This was apparently the first accurate report (by Crick et al., 1961) on the currently known genetic code (central dogma of molecular biology) in terms of amino acid sequence/polypeptide/protein which is basically the end-product of nucleic acid (DNA). Although the earlier work on exploring these codes defined them as non-universal and non-overlapping, the truth of the matter is, that if the code was universal, all the genetic codes were practically impossible. For example, most of the abnormal forms of human proteins like haemoglobin show just a single amino acid change implicating their non-universal nature and non-overlap. Thus, there arose a need to have an arrangement which genuinely shows how to select the correct and coded amino acid “triplets” (protein units). The options suggested in this context were (i) To separate the codes by “commas” or (ii) Making a fixed start point and working along the sequences of bases, say, 3-4 at a time. The second was widely accepted later.
Crick and the team carried out their genetic experimentations on the structural gene of the virus T4 Bacteriophage, which attacks bacteria (E.coli). An exhaustive analyses on the induced mutations (using acridine orange as mutagen) in this gene and their functional implications in culture assays, led to the characterization of mutants as “leaky” or “completely lacking” a particular amino acid. Also, it was found to be possible to revert a mutation, by putting back the amino acid, and by adding a second mutation in vicinity which suppresses this mutation. These mutations proved to be helpful in reading the correct genetic code. It was therefore inferred by these experiments that a protein and its units can be read along as “triplet codes” starting from a point of initiation. Also, the suppressors do not extend along the entire gene, implicating that there could be some termination sequences, or a “nonsense” mutation or probably some complicated protein structure.
The report states a rigorous statement on the theory of genetic code regarding how it is read and the direction in which it is read.
Further studies on deletion mutant or acridine orange mutant conclusively proved that the sequence is read in groups from a fixed “start point” located on the left side of the cistron. But, if this “reading frame” shifts, then, the reading may become erroneous. It is therefore critical to start reading these “triplets” at the right point. Otherwise the reading becomes, what is termed to be “non-sense” or unacceptable or may act as the end of the chain of the amino acid sequence. The analysis of deletion mutations proved and postulated the idea that the sequence is read in groups from a fixed start point and the operation of each functional unit is not affected by the alteration in any other neighbouring cistron. With some standing doubts but with great hopes to solve the protein puzzle in their minds, they reported that the coding sequence is probably three and not the multiple of three.
To summarize, this report depicts the genetic code in terms of amino acid sequence/polypeptide/ protein which is basically the end product of the fundamental genetic material (DNA) in any living cell. Several characteristic features of this genetic code are described and highlighted in this report.
i. A group of three bases codes for a single amino acid.
ii. A code cannot overlap and should be correctly read.
iii. The start point of this sequence is fixed.
iv. The code is “degenerate” i.e., one particular amino acid can be coded by one of the several triplets.