DNA found in the cell's nucleus, contains the genetic code. In actuality, there are four nitrogenous bases in DNA--Adenine, Guanine, Cytosine and Thymine. These pair according to the rules--A pairs with T and C pairs with G. DNA is a double--stranded molecule in the shape of a double helix. Either side of the DNA molecule can serve as a template for messenger RNA to copy in a process called transcription. If the DNA code reads TAC-GGC-TTA, then the complementary mRNA code would be AUG -CCG-AAU. There is no thymine in RNA, so it is substituted with the base Uracil(U). In the genetic code chart, each triplet or codon corresponds to a particular amino acid. For example, AUG in the genetic code chart is the amino acid methionine. It also is the signal to start translation at the ribosome. This results in each triplet be translated and the appropriate amino acid being brought by transfer RNA to the ribosome. A long chain of amino acids, joined with the help of enzymes and peptide bonds, forms a polypeptide. Eventually, a stop codon is reached, for example UGA, and the polypeptide detaches from the ribosome and folds into a functional protein.
Now there are four Nitrogeneous bases in our DNA and DNA codes for proteins. As we already know that there are 20 amino acid(essential amino acid). Let's us consider the following possibiities
Now if we consider the genetic code to be single then it would be impossible as there are only four bases and that cannot code for 20 amino acid.
Case II :
If the genetic code was a double code that an amino acid is code by 2-nitrogeneous bases, then it can formed, 16 codon (4^2), still it is not sufficient enough to code 20 amino acid, so the genetic codon can't be 2
If the genetic code was 3, that is 3 amino acid taken at a time then it will formed 64 codon (4^3). But we have only 20 amino acid. (let's keep for time being)
If the genetic code was 4, that is amino acid taken at a time then it will formed 256 codon (4^4). But we have only 20 amino acid which is way too more. So it can't be 4.
So the only possibility is for the codon to have 3 nitrogenous bases. George Gamow postulated that a three-letter code must be employed to encode the 20 standard amino acids used by living cells to encode proteins . More over all the codon are specific, they will code for a single amino aicd. But an amino aicd can be coded by different codon, so 64 codon codes for the 2 amino aicd.
Hence the genetic code is a triplet codon.