The nitrogen bases vary from one DNA segment to another in arrangement so the information is based upon the sequence of their arrangement. The sequence of nucleotides is then the code in which the DNA sends the message. How is the message sent? The DNA molecule replicates in a particular segment and the replicated segment in the form of RNA now called mRNA carries the genetic message in the form of nucleotide sequence (for details see protein synthesis). The sequence of nitrogen bases has been found to be identical to the linear sequence of amino acids in a peptide chain. In other words there is co linearity between mRNA and polypeptide chain.
It is thus clear that the genetic code is in the language of nitrogen bases and the reading of the code is in their sequence of arrangement. Amino acids involved in protein synthesis A large number of amino acids – almost up to 150 are found in nature. Of these however quite a number of them remain as free amino acids in the sense they do not join together to form a molecule of protein.
It has been found out that only about a little more than 20 amino acids are protein amino acids in that all the known proteins are made up of only these amino acids. Some amino acids however are found in an almost all protein chains while a few others are found only in very few proteins. For example cystine is present only in proteins which have a disulphide linkage. Hydroxylproline is another amino acid found only in few proteins.