Genetic code

Central Dogma Of Protein Synthesis: (By Crick)





Genetic Code:

Dictionary of nucleotide base combinations which determine sequence of amino acids in proteins.



A sequence of 3 nucleotide bases on m-RNA which

                           codes for a single amino acid. E.g. AUG codes for


(Number of Codons= 4 x 4 x 4 (Since there are 4 different nucleotide bases Purines: A, G Pyrimidines: C, U NOT T)

                                    = 64

Holley, Khurana and Nirenberg:

Nobel for deciphering/ cracking the                   

                                                           genetic code

61 sense codons ― 20 standardamino acids

3 non-sense/stop codons― UAG-Amber



(Selenocysteine is coded for by UGA, though it is a stop codon which depends on availability of selenium)


Read on m-RNA from 5’→3’ without gaps

One codon → One amino acid

Exception: AUG (start codon) →Methionine, but if AUG not available, GUG→Methionine

One amino acid→Multiple codons E.g.: GGG, GGA, GGC, GGU code for glycine

Same in all organisms from bacteria to mammals (exceptions are found in yeast mitochondria and mycoplasma)

Wobble Hypothesis:

  1. Since genetic code is degenerate, 61 codons code for 20 amino acids.
  2. BUT the number of t-RNA is NOT 61, but less. This is due to wobbling of 3rd base of codon.
  3. The 3rd base of t-RNA may not be complementary. Actual pairing occurs at 1st two positions only.

m-RNA:   GUU         GUC       GUA       GUG

t-RNA:CAA          CAA       CAA        CAA

Amino:     Val            ValValVal


  • Single Gene Mutation:

Sickle Cell Anaemia: GAG → GUG,

Glu → Val ( At 6th position of β Chain of Hb)

Sickling under low oxygen tension

  • Frame Shift Mutation:

Addition/Deletion results in an altered reading frame resulting in:

  1. Altered amino acid sequence
  2. Premature termination of peptide chain resulting in dysfunctional/non-functional protein.

— Jay Shah

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