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Sagot :
Answer:
The aspect of translation that makes it so challenging for the cell is that it involves too many steps and too many molecules and organelles. mRNA associated with rRNA and tRNA. rRNA reading mRNA in a specific direction. Codon sequences indicating where to start and where to finish the synthesis. Different codons codifying for different aminoacids and tRNA recognizing those codons to add the correct amino acid to the chain. If the protein is destined to exportation, RER and Golgi complex are also involved in the process.
Explanation:
In protein synthesis, the first step is to synthesize messenger RNA, mRNA. The transcription is the coping process of the DNA section, and it happens in the nucleus. After that, the translation occurs when the formed mRNA moves to the cytoplasm through the nucleus membrane pores.
Once in the cytoplasm, it begins the translation process. mRNA meets a free ribosome, which is the primary structure for protein synthesis. Ribosomes are composed of two subunits, a big one and a small one. They also associate with different tRNA that carrying amino acids. Ribosomes might be floating freely in the cytoplasm or can be attached to the rough endoplasmic reticulum.
The ribosome reads mRNA in the 5´ to 3´ direction, and, according to the codons that are being readen, tRNA transfers the correct amino acids to build the polypeptide chain. Each tRNA has two important sites. One of them that couples with the codon of the mRNA molecule, named anticodon. The other site couples with an amino acid through the action of the aminoacyl-tRNA synthase enzyme. The whole complex, amino acid + enzyme + tRNA is named aminoacyl-tRNA. Considering that there are twenty amino acids available, there are also twenty complexes of aminoacyl-tRNA, one for each amino acid. tRNA allows amino acids to align according to the nucleotidic sequence in the mRNA molecule.
Once the new amino acid links to the growing peptidic chain, the binding between the amino acid and the tRNA molecule breaks. The tRNA is now free to join another amino acid and repeat the cycle.
The protein is synthesized from the amino terminus to the carboxy terminus, while the added amino acids to the chain are coded by a codon formed by three bases in the mRNA. mARNs also have a start and end codon that are the signals of the synthesis initiation and finish. When the ribosome reaches the end codon, protein synthesis is over.
And if the protein is destined for exportation or cell membrane, its synthesis also involved organelles like the Rough endoplasmic reticulum and the Golgi complex.
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