Genetic Code and its Co-relation with Wobble - Hypothesis

Genetic Code● Consists of a “TRIPLET “, i.e. A sequence of 3 bases codes for a

particular amino acid .

● There are 4³ or 64 possible combinations .

● Group of 3 bases coding for an amino acid in mRNA is called a codon

PROPERTIES :

● Degenerate : An amino acid can be coded for by more than one codon. Anti codon shows Wobble .

● Unambiguous : Each codon indicates a single , specific amino acid .

● Non-overlapping : When translated , the reading frame is advanced 3 bases at a time .

Genetic Code

Contd. .● The base sequence is read from a fixed point , without any

punctuations.

● Out of 64 codons , 61 codons specify amino acid , while remaining 3 are nonsense or stop codons.

● The code is degenerate - except for Trp and Met,each amino acid is coded by two or more codons .

● Codons are recognized by aminoacyl-tRNAs.

● Aminoacyl-tRNAs activate the amino acid for transfer to peptide

● Aminoacyl-tRNA synthetases does the critical job : linking the right amino acid with "cognate" tRNA

● Reaction:amino acid + ATP → aminoacyl-AMP + Ppiaminoacyl-AMP + tRNA → aminoacyl-tRNA + AMPSum of 1 and 2: amino acid + tRNA + ATP → aminoacyl-tRNA + AMP + PPi

Wobble in the anti codon :

● In 1966, Francis Crick ,proposed “the Wobble hypothesis”.According to this hypothesis, only the first two bases of the codon have a precise pairing with the bases of the anticodon of mRNA, while the pairing between the third bases of codon and anticodon may Wobble (non specific).

● The pairing between the first two bases is normal, while between G and U is against the normal pairing pattern. This unusual bonding between G and U is called Wobble pairing.

● Inosine is the fifth base in the anti codon.

● Inosine arises through enzymatic modification of adenine by ADAR-Adenosine Deanimase RNA specific .

● Inosine is commonly found in tRNAs and is essential for proper translation of the genetic code in wobble base pairs.

● Advantage of wobble: dissociation of tRNA from mRNA is faster