GENETIC CODE

GENETIC CODE

Term Given by George Gamow.

Discovered by Nirenberg, Matthaei and Khorana.

The relationship between the sequence of amino acids in a polypeptide chain and nucleotide sequence of DNA or m-RNA is called genetic code.

There occur 20 types of amino acids which participate in protein synthesis. DNA contains information for the synthesis of any types of polypeptide chain. In the process of transcription, information is transfered from DNA to m-RNA in the form of complementary N2- base sequences.

m-RNA contains code for each amino acid and it is called codon. A codon is the nucleotide sequence on m-RNA which codes for particular amino acid; whereas the genetic code is the sequence of nucleotides on m-RNA molecule, which contains information for the synthesis of polypeptide chain.

Triplet Code :-

The main problem of genetic code was to determine the exact number of nucleotide in a codon which codes for one amino acid.

There are four types of N2-bases in m-RNA (A, U, G, C) for 20 types of amino acids.

If genetic code is singlet i.e. codon is the combination of only one nitrogen base, then only four codons are possible A, C, G and U. These are insufficient to code for 20 types of amino acids.

Singlet code -  (4^{1  =}4 x1= 4) codons

If genetic code is doublet (i.e. codon is the combination of two nitrogen bases) then 16 codons are formed.

Doublet code -  (4²=4x4=16)  codons.

16 codons are insufficient for 20 amino acid.

Gamow (1954) pointed out the possibility of three letters code (Triplet code).

Genetic code is triplet i.e. one codon consists of three nitrogen bases.

Triplet code -  (4³=4x4x4=64)  codons.

 In this case there occurs 64 codons in dictionary of genetic code.

64 codons are sufficient to code 20 types of amino acids.

The chemical method developed by Har Gobind Khorana was instrumental in synthesising RNA molecules with defined combinations of bases (homopolymers and copolymers). Marshall Nirenberg's cell-free system for protein synthesis finally helped the code to be deciphered. Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpful in polymerising RNA with defined sequences in a template independent manner (enzymatic synthesis of RNA). 

Characteristics of Genetic Code:- 

i. Triplet in Nature- 

A codon is composed of three adjacent nitrogen bases  which specific one amino acid in poly chain.

For example:-

In m-RNA if there are total 90 N2-bases.

Then this m-RNA determine 30 amino acids in polypeptide chain.

In above example, number of nitrogen base are 90 so codons = 30 and 30 codons decide 30 amino acid in polypeptide chain.

ii. Universality:- 

The genetic code is applicable universally. The same genetic code is present in all kinds of living organism Inicluding viruses, bacteria unicellular and multicellular organisms.

iii. Non-Ambiguous and specific unamfiguous:-

Genetic code is non ambiguous i.e. one codon specifies only one amino acid and not any other.

In this case one codon never code for two different amino acids. Exception GUG codon which codes both valine and methionine amino acids.

iv. Non-Overlapping:-

A nitrogen base is a constituent of only one codon.

v. Comma less:- 

There is no punctuation (comma) between the adjacent codon i.e. each codon is immediately followd by the  next codon.

If a nucleotide is deleted or added, the whole genetic code read differently.

A polypeptide chain having  50 amino acids shall be specialized by a linear sequence of 150 nucleotides.

If a nucleotide is added in the middle of this sequence, the first 25 amino acids of polypeptide will be same but next 25 amino acids will be different.

vi. Degeneracy of Genetic code :-

There are 64 codons for 20 types of amino acids, so most of the amino acids (except two) can be coded by more than one codon. Single amino acid coded by more than one codon  is called "Degeneracy of genetic code". This incident was discovered by Baurnfield and Nirenberg.

Only two amino acids Tryptophan and Methionine are specified by single codon.

UGG for Tryptophan

AUG for Methionine.

All the other amino acids are specified or coded by 2 to 6 codons. 

Leucine, serine and arginine are coded or specified by 6-codons.

Leucine = CUU, CUC, CUA,  CUG, UUA & UUG.

 Serine = UCU, UCC, UCA, UCG, AGU, AGC.

Arginine = CGU, CGC, CGA, CGG, AGA, AGG.

Degeneracy of genetic code is related to third position (3'- end of triplet codon) of codon. The third bases is described as "Wobbly base".

Chain Initiation and Chain Termination Codon - 

Polypeptide chain synthesis is signalled by two initiation codon is AUG or GUG.

AUG codes methionine amino acid in eukaryotes and in prokaryotes AUG codes N-formyl methionine.

Some times GUG also functions as start codon it codes for valine amino acid normally but when it is present at starting position it codes for methionine amino acid.

Out of 64-codond 3-codons are stopping or Nonsense Codons or termination codon.

UAA (Ochre)

UAG (Amber)

UGA (Opal)

All these are Non-Sense codons or Stop codons.

So only 61 codons are sense codons which specify 20 amino acid.

WOBBLE HYPOTHESIS

It is propounded by CRICK.

Normally an anticodon recognises only one codon, but sometimes an anticodon recognises more than one codon.

 This is known as Wobbling. Wobbling normally occurs for third nucleotide of codon.

For eg. anticodon AAG can recognise two codons i.e. UUU and UUC, both stands for phenyl alanine.

Types of m-RNA- m-RNA is of 2 types:-

1. Monocistronic-  The mRNA, in which genetic signal is present for the formation of only one polypeptide chains eg. Eukaryotes.

2. Polycistronic- The  m-RNA, in which genetic signal is present for the formation of more than  one polypeptide chains eg. Prokaryotes.

Non sense codons are found in middle position in polycistronic m-RNA.

CENTRAL DOGMA

Central dogma was given by Crick.

The formation (production) of m-RNA from DNA and then synthesis of protein from it, is known as Central Dogma.

It means, it includes transcription and translation.

                           

Reverse Transcription:- 

The formation of DNA from RNA is known as Reverse - transcription. It was discovered by Temin and Baltimore in Rous - Sarcoma virus. So it is also called Teminism.

ss-RNA of Rous-Sarcoma virus (Retro virus) produces ds-DNA in host's cell with the help of enzyme reverse transcriptase (DNA-polymerase). This DNA is called 

c-DNA (Complementary DNA).

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