The role of amino acids in protein biosynthesis

The blueprint for synthesising a protein is stored within the coding DNA (cDNA) of the genes in the chromosomes. In order to encode the information to incorporate one of the twenty amino acids likely to be present in a protein, one or two purine or pyrimidine bases are not sufficient since there would be too few unique combinations of bases. It has been proved conclusively that each amino-acid residue in a protein is encoded by a triad or codon of purine/pyrimidine bases in the gene. The sequences of bases in the cDNA and messenger RNA (mRNA) are complementary, with the important exception that some sections of cDNA are not transcribed into RNA. Thus, the bases A, C, G and T in cDNA become U, G, C and A in mRNA. The sections of DNA that are transcribed are known as exons, whereas untranscribed sections are known as introns. 

For some considerable time, it was thought that the genetic code was universal, especially because genetic-engineering experiments showed repeatedly that eukaryotic genes could be expressed in bacteria such as E. coli. More recently, however, it has been found that mitochondria have their own genetic code and protein-synthetic machinery. This has led to discussions about the evolutionary origin of mitochondria, a topic that cannot be pursued here. 

The synthesis of a protein requires the mRNA as a template containing the full sequence of codons, including the codon to terminate synthesis. The ribosomes, which orchestrate protein synthesis, read the mRNA in the 5 — 3 direction. (The 5 end has a phosphate group on the 5 -carbon atom of a ribose moiety whereas the 3 end has a phospate group on the 3 -carbon atom of ribose). Protein biosynthesis requires a transfer ribonucleic acid (tRNA) to convey an amino acid to the growing peptide chain. tRNAs are specific for each codon and contain 60-95 nucleotides, a few of which have unusual structures. The 3 end of the tRNA has the sequence 

It must be appreciated that the foregoing is only a skeletal account of a very complex process involving initiation factors, elongation factors and release factors. In addition, the remarkable structures of tRNAs have not been discussed here. 

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