6. Amino acids are the structural @# units monomers that make up proteins. They join together to form short polymers chains called peptides or longer chains called either polypeptides or proteins. These chains are linear and unbranched, with each amino acid residue within the chain attached to two neighboring amino acids. The process of making proteins encoded by DNA/RNA genetic material is called translation and involves the step-by-step addition of amino acids to a growing protein chain by a ribozyme that is called a ribosome The order in which the amino acids are added is read through the genetic code from an mRNA template, which is an RNA copy of one of the organism's genes. Twenty-two amino acids are naturally incorporated into polypeptides and are called proteinogenic or natural amino acids. Of these, 20 are encoded by the universal genetic code. The remaining 2, selenocysteine and pyrollysine, are incorporated into proteins by unique synthetic mechanisms. Selenocysteine is incorporated when the mRNA being translated includes a SECIS element, which causes the UGA codon to encode selenocysteine instead of a step codon. An unknown amino acid has 0.032% by wt. sulphur. If each molecule has one S atom only, No. of molecules in 1gm of amino acid.Pyrollysine is used by some methanogenic archaea in enzymes that they use to produce methane. It is coded for with the codon UAG, which is normally a stop codon in other organisms. This UAG codon is followed by a PYLIS. Several independent evolutionary studies, using different types of data, have suggested that Gly, Ala, Asp, Val, Ser, Pro, Glu, Leu, Thr (i.e. G, A, D, V, S, P, E, L, T) may belong to a group of amino acids that constituted the early genetic code, whereas Cys, Met, Tyr, Trp, His, Phe (i.e. C, M, Y, W, H, F) may belong to a group of amino acids that constituted later additions of the genetic code.