Genetic Code


The process of translation requires the transfer of genetic information from a polymer of nucleotides to synthesize a polymer of amino acids. Neither does any complementarity exist between nucleotides and amino acids, nor could any be drawn theoretically. There existed ample shreds of evidence, though, to support the notion that changes in nucleic acids (genetic material) were responsible for the change in amino acids in proteins.

Term Given by George Gamow
Discovered by Nirenberg, Matthaei and Khorana.

Codon:- is the nucleotide sequence on mRNA which codes for a particular amino acid. 

Genetic code:- is the sequence of nucleotides on mRNA which codes for the whole polypeptide chain.







Triplet code:- by Gamow



Marshall Nirenberg’s and Matthaei



synthesized protein in the cell-free system. They enzymatically synthesized RNA by Severo Ochoa enzyme (polynucleotide phosphorylase ) This enzyme form RNA without DNA template. First discovered codon UUU.









The chemical method developed by Har Gobind Khorana:- Was instrumental in synthesizing RNA with defined combinations of bases (homopolymers and copolymers). 







CHARACTERISTICS OF GENETIC CODE 


1. Triplet in nature:-

A codon is composed of three adjacent nitrogen bases which specify one amino acid in the polypeptide chain For e.g. In m-RNA number of 
Nitrogen bases = 90
 So codons = 30 
 Decide amino acids = 30


2. Nearly Universal:-


In all kinds of living organisms the same codon code the same amino acid. For e.g. in Bacteria:- UUU code Phenylalanine Plants:- UUU code Phenylalanine Animals:- UUU code Phenylalanine 
Exception:- Mitochondrial codon Some Protozoans


3. Non-overlapping:- 

A nitrogen base is a constituent of only one codon 
Exception:- some viruses


Non-overlapping              Overlapping 

1. AUG 
2. CUA 
3. GUC



4. Commaless :- 

There is no punctuation (comma) between the adjacent codon i.e. each codon is immediately followed by the next codon. If a nucleotide is deleted or added, the whole genetic code read differently


5. Non-ambiguous:- 

one codon code only one amino acid and not any other e.g. UUU code:- only Phenylalanine GGG code:- only Glycine Exception GUG code:- both valine and methionine amino acids. 

Degeneracy of genetic code:- 

Single amino acid coded by more than one codon.
eg



Leucine :- 6 codon
Serine :- 6 codon
Arginine :- 6 codon   

Exception
Methionine :- 1 codon 
Tryptophan :- 1 codon



WOBBLE HYPOTHESIS 


It was propounded by CRICK 
•Sometimes an anticodon recognizes more than one codon. 
•Wobbling normally occurs for the third nucleotide of codon 
•Wobble base Codon (m-RNA ):- 5’ Third N base 3’ First N base 




MONOCISTRONIC

The m - RNA in which genetic signal is present for the formation of only one polypeptide chain. eg. Mostly in Eukaryotes




POLYCISTRONIC

The m-RNA, in which a genetic signal is present for the formation of more than one polypeptide chain. eg. Mostly in Prokaryotes. 



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