12th Class Biology Genetics Genetic Code

Genetic Code

Category : 12th Class

Defined as structure of nitrogen bases(nucleotides) in mRNA molecule which contain the information for the synthesis of protein molecule. It is discovered by frame shift mutation by Crick.

Codon is the sequence of nitrogen bases (nucleotides) in mRNA, which codes for a single amino acid. Nirenberg and Mathaei (1961) experimentally proved that a single amino acid is determined by a sequence of three nitrogen bases which is known as triplet code. Khorana has got Nobel prize on genetic code.

Salient Features

Triplet : A single amino acid is specified by a sequence of three nucleotides in mRNA i.e., called codon. Due to triplet nature, it consist 64 codon.

Universal : A codon specifies the same amino acid in all organisms from viruses to human beings.

Commaless : There is no pause, so it reads continously.

Non-overlapping : No overlapping between adjacent nucleotide.

Initiation codon : The synthesis of polypeptide chain initiated by initiation codon, which located beginning the cistron i.e., AUG or GUG, which codes to methionine and valine amino acid respectively.

Termination codon : Termination is done by codon. These are UAA, UGA or UAG which does not code to any amino acid. These are also called nonsense codon.

Degeneracy : A single amino acid may be specified by many codon i.e., called degeneracy. Degeneracy is due to the last base in codon, which is known as wobble base. Thus first two codon are more important to determining the amino acid and third one is differ without affecting the coding i.e., known wobble hypothesis, (proposed by Crick) which establishes a economy of tRNA molecule and put forwarded by Crick. Degeneracy of genetic code was discovered by Berrfield and Nirenberg.

 

The Genetic Code Dictionary

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