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Deciphering the Code

Deciphering the Code -

Prior to the breakthrough Nirenberg-Matthaei experiments of 1961, work on the genetic code was in the area of theoretical speculations and development of concepts.

This period also has its importance because it laid the theoretical foundations for subsequent experimental work.

The physicist George Gamow proposed the diamond code (1954) and the triangle code (1955) and gave a comprehensive theoretical framework to the different facets of the genetic code.

Gamow's proposals dealt with the general features of the genetic code as recognized today.

The main features of his proposals were: i) A triplet codon correspond into one amino acid of the polypeptide chain.

ii) Direct template translation by codon amino acid pairing.

iii) Translation of the code in an overlapping manner.

iv) Degeneracy of the code, i.e. an amino acid being coded by more than one codon.

v) Colinearity of nucleic acid and the primary protein synthesized.

vi) Universality of the code, i.e. the code being essentially the same for different organisms.

In 1957 Brenner showed that the overlapping triplet code is an impossibility, and subsequent work has shown that the code is a non­ overlapping one (with the exceptions mentioned previously).

Gamow's idea of direct template relationship between nucleic acid was challenged when Crick proposed his adaptor hypothesis.

According to this hypothesis adaptor molecules intervene between nucleic acid and amino acids during translation.

It is now known that tRNA molecules act as adaptors between codons of mRNA and amino acids of the resulting polypeptide chain. An offshoot of the adaptor hypothesis is the comma free code.

The in vitro approach. In principle the most direct way to determine the code would be to determine the sequence of amino acids in a protein and the sequence of nucleotides in the mRNA specifying the proteins.

Although this can be done today, no adequate methods were available in the early 1960s.

Therefore, more indirect methods had to be employed. The discovery of the enzyme polynucleotide phosphorylase in 1955 by Grunberg Manago and

Ochoa made possible the synthesis of polynucleotides containing only a single type of nucleotide repeated several times. Thus it became possible to synthesize polynucleotides containing only U, A, C or G nucleotides.

Polynucleotide phosphorylase differs from RNA polymerase used to transcribe mRNA from DNA in that:

(i) it does not require a template or primer (ii) the activated substrates are ribonucleotide diphosphates and not triphosphates, and (iii) orthophosphate (Pi) is produced instead of pyrophosphate (PPi).

The breaking of the genetic code was made possible by the use of synthetic polynucleotides and trinucleotides.

The different types of techniques used include the use of polymers containing a single type of nucleotide (homopolymers),

the use of mixed polymers containing more than one type of nucleotide (heteropolymers) in random or defined sequences and the use of trinucleotides ('minimessengers') in filter binding.
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