Biology NUCLEIC ACIDS (RNA)
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NUCLEIC ACIDS (RNA)

Genetic Code :

`->` Term Given by George Gamow.

`->` Discovered by Nirenberg, Mathai 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 `N_2` -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 ; wherease 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 `N_2`-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 `= 41= 4 xx 1 = 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 = 4 xx 4 = 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 = 4 xx 4 xx 4 = 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.

`->` H.G. Khorana artificially synthesized an mRNA.

`->` Severo ochoa enzyme (RNA polymerase enzyme) is also helpful in polymerising RNA in a template independent manner.

Characteristics of Genetic Code :-

(i) Triplet in Nature :-

`->` A codon is composed of three adjacent nitrogen bases which specifies the one amino acid chain.

For Ex.:
# In m-RNA if there are total `90 N_2` - bases.

#Then this m-RNA determines `30` amino acids in polypeptide chain.

# In above example, number of nitrogen bases are `90` so codons `=> 30` and `30` codons decide `30` amino acids in polypeptide chain.

(ii) Universality :-

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

(iii) Non - Ambiguous :-

`->` 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 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 followed 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 Baumfield and Nirenberg .

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

`[tt( ( text (UGC for Tryptophan) ) , ( text (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 base is described as "Wobbly base".




Chain Initiation and Chain Termination Codon :-

`->` Polypeptide chain synthesis is signalled by two initiation codons AUG or GUG.


`->` AUG codes methionine amino acid in eukaryotes and in prokaryotes AUG codes N-formyl methonine.
`->` 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` codons `3`-codons are stopping or nonsense or termination codon .
Nonsense codons do not specify any amino acid.

`tt( ( text(UAA {Ochre} )) , (UAG {Amber}) , (UGA {Opal}) ) }` Non-Sense Codons or Stop codons

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