5.12.1 Mechanism of Translation

. . Mechanism of Translation The cellular factory responsible for synthesizing protein is the ribosome. The ribosome consists of structural RNAs and about different proteins. In inactive state, it exists as two subunits; large subunit and small subunit. When the subunit encounters an mRNA, the process of translation of the mRNA to protein begins. The prokaryotic ribosome ( S) consists of two subunits, the larger subunit ( S) and smaller subunit ( S). The ribosomes of eukaryotes ( S) are larger, consisting of S and S sub units. ‘S’ denotes the sedimentation coefficient which is expressed as Svedberg unit (S). One of the alternative ways of dividing up a sequence of bases in DNA or RNA into codons is called reading frame . Any sequence of DNA or RNA, beginning with a start codon and which can be translated into a protein is known as an Open Reading Frame (ORF) . A translational unit in mRNA is the sequence of RNA that is flanked by the start codon and the stop codon and codes for polypeptides. mRNA also have some additional sequences that are not translated and are referred to as Untranslated Regions (UTR). UTRs are present at both ' end (before start codon) and at ' end (after stop codon). The start codon (AUG) begins the coding sequence and is read by a special tRNA that carries methionine (met). The initiator tRNA charged with methionine binds to the AUG start codon. In prokaryotes, N - formyl methionine (f met ) is attached to the initiator tRNA whereas in eukaryotes unmodified methionine is used. The ' end XII Std Zoology Chapter XII Std Zoology Chapter Molecular Genetics of the mRNA of prokaryotes has a special sequence which precedes the initial AUG start codon of mRNA. This ribosome binding site is called the Shine – Dalgarno sequence or S-D sequence . This sequences base-pairs with a region of the 16Sr RNA of the small ribosomal subunit facilitating initiation. The subunits of the ribosomes ( S and S)