This is very good question! I will talk about the nucleotidesequence in human being only. I will try to answer the question,which may be edited further. Three nucleotides denote the aminoacids. There are four types of nucleotides. So 4*4*4 gives you 64options. They are enough to denote the 20 amino acids and start andstop codons for synthesis of the proteins. 2 necleotides will give4*4, that means only 16 options. This sequence of amino acids isunique to every individual. There is little difference in thesequence of amino acids. But this difference makes every individualas unique person. So every body is different and special. If theorder of nucleotides changes, the sequence of amino acids will alsochange. This will invite the immunological response. Why such orderof amino acids is so important to the body is not exactly known tome. That is probably to recognize the self and non-selfprotein.