Genetic Code

Central dogma of life explains genetic information transfer. Here, the Linear Genetic Sequence  viz. A, T, G and C of polypeptide chain are integrated with 20 amino acids. DNA transcripts genetic information in to mRNA and are then translated in to protein. During the translation of protein, RNA gets attached to the polypeptides through cohesion molecules called ‘adapters’. Special adaptor molecules are referred  as transfer RNA (tRNA). They are capable of recognizing three nucleotides which are consecutively called as codons. There  are 4 nucleotides and the number of possible combinations are 64.

History of genetic code

Francis Crick was the first person to discover the wobble concept of ‘genetic code’ in the year 1966. He observed that hydrogen bond is established between anticodons in 5’ position with codons of 3’position. For instance, U codes either adenine or guanine pairing. Crick postulated Wobble rule to crack genetic code.

Wobble rule

Wobble rule allows base pairing only between purine – purine base or pyrimidine – pyrimidine base. This helps in predictions  between ribose – ribose structure.

Wobble rule will not allow a tRNA molecule to identify the entire four different codons. Only three codons shall be identified if and only one (inosine) enters anticodon 5’ position. For instance, there are three serine codons formed by three tRNA. Their confirmation is predictable by stacking interactions between the bases.

Genetic code cracking

One great achievement of molecular biology is cracking genetic code. Till the discovery of mRNA translation of protein in the year 1966, there was no hope or thought of genetic code. Detailed knowledge of genetic code is possible only by proper understanding of codons coding for specific amino acid sequence of polypeptide chain ultimately to form protein. DNA sequencing methods were introduced by Sanger and many others. These methods are primitive. Right after one year, the discovery of mRNA resulted in production of artificial or synthetic messenger RNA. Artificial mRNA is made from cell free extracts by in vivo method. Thus, synthetic mRNA transmitted the genetic informations for the synthesis of protein which helped to crack the genetic code.

Codons’ direct chain termination

Codons like UAA, UGA and UAG do not correspond to any of the 20 amino acids which terminate the polypeptide chain. It is because these codons are not recognized by tRNAs. However, they are recognized by specific proteins. These specific proteins act as release factors. For instance, one such release factor in bacteria is RF1, RF2 and eRF1 in eukaryotes. Ribosome is the place where mRNA translates into protein. Hence, release factors enter the ribosomes and activate the hydrolysis of peptidal tRNA there by synthesizing new proteins as release factors.