Components of Genetic Translation,Protein Synthesis,Anticodon,Prokaryotic Eukaryotic Ribosomes,Translation System,Aminoacyl,tRNA Synthetase

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	Components of Genetic Translation - All the amino acids that eventually appear in the finished protein must be present at the time of protein synthesis. Even if one amino acid is not available, then the translation stops. Transfer RNA (tRNA):At least one specific type of tRNA is required per amino acid. In humans there are at least 50 species of tRNA, whereas bacteria contain 30 to 40 species. Because there are only 20 different amino acids commonly carried by tRNAs, some amino acids have more than one specific tRNA molecule.
This is particularly true of those amino acids that are coded by several codons. The tRNA molecule has the following two important sites which are essential for protein synthesis. Amino acid attachment site Each tRNA molecule has an attachment site for a specific amino acid at its 3’ -end. When a tRNA has a covalently attached amino acid, it is said to be charged and the amino acid is called as uncharged. Anticodon Each tRNA molecule also contains a 3-base nucleotide sequence, the anticodon, that recognizes a specific codon on the mRNA.
Aminoacyl-tRNA Synthetase: This family of enzymes is required for attachment of amino acids to their corresponding tRNAs. Each member of this family recognizes a specific amino acid and the tRNAs that correspond to that amino acid. Each aminoacyl tRNA synthetase catalyses a two step reaction that results in the covalent attachment of an amino acid to the 3' end of its corresponding tRNA. The overall reaction requires A TP which is cleaved to AMP and PPi. The extreme specificity of the synthesis in recognizing these two structures is largely responsible for the high fidelity of translation.
Functionally Competent Ribosomes: Ribosomes are large complexes of protein and rRNA. They consist of two subunits. One large and one small whose relative sizes are generally given in Svedberg units. Prokaryotic and eukaryotic ribosomes are similar in structure and function but different in size. Ribosomal RNA (rRNA) : Prokaryotic ribosomes contain 3 molecules of rRNA (5S, 23S and 16S RNA) and 53 protein whereas eukaryotic ribosomes contain 4 molecules of rRNA (5S, 28S, 5.8S and 18.5S RNA) and 80 proteins. The rRNAs have extensive regions of secondary structure arising from the base pairing of complementary sequences of nucleotides in different portions of the molecules.
	Ribosomal proteins are present in considerably greater number in eukaryotic ribosomes than in prokaryotes and playa number of roles in the structure and function of the ribosome and its interactions with other components of the translation system. The ribosome has two binding sites for tRNA molecules, the A and the P sites, each of which extends over both subunits. Together, they cover two neighbouring codons. During translation, the A site binds to an incoming aminoacyl tRNA as directed 'by the codon currently occupying this site. This codon specifies the next amino acid to be added to the growing peptide chain.

The P site codon is occupied by peptidyl-tRNA.This tRNA carries the chain of amino acids that have already been synthesized. In eukaryotic cells, the ribosomes are either free in the cytosol or in close association with ER.

The RER associated ribosomes are responsible for synthesizing proteins that are to be exported from the cell as well as those that are destined to become integrated into the plasma, ER, golgi membrane or lysosome.A TP and GTP are required as sources of energy.

Cleavage of four high energy bonds is required for the addition of one amino acid to the growing polypeptide chain.

Two A TP are utilized in the aminoacyl-tRNA synthetase reaction (one for removing PPi and one for the subsequent hydrolysis of the PPi to inorganic phosphate by pyrophosphates) and two GTP are also utilized (one for binding the aminoacyl-tRNA to the A site, and one for the translation step).