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  Processing of tRNA Molecules

Processing of tRNA Molecules:

Most cells comprise 40 to 50 different tRNAs. Transfer RNA’s (tRNA) are derived from longer RNA precursors of enzymatic elimination of extranucleotide units from the 5' and 3' ends.

1. Formation of the 3'-OH terminus:

This procedure includes the action of an endonuclease which recognizes a hairpin loop at the 3' end known as RNAse D, which stops two bases at short of CCA terminus, however it later removes these two bases after the 5' end is processed. This enzymatic digestion leaves the molecule known as pre-tRNA.

2. Formation of the 5'-P terminus:

The 5'-P terminus is created via an enzyme called RNAse P that removes excess RNA from the 5' end of a precursor molecule via an endonucleolytic cleavage which generates the correct 5' end.

3. Production of modified bases:

The last modification is to generate the altered bases in the tRNA.
In tRNA, two uridines are transformed to one guanosine to methyl guanosine (MG), pseudo uridine (Ψ ), one adenine to isopentyladenine (IPA) etc.
 

1976_modification of trna molecules.jpg

                                                             Diagram: Modification of tRNA molecules

Role of tRNA in protein synthesis:

Transfer RNA (tRNA) is the smallest polymeric RNA. These molecules seem to be produced via the nuclear processing of a precursor molecule.

The tRNA molecules serve some functions, the most significant of which is to activate amino acids for protein synthesis. The function of tRNA (transfer RNA) is to bind the particular aminoacids, one may think that there are 20 types of tRNAs (that is as many as the constituent amino acids of proteins). Because the code is degenerate (that is there is more than one codon for an aminoacid). There is as well more than one tRNA for a particular aminoacid. Hence there are usually various tRNAs particular for similar aminoacid (Sometimes upto 4 or 5); they are known as isoacceptor tRNA’s. These several tRNA’s, able of binding similar amino acid, different in their nucleotide sequence, they can either have similar anticodon and hence recognize similar codon (or) have dissimilar anticodons and so permit the incorporation of the aminoacid in response to multiple codons specifying similar aminoacid.

As explained earlier, each tRNA is particular for an aminoacid that is it can bind (or accept) only that specific aminoacid. So, tRNAAla denotes a tRNA that is specific for alanine.

Codon is built up of 3 bases, and is exists in mRNA. The tRNA consists of anticodon that is complementary (opposite) to codons in mRNA.

Table: The differences between replication and transcription

1961_diff repliction & transcription.jpg

 

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