04 Molecular Basis of Inheritance - part 07 - Characterestic of Genetic code:

04 Molecular Basis of Inheritance - part 07 - Characterestic of Genetic code:


Characterestic of Genetic code:
  • Genetic code of DNA has certain  fundamental characteristics ­ 
i. Genetic code is a triplet code: 
  • Sequence of three consecutive bases constitute codon, which specifies one particular amino acid. 
  • Base sequence in a codon is always in 5'- 3' direction. 
  • In every living organism genetic code is a triplet code. 
ii. Genetic code has distinct polarity : 
  • Genetic code shows definite polarity i.e. direction. It, therefore, is always read in 5' - 3' direction and not in 3' 5' direction. 
  • Otherwise message will change e.g. 5' AUG 3'. 
iii. Genetic code is non-overlapping : 
  • Code is non overlapping i.e. each single base is a part of only one codon. Adjacent codons do not overlap. 
  • If non-overlapping, then with 6 consequtive bases only two amino acid molecules will be in the chain Had it been overlapping type, with 6 bases, there would be 4 amino acid molecules in a chain. 
  • Experimental evidence is in favour of non-overlapping nature.
iv. Genetic code is commaless : 
  • There is no gap or punctuation mark between successive/ consecutive codons. 
v. Genetic code has degeneracy : 
  • Usually single amino acid is encoded by single codon. However, some amino acids are encoded by more than one codons.
  •  e.g. Cysteine has two codons, while isoleucin has three codons. This is called degeneracy of the code. 
  • Degeneracy of the code is explained by Wobble hypothesis. 
  • Here, the first two bases in different codons are identical but the third one, varies. 
vi. Genetic code is universal : 
  • By and large in all living organisms the specific codon specifies same amino acid. 
  • e.g. codon AUG always specifies amino acid methionine in all organisms from bacteria up to humans. 
vii. Genetic code is non-ambiguous : 
  • Specific amino acid is encoded by a particular codon. Alternatively, two different amino acids will never be encoded by the same codon. 
viii. Initiation codon and termination codon: 
  • AUG is always an initiation codon in any and every mRNA. 
  • AUG codes for amino acid methionine. 
  • Out of 64 codons, three codons viz. UAA, UAG and UGA are termination codons which terminate/ stop the process of elongation of polypeptide chain, as they do not code for any amino acid. 
ix. Universal : 
  • Usually in all organisms the specific codon specifies same amino acid. 
x. Codon and anticodon : 
  • Codon is a part of DNA e.g. AUG is codon. It is always represented as 5' AUG 3'. 
  • Anticodon is a part of tRNA. It is always represented as 3'UAC 5'.
  • It is possible to predict sequence of codon on mRNA by studying the sequence of amino acids in a polypeptide chain.


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