactions the the ribosome. At initiation, the ribosomerecognizes the beginning point in a segment that mRNA and binds amolecule that tRNA bearing a solitary amino acid. In every bacterialproteins, this very first amino acid isN-formylmethionine. In elongation, (more...)


Figure 10-40

A gene of E. Coli being all at once transcribedand translated. (Electron micrograph by O. L. Miller, Jr., andBarbara A. Hamkalo.)


Protein processing

Even after ~ mRNA has been properly translated into its protein product,processing may continue. For example, membrane protein or proteins the aresecreted indigenous the cell room synthesized v a brief leader peptide, dubbed asignal sequence, at theamino-terminal (N-terminal) end. This signal succession is a stretch that 15 to 25amino acids, many of which room hydrophobic. It allows for recognition by factorsand protein receptors the mediate transport with the cell membrane; in thisprocess, the signal sequence is cleaved by a peptidase (Figure 10-41). (A comparable phenomenon exists because that certainbacterial proteins that room secreted.) Moreover, several tiny peptide hormones,such as corticotropin (ACTH), result from the particular cleavage the a single,large polypeptide precursor.


Figure 10-41

Signal sequences. Proteins destined to it is in secreted from the cell havean amino-terminal sequence the is wealthy in hydrophobic residues.This signal sequence binding to the membrane and also draws the remainderof the protein v the lipid bilayer. The signal (more...)


Protein splicing

An extraordinary procedure that splices out an inner segment of particular proteinshas been explained in a variety of organisms, consisting of prokaryotes andeukaryotes. This inner segment is termed an intervening proteinsequence, or IVPS. The essential facet the this processis the development of a new peptide bond in between the 2 sequences flanking theIVPS. This reaction is autocatalytic and can take place in vitro. Figure 10-42 depicts protein splicing inschematic form. Interestingly, every IVPS segments learned so far contain anendonuclease activity, return this activity is unrelated to theprotein-splicing reaction.

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Figure 10-42

Protein splicing results in the removed of an inner segment (IVPS)and the development of a new peptide bond that links the 2 regionsthat initially flanked the IVPS.


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