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Protein synthesis is the process of producing proteins using information encoded in DNA, located in the nucleus of the cell. Cells perform two processes to convert the information in DNA into proteins.

What Is The Site Of Protein Synthesis In The Cell

What Is The Site Of Protein Synthesis In The Cell

First, in a process called transcription, the coding region of a gene is copied into a single-stranded ribonucleic acid (RNA) version of double-stranded DNA. This is accomplished by RNA polymerase, a large enzyme that catalyzes the joining of nucleotides into RNA chains using DNA as a template. RNA is further processed into messenger RNA (mRNA) before being transported to the cytoplasm.

Rna Holds The Reins In Bacteria: U M Researchers Observe Rna Controlling Protein Synthesis

After processing, the mRNA is transported through the nuclear pore to the cytoplasm, where the translation machinery (i.e., ribosomes, eukaryotic initiation factors eIF4E and eIF4G, and Poly(A) binding proteins) performs the second process: translation, in which the ribosomes The amino acids are assembled in the order specified by the mRNA sequence.

Protein synthesis is a key cellular process in prokaryotes and eukaryotes. This is accomplished by the ribosome, an evolutionarily conserved ribonucleoprotein complex, and is assisted by many other proteins and RNA molecules. Together they synthesize all the proteins required for various biological functions. Protein synthesis can be divided into 3 phases: initiation, elongation, and termination. Each stage has different proteins and RNA molecules that play a role in efficient catalysis. Ribosomes also have three major sites: the acceptor site (A site), the peptidyl transfer site (P site), and the exit site (E site), which contains tRNA, facilitating catalysis.

Initiation begins with the 30S subunit bound to initiation factor 3 (IF-3). IF-3 binding prevents premature binding of 50S units and also functions to guide the mRNA strand. The mRNA binds to this complex with the assistance of Shine-Dalgarno sequences. This sequence is a string of 9 nucleotide bases upstream of the mRNA start codon AUG. It is complementary to sequences on the 16S rRNA of the 30S subunit and helps align the mRNA with the 30S. Next, IF-1 binds to the A site on 30S, which is the first site that all charged tRNAs bind to. IF-1 effectively prevents premature binding of tRNA at the A site before the ribosome is fully assembled.

IF-2 delivers the first tRNA to the P site, where the peptidyl transfer reaction occurs. In bacteria, the first tRNA is always an N-formyl-modified methionine, encoded by the AUG start codon. Once more amino acids are added to the nascent peptide chain, the formyl group is removed downstream. At this stage, the 30S preinitiation complex is fully assembled, attracting the 50S subunit to spontaneously assemble with it. IF-2 is a GTP-binding protein, and hydrolysis of GTP releases all initiation factors from the newly assembled initiation complex. The 70S-mRNA-f-met tRNA complex is now ready for protein synthesis.

Question Video: Understanding The Function Of Ribosomes

After the 70S complex assembles with the initiating tRNA at the P site, the ribosome begins scanning the mRNA sequence. Each codon corresponds to a specific amino acid and is delivered to the ribosome via a thermolabile elongation factor (EF-Tu). EF-Tu forms a complex with a charged tRNA molecule, places it on the mRNA, and is then separated from 70S by GTP hydrolysis.

The GTP-bound state of EF-Tu is critical for efficient tRNA delivery, so cells have evolved a mechanism to recycle EF-Tu by using another protein called thermostable elongation factors (EF-Ts). EF-Ts function as guanine nucleotide exchange factors, effectively releasing GDP from EF-Tu so that new GTP molecules can be bound. When EF-Tu binds to another GTP molecule, it can form the tRNA-EF-Tu-GTP complex again and continue the tRNA delivery process. Once a charged tRNA is present at both the A and P sites, a peptide bond is formed between the two amino acids through nucleophilic attack of the A site amino acid on the P site amino acid. At this stage, the A site contains a tRNA with a growing peptide chain, while the P site has an empty tRNA.

Another GTP-binding protein, elongation factor G (EF-G), catalyzes the movement of tRNA along the assembly line. This is called translocation and clears the A site for further peptidyl transfer reactions. Once EF-G binds to the ribosome, GTP hydrolysis causes a conformational shift in the ribosome, allowing the tRNA to move downward from the A and P sites to the P and E sites. The E site is the exit site and empty tRNAs diffuse back into the cytoplasm where they are replenished by tRNA synthetase. Once EF-G undergoes translocation, the A site is ready to accept new tRNA. Thus, the elongation cycle continues to provide growing nascent peptides until a stop codon is encountered.

What Is The Site Of Protein Synthesis In The Cell

Once the stop codon is reached on the mRNA strand, there are no longer tRNA molecules that can base pair with the mRNA’s complement. In contrast, release factors 1 and 2 (RF-1/RF-2) recognize the stop codon and bind to 70S. This triggers hydrolysis of the P-site peptide chain and releases the peptide into the cytoplasm for further processing and folding. RF-3 is a GTP-binding protein that binds to 70S and triggers the release of RF-1/RF-2 via GTP hydrolysis. During this stage, the 70S ribosome binds the mRNA and empty tRNA. In this state, 70S cannot undergo protein synthesis and therefore must be recycled. This function is performed by ribosome recycling factors (RRF) and EF-G, which bind to ribosomes and cause their dissociation via GTP hydrolysis. Once the 30S and 50S subunits are free, IF-3 rebinds the 30S to prevent premature formation of the 70S and the initiation cycle can begin again.

Translation And Protein Synthesis

The structural complexity of ribosomes and their core biological functions make them prime targets for inhibition. Given the differences between prokaryotic 70S ribosomes and eukaryotic 80S ribosomes, organisms have evolved small molecules that can selectively target 70S ribosomes and 80S ribosomes, selectively killing their targets. These inhibitors target nearly every stage of protein synthesis, and modern X-ray crystallography allows us to fully understand their binding modes and mechanisms of action. Many 70S inhibitors serve as clinically effective antibiotics because they exhibit selective toxicity against bacterial cells. It should be noted that many inhibitors inhibit multiple steps of protein synthesis, enhancing its antimicrobial activity. Some key inhibitors of protein synthesis in prokaryotes are discussed below. This stunning work of art (Figure 5.7.1) illustrates a process that occurs in the cells of all living things: the production of proteins. This process is called protein synthesis and it actually consists of two processes –

, where translation takes place. During translation, the genetic code in the mRNA is read and used to make polypeptides. These two processes can be summarized by the central dogma of molecular biology: DNA → RNA → protein.

Transcription is the first part of the central dogma of molecular biology: DNA → RNA. It is the transfer of genetic instructions from DNA to mRNA. During transcription, an mRNA becomes complementary to a DNA strand. You can see how this happens in Figure 5.7.2.

Figure 5.7.2 Transcription uses the sequence of bases in a DNA strand to form the complementary strand of mRNA. A triplet is a group of three consecutive nucleotide bases in DNA. Codons are complementary sets of bases in mRNA.

Translation (protein Synthesis)

Transcription begins when RNA polymerase binds to a region of the gene called the promoter sequence. This signals the DNA to unwind so that the enzyme can “read” the DNA’s bases. The two DNA strands are named according to whether they serve as templates for RNA. The strand used as a template is called the template strand and may also be called the antisense strand. The sequence of bases on the opposite strand of DNA is called the non-coding or sense strand. Once the DNA is opened and RNA polymerase is attached, RNA polymerase moves along the DNA, adding RNA nucleotides to the growing mRNA strand. The DNA template strand is used to create the mRNA through complementary base pairing. Once the mRNA strand is complete, it separates from the DNA. The result is an mRNA strand that is nearly identical to the coding strand of DNA – the only difference is that DNA uses the base thymine, while mRNA uses uracil instead of thymine

Not ready for translation yet. At this stage it is called pre-mRNA and must undergo more processing before leaving the nucleus as mature mRNA. Processing can include splicing, editing and polyadenylation. These processes modify the mRNA in a variety of ways. This modification allows a single gene to be used to make multiple proteins.

Translation is the second part of the central dogma of molecular biology: RNA → protein. This is the process in which the genetic code

What Is The Site Of Protein Synthesis In The Cell

After being transcribed in the nucleus, the mRNA enters the cytoplasm through the nuclear pores. The region on the mRNA that contains the methylation cap and start codon, small

Stages Of Translation (article)

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