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Protein synthesis is the process of producing proteins using information coded by DNA, located in the nucleus of the cell. Two processes are carried out in order for cells to convert the information in DNA into proteins.
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The Site Of Protein Synthesis Is The
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 an RNA chain using DNA as a template. The RNA is further processed into messenger RNA (mRNA) before being transported to the cytoplasm.
Using Your Understanding Of The Role The Nucleus Plays In Cellular Function, Select The Best Explanation
After processing, the mRNA is transported through the nuclear pore into the cytoplasm, where the translation machinery (i.e., the ribosome, the eukaryotic initiation factors eIF4E and eIF4G, and poly(A)-binding protein) carries out a second process, translation, during which the ribosomes assemble the amino acids in the order dictated by mRNA sequence.
Protein synthesis is a critical cellular process in prokaryotes and eukaryotes. This is carried out by the ribosome, an evolutionarily conserved ribonucleoprotein complex, with the help of many other proteins and RNA molecules. Together they synthesize all the proteins needed for various biological functions. Protein synthesis can be divided into 3 phases: initiation, elongation and termination. Each stage has different protein and RNA molecules that play a role in efficient catalysis. The ribosome also has three main sites: an acceptor site (A site), a peptidyl transfer site (P site), and an exit site (E site) where tRNA resides, facilitating catalysis.
Initiation begins with the 30S subunit to which initiation factor 3 (IF-3) is bound. IF-3 binding prevents premature binding of the 50S unit and also plays a role in mRNA strand guidance. mRNA binds to this complex with the help of the Shine-Dalgarn sequence. This sequence is a sequence of 9 nucleotide bases upstream of the AUG start codon on the mRNA. It is complementary to the 16S rRNA sequence of the 30S subunit and helps align the mRNA to the 30S. Next, IF-1 binds to the A site on 30S, which is where all charged tRNAs first bind. IF-1 effectively blocks the premature binding of tRNA at the A site before the ribosome is fully assembled.
IF-2 delivers the first tRNA to the P site, the site where peptidyl transfer reactions take place. In bacteria, the first tRNA is always an N-formyl modified methionine, encoded by the AUG start codon. The formyl group is removed downstream after more amino acids have been added to the nascent peptide chains. At this stage, the 30S preinitiation complex is fully assembled which attracts the 50S subunit to spontaneously assemble with it. IF-2 is a GTP-binding protein, and GTP hydrolysis releases all initiation factors from the freshly assembled initiation complex. The 70S-mRNA-f-met tRNA complex is now ready for protein synthesis.
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After the 70S complex binds to the initiator tRNA at the P site, the ribosome begins to scan the mRNA sequence. Each codon corresponds to a specific amino acid, which is delivered to the ribosome by the thermostable elongation factor (EF-Tu). EF-Tu forms a complex with the charged tRNA molecule, places it on the mRNA and then detaches from the 70S by GTP hydrolysis.
The GTP-bound state of EF-Tu is essential for efficient tRNA delivery, so the cell has evolved a mechanism to recycle EF-Tu using another protein called thermostable elongation factor (EF-Ts). EF-Ts serve as a guanine nucleotide exchange factor, effectively releasing GDP from EF-Tu so that a new molecule of GTP can bind. When EF-Tu binds another GTP molecule, it can re-form the tRNA-EF-Tu-GTP complex and resume the tRNA delivery process. After both site A and site P have charged tRNAs present, a peptide bond is formed between the two amino acids through nucleophilic attack of the amino acid of site A on the amino acid of site P. At this stage, site A contains a tRNA with a growing peptide chain and site P has an empty tRNA .
Another GTP-binding protein, elongation factor G (EF-G), catalyzes the movement of tRNA along the conveyor belt. This is called translocation and vacates the A site for further peptidyl transfer reactions. After EF-G binds to the ribosome, GTP hydrolysis causes a conformational shift of the ribosome so that the tRNAs move down 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 cytosol where they are refilled by tRNA synthetases. After EF-G has translocated, the A site is ready to accept the new tRNA. Thus, the elongation cycle continues to generate a growing nascent peptide, until a stop codon is encountered.
Once the stop codon on the mRNA strand is reached, there are no more tRNA molecules that can complementarily pair with the mRNA. Instead, release factors 1 and 2 (RF-1/RF-2) recognize stop codons and bind to 70S. This initiates hydrolysis of the peptide chain at the P-site and releases the peptide into the cytosol for further processing and folding. RF-3, a GTP-binding protein, binds to 70S and triggers the release of RF-1/RF-2 through GTP hydrolysis. At this stage, the 70S ribosome has bound mRNA and empty tRNA. In this state, 70S cannot carry out protein synthesis and must therefore be recycled. This function is performed by ribosome recycling factors (RRF) and EF-G, which bind to ribosomes and cause their dissociation through GTP hydrolysis. Once the 30S and 50S subunits are released, IF-3 rebinds 30S to prevent premature formation of 70S and the initiation cycle can begin anew.
Ribosome Profiling Reveals The What, When, Where And How Of Protein Synthesis
The very structural complexity of the ribosome, together with its central biological function, makes it a prime target for inhibition. Given the differences between prokaryotic 70S ribosomes and eukaryotic 80S ribosomes, organisms have developed small molecules that can selectively target 70S ribosomes and 80S ribosomes to selectively kill their target. These inhibitors target almost every stage of protein synthesis, and modern X-ray crystallography gives us a comprehensive understanding of their binding modes and mechanisms of action. Many 70S inhibitors serve as potent antibiotics in the clinic since they exhibit selective toxicity toward bacterial cells. It should be noted that many inhibitors inhibit multiple steps of protein synthesis, increasing their antimicrobial activity. Some of the key inhibitors of protein synthesis in prokaryotes are discussed below. This incredible work of art (Figure 5.7.1) shows a process that takes place in the cells of all living things: the production of proteinases. This process is called protein synthesis and it actually consists of two processes —
, where the translation takes place. During translation, the genetic code in mRNA is read and used to make polypeptides. These two processes are 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 in DNA to mRNA. During transcription, a strand of mRNA is formed to complement the strand of DNA. You can see how this happens in Figure 5.7.2.
Figure 5.7.2 Transcription uses a sequence of bases in a strand of DNA to make a complementary strand of mRNA. Triplets are groups of three consecutive nucleotide bases in DNA. Codons are complementary groups of bases in mRNA.
How Do Antibiotics Affect Protein Synthesis?
Transcription begins when the enzyme 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 bases. The two strands of DNA are named based on whether or not they will be used as a template for RNA. The strand used as a template is called a template strand or it can also be called an a ntisense string. The sequence of bases on the opposite DNA strand is called the non-coding or sense strand. After the DNA is opened and RNA polymerase attaches, RNA polymerase moves along the DNA, adding RNA nucleotides to the growing mRNA strand. The DNA template strand is used to create mRNA through complementary base pairing. Once the mRNA strand is complete, it is separated from the DNA. The result is an mRNA strand that is almost identical to the coding strand of DNA – the only difference is that DNA uses the base thymine and mRNA uses uracil instead of thymine
Not yet ready for translation. At this stage it is called pre-mRNA and must undergo additional processing before leaving the nucleus as a mature mRNA. Processing may include splicing, editing and polyadenylation. These processes modify the mRNA in a variety of ways. Such modifications allow a single gene to be used to make more than one protein.
Translation is the second part of the central dogma of molecular biology: RNA → Protein. It is the process in which the genetic code
After transcription in the nucleus, the mRNA exits through the nuclear pore and enters the cytoplasm. In the region on the mRNA that contains the methylated cap and the start codon, a small
Ribosome Definition And Examples
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