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Section: Genetics » The Genetic Code
 
 
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  Second genetic code, and second half of the genetic code
 
     
 
Content
⇒ The Genetic Code
⇒ Properties of genetic code 
⇒ Chain initiation and chain termination codons
⇒ Synonym codons and degeneracy
⇒ Mutations and the genetic code 
⇒ Wobble hypothesis
⇒ New genetic codes in mitochondria and ciliate protozoa
⇒ Suppressor mutations, base substitutions and suppressor tRNAs
⇒ Second genetic code, and second half of the genetic code
⇒ Recoding of the genetic code
'Second Genetic Code' and 'Second Half of the Genetic Code'
In the next main topic we will discuss about the 'Second Genetic Code' and 'Second Half of the Genetic Code'. A distinction, therefore, needs to be made of these two expressions from 'The Genetic Code' described in this section. As already mentioned earlier in this section, the genetic code is meant for translation of the language of nucleic acid into that of protein, using triplet codons. However, the 'Second Genetic Code' refers to the mechanism involved in the interaction of specific tRNAs with corresponding amino acyl synthetase enzymes (how does an enzyme recognize its tRNA or vice versa ?) and the 'Second Half of the Genetic Code' refers to the protein folding problem or to the rules, which govern the formation of a three dimensional structure from the primary structure (amino acid sequence) of a protein. Both these problems remain only partly resolved and continue to receive attention of molecular biologists.

X-ray crystallographic structures of several 'tRNA-synthetase complexes' representing intermediates in the formation of 'amino-acyl tRNAs' have been studied. The most important of these is a complex between 'glutamyl-tRNA synthetase' with tRNAGln (tRNA for glutamine) and ATP. It was shown that there are multiple points of contact all along the inner side of the L-shaped structure of tRNA, which helps in recognition of tRNA by synthetase enzyme and in the formation of linkage between tRNA and amino acid (consult Expression of Gene : Protein Synthesis 1.  Proteins and Protein Synthesis Apparatus (tRNA and Ribosomes) for more details).
 
     






     
     
 
 
     
 
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