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Translation (biology)

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Christian Klar (klar94@web.de) (22/12/2018)


Entry done as an activity of the course: A General Understanding of Information, held in Munich University of Applied Sciences



Translation is the process in which specific proteins are created after the information provided by an RNA. The basic process of protein production is addition of one amino acid at a time to the end of a protein. Small parts of a mRNA string connect one after another connect to different tRNAs. Each tRNA contains a matching amino acid. The different amino acids are connected to a string (protein) in the pattern provided by mRNA and thus DNA. This operation is performed by a ribosome. A ribosome is made up of two subunits, a small subunit and a large subunit. These subunits come together before translation of mRNA into a protein to provide a location for translation to be carried out.


These are the steps of translation:


1) Initiation: A small ribosomal subunit attaches itself to the end of the mRNA, where the 5’ cap is placed. It moves along the mRNA until it finds a start codon. At this codon the first tRNA joins in. Meanwhile a larger ribosomal subunit (50S) combines with the smaller subunit to form a ribosome, around a small area of the mRNA string. The first aa-tRNA is attached at the start codon is now in the ribosomes A-site. It contains the corresponding codon of the start codon.

each codon has a 3&#39; and a 5&#39; end. for example G(3&#39;) and U(5&#39;)RNA devided into Codons. Each has a 3' and a 5' end.


2) ElongationIn the A-site the next codon is engaged by its corresponding aa-tRNA. Here the amino acid binds covalently to the one next to it. The ribosome then moves to the next mRNA codon to continue the process. Resulting the current aa-tRNA to get from the A-site into the P-site of the ribosome. The amino acid here breaks its covalent bond to the tRNA, connecting to th chain of amino acids called protein. The tRNA without amino acid goes on into the E-site where it releases from the mRNA out of the ribosome. The ribosome continues the Elongation process further, down the mRNA string, producing the protein, that runs out of the subunit and towards the stop codon. [That whole paragraphe sounds a bit too specific. Make the corresponding parts oif the sites and tRNA more clear][better?]


3) Termination. When a stop codon is reached by the A-site of the ribosome, the ribosome releases the polypeptide resulting in ending of the synthesis with this ribosome. No more tRNA molecules are set onto the mRNA string in this ribosome.


Diagram showing the translation of mRNA and the synthesis of proteins by a ribosome



[I would put the source of the picture right here]

The rate of translation varies; it is significantly higher in prokaryotic cells (up to 17-21 amino acid residues per second) than in eukaryotic cells (up to 6-9 amino acid residues per second).


After the new amino acid is added to the chain, and after the mRNA is released out of the nucleus and into the ribosome's core, the energy provided by the hydrolysis of a GTP bound to a translocase, moves the ribosome down one codon towards the 3' end.


Even though the ribosomes are usually considered accurate and processive machines, the translation process is subject to errors that can lead either to the synthesis of erroneous proteins or to the premature abandonment of translation. Repair enzymes recognize structural imperfections between improperly paired nucleotides, cutting out the wrong ones and putting the right ones in their place. But some replication errors make it past these mechanisms, thus becoming permanent mutations. These altered nucleotide sequences can then be passed down from one cellular generation to the next, and if they occur in cells that give rise to gametes, they can even be transmitted to subsequent organismal generations. Moreover, when the genes for the DNA repair enzymes themselves become mutated, mistakes begin accumulating at a much higher rate. In eukaryotes, such mutations can lead to cancer. (nature.com) [What are these errors? And where exatly does the mistake occur?][better?]


Codon


A codon is a set of three nucleotides in a row on a DNA or RNA string.


small ribosomal subunits (30S)


The small subunit is responsible for the binding and the reading of the mRNA during translation. It is referred to as the 30S subunit. Together with the larger 50S subunit it can form a 70S ribosome.


larger ribosomal subunit (50S)


50S includes the activity that catalyses peptide bond formation, provides a binding site for the G-protein factors and helps protein folding after synthesis.


ribosome


The ribosome is a complex molecular machine, found within all living cells, that serves as the site of biological protein synthesis (translation). It is the "factory" where amino acids are assembled into proteins. Ribosomes link amino acids together in the order specified by messenger RNA (mRNA) molecules. Ribosomes consist of two major components: the small ribosomal subunit (30s), which reads the RNA, and the large subunit (50S), which joins amino acids to form a polypeptide chain.


It has 3 sites in general, all of whom have a different duty. The A-site (for aminoacyl) is a binding site for charged t-RNA molecules. Second binding site for tRNA would be the P-site (for peptidyl). Last of the three sites is the E-site (for exit). When an aminoacyl-tRNA initially binds to its corresponding codon on the mRNA, it is in the A site. Then, a peptide bond forms between the amino acid of the tRNA in the A site and the amino acid of the charged tRNA in the P site. The growing polypeptide chain is transferred to the tRNA in the A site. Translocation occurs, moving the tRNA in the P site, now without an amino acid, to the E site; the tRNA that was in the A site, now charged with the polypeptide chain, is moved to the P site. The tRNA in the E site leaves and another aminoacyl-tRNA enters the A site to repeat the process. An amino acid chain is linked by peptide bonds.

[Try reverring to the picture when you describe the different functionalities. Make a lot of things more clear]


stop codon


In the genetic code, a stop codon (or termination codon) is a nucleotide triplet within messenger RNA that signals a termination of translation into proteins.


Overall, RNA helps synthesize, regulate, and process proteins; it therefore plays a fundamental role in performing functions within a cell.

 


Different types of RNA


Messenger Ribonucleic acid (mRNA)


Messenger RNA (mRNA) carries information about a protein sequence to the ribosomes, the protein synthesis factories in the cell. It is coded so that every three nucleotides (a codon) corresponds to one amino acid. In eukaryotic cells, once precursor mRNA (pre-mRNA) has been transcribed from DNA, it is processed to mature mRNA. This removes its introns—non-coding sections of the pre-mRNA. The mRNA is then exported from the nucleus to the cytoplasm, where it is bound to ribosomes and translated with the help of tRNA. In prokaryotic cells, which do not have nucleus and cytoplasm compartments, mRNA can bind to ribosomes while it is being transcribed from DNA.

 


Transfer Ribonucleic acid (tRNA/sRNA)


tRNA is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length, that serves as the physical link between the mRNA and the amino acid to the protein. tRNA does this by carrying an amino acid to the protein synthetic machinery of a cell (ribosome) in a messenger RNA. When the tRNA has an amino acid linked to it, the tRNA is termed "charged" and can also be called aa-tRNA.


Aminoacyl tRNA synthetases (enzymes) catalyse the covalent bonding between specific tRNAs and the amino acids that their anticodon sequences call for. The product of this reaction is an aa-tRNA. This process can be called aminoacylation. There is normally a single aminoacyl tRNA synthetase for each amino acid, despite the fact that there can be more than one tRNA, and more than one anticodon, for an amino acid. Certain organisms can have one or more aminoacyl tRNA synthetases missing. This leads to charging of the tRNA by a chemically related amino acid. An enzyme or enzymes modify the charged amino acid to the final one.



aminoacyl-tRNA (aa-tRNA also referred to as charged)


Aminoacyl-tRNA is tRNA to which its cognated amino acid is chemically bonded (charged). The aa-tRNA, along with some elongation factors, deliver the amino acid to the ribosome for incorporation into the polypeptide chain that is being produced. A specific cognate amino acid is charged or amino acylated to each tRNA by aminoacyl tRNA synthetase. This matching is crucial, since it ensures that only the particular amino acid matching the anticodon of the tRNA, and in turn matching the codon of the mRNA, is used in protein synthesis.


Ribosomal ribonucleic acid (rRNA)


The rRNA is one of only a few gene products present in all cells and are widely used for working out evolutionary relationships among organisms, since they are of ancient origin and are found in all known forms of life. It is created in the nucleus by transcription, where it also is modified. Some parts are deleted (internal transcribed spacers) from its RNA form and the rest is enzymatically modified. Afterwards the ribosome binds mRNA and carries out protein synthesis. The new Proteins belong to the group of ribonucleoproteins.


Transfer-messenger RNA (tmRNA)


Transfer-messenger RNA (tmRNA) is found in many bacteria and plastids. It tags proteins encoded by mRNAs that lack stop codons for degradation and prevents the ribosome from stalling.


micro ribonucleic acid (miRNA)


A microRNA is a small non-coding RNA molecule, found in plants, animals and some viruses, that functions in RNA silencing (having a negative regulating effect on gene expression) and post-transcriptional regulation of gene Expression.


References





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MrTomatensenf,
Dec 22, 2017, 2:19 AM
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