Question

If the genetic code did evolve over time, when would this have appeared to have happened, and what observations demonstrate that the genetic code was subject to natural selection sometime in the past ("frozen adaptation" as opposed "frozen accident")? What evidence can support either viewpoint?

Answer 1

Genetic Code – The codons are present in mRNA in the form of triplets i.e., three bases together for a particular amino acid. Some 64 types of codons are found in mRNA, one of them is the initiator, three are terminators and rest code for any one amino acid out of 20 amino acids found in the protein.

The important characteristics of Genetic code are as under:-

i. The Code is triplet.

ii. The Code is degenerate.

iii. The Code is non-overlapping.

iv. The Code is comma-less.

v. The Code is non-ambiguous and

vi. The Code is universal – While the very existence of mRNA codon dictionary has been mainly as an outcome from the work on micro-organisms E.coli, there is no doubt that in all kinds of living organisms the same genetic code is used that means genetic code is universal.

Evolution of Genetic code:-

The Genetic code has evolved with the evolution of life.

Codon reassignment – This process requires both the elimination of tRNAs for sense codons or release factors for termination codons. Thus creating novel tRNAs and release factors that recognizes the concerned codon.

The three main concepts on the evolution of the code are as under:-

i. the Stereochemical theory – According to this theory the codon assignments are dictated by physic – chemical affinity between amino acids and the anticodons.

ii. the Coevolution theory – According to this theory the code assignments coevolved with amino acid biosynthesis pathways.

iii. the error minimization theory – According to this theory the code was evolved as a result of natural selection to minimize the adverse effect of point mutations and errors during translation.

These theories are not mutually exclusive and are inter-related with each other. These theories are also compatible with frozen adaptation hypothesis over frozen accident hypothesis.

Frozen accident hypothesis- This hypothesis assumes that the genetic code was formed through a random combination of its components which are formed by an abiotic route proposed by Crick in 1968. The Genetic code having high evolutionary potential made it successful in competition with all other systems and it emerged as sole universal system to the present date. But taking into account the complex nature of protein synthesis machinery and age of our universe there is a very low probability of the correctness of this hypothesis.

Frozen adaptation hypothesis – According to this hypothesis the Genetic code are co-adapted with or ‘frozen’ by the protein coding genes that they translate therefore cannot easily change by natural selection. The standard Genetic code exhibits non- random pattern that corrects common errors in protein coding genes during the transmission of information. The repeated selection over small successive changes produces Genetic code that is substantially error correcting.

• From the study of Genetic code for various species of extant organisms it is assumed that the Genetic code had started from a limited number of amino acids and evolved to the universal Genetic code of most extant organisms.
• The simple mitochondrial genetic system with small genome size is interesting as the model case of evolution of genetic code since it is greatly deviated from the universal Genetic code and also it allows larger variation of genetic code than other genetic systems.