The dictionary of coded language of genetic information is called cryptogram. The simplest and smallest possible code is a single code or singlet code which specifies that one amino acid is synthesized by one nucleotide. Singlet code is not possible for coding 20 different amino acids.
Then how many letters would be used as a code for a single amino acid? In addition to the single code, if a code of two alphabets of RNA is used, only 42 = 16 codes are possible. There are a total of 20 essential amino acids; hence the use of double code gives only 16 possible combinations (code).
Therefore, use of a triple code i.e. a code of three nucleotide bases, can serve the purpose. By using triple codes we get 43 = 64 codons or triplets. Therefore, 64 codes are enough for 20 amino acids. The possible singlet, doublet and triplet codes represented in terms of mRNA language are illustrated in Table 7.1.
Crick (1961) provided the first experimental evidence in support to the concept of triplet code of mRNA. When they inserted or deleted single or double base pairs in a particular region of DNA of T4 phage of E. coli, the bacteriophage ceased the normal function. In addition, when three base pairs were added or deleted in the T4 DNA, the bacteriophage performed normal function.
Based on this experiment they concluded that the genetic code is a triplet code because due to addition or deletion of single or double base pairs the reading sequence was changed, whereas it was returned to normal with addition of a third nucleotide. They also suggested that many amino acids are specified by more than one triplet code i.e. the code is degenerate.
