ABSTRACT
The protein structure in DNA is coded by the genetic code, with the exception of some viruses that use RNA for this purpose. A very complex cell apparatus has evolved to translate the language of nucleotides in DNA into the language of amino acids in proteins. A multistep biosynthesis of the polypeptide chain starts with the transcription of DNA into RNA. The transcribed RNA, called the primary transcript, is then transformed in a series of molecular events into the mRNA, which is used as a template for the next step of the biosynthesis, termed translation. This step takes place in the ribosome and is assisted by a large number of accessory proteins, tRNAs, and energy-providing molecules. The three-step procedure of the protein biosynthesis is basically the same in all organisms. All the steps of the protein biosynthesis are strictly regulated in order to get the proper repertoire of functional proteins present in the cells and in the extracellular solutions. Recently, techniques have been developed to additionally include into the protein sequence some of the noncoded amino acids in order to design and produce proteins with novel functions. In addition to the proteins, the majority of physiologically important peptides are also synthesized via a transcription and translation process; however, some of the important peptides are assembled nonribosomally via a series of enzyme-catalyzed reactions.
