♦️Genetic Code

♦️Genetic Code

(1) Term genetic code was given by George Gamow (1954). He was the first to propose the triplet code (one codon consists of three nitrogen bases).
(2) The relationship between the sequence of amino acids in a polypeptide chain and nucleotide sequence of DNA or mRNA is called genetic code.
(3) 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 transfers from DNA to m-RNA in the form of complementary N2-base sequence.
(4) A codon is the nucleotide sequence in m-RNA which codes for particular amino acid; whereas the genetic code is the sequence of nucleotides in m-RNA molecule, which contains information for the synthesis of polypeptide chain.
(5) 61 out of 64 codons code for only 20 amino acids.
(6) The main problem of genetic code was to determine the exact number of nucleotide in a codon which codes for one amino acid.

🔷Characteristics of genetic code

(1) Triplet in nature
(a) A codon is composed of three adjacent nitrogen bases which specify one amino acid in polypeptide chain.
(b) For example- In m-RNA if there are total 90 N2 – bases. Then this m-RNA determines 30 amino acids in polypeptide chain.

(2) Univerality
(a) The genetic code is applicable universally.
(b) The same genetic code is present in all kinds of living organism including viruses, bacteria, unicellular and multicellular organisms. In all these organisms, triplet code for specific amino acid.

(3) Non-ambiguous
(a) Genetic code is non ambiguous i.e. one codon specifies only one amino acid and not any other.
(b) In this case one codon never code two different amino acids. Exception GUG codon which code both valine and methionine amino acid.

(4) Non-overlapping
(a) A nitrogen base is a constituent of only one codon.

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

(6) Degeneracy of genetic code
(a) Only two amino acids – tryptophan and methionine are specified by single codon.
UGG for tryptophan
AUG for methionine

(b) All the other amino acids are specified or coded by 2 to 6 codons.
(c) Leucine, serine and arginine are coded or specified by 6-codons.
(d) Degeneracy of genetic code is related to third position (3’-end of triplet codon) of codon. The third base is described as ‘Wobble base’.

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