Ribosome

Ribosome is important for protein synthesis in which mRNA is translated into polypeptide. It has a complex structure which a large proportion is made up of ribosomal RNA (rRNA) and some proteins. In prokaryotes, each ribosome has a large 50S and small 30S ribosomal subunits. The two subunits combine to made up a total density of 70S. 16S rRNA and 21 proteins assemble to form 50S large subunit whereas 5S rRNA, 23S rRNA and 34 proteins combine to form 30S small subunit. In addition, there are three positions namely aminoacyl site (A-site), peptidyl site (P-site) and exit site (E-site) for holding amino acids. During the initiation stage of translation, the Shine-Dalgarno sequence near the 5’ end of mRNA binds to 16S rRNA in 30S small subunit. With the aid of some initiation factors, initiator tRNA N-formylmethionine binds to the start codon on mRNA and large 50S subunit also associates with the complex to form an mRNA-ribosome complex. The initiator tRNA is positioned at the P-site. Elongation of amino acids begin when tRNA enters the A-site of the ribosome Peptidyl transferase in ribosome facilitates the bond formation between the amino acid on tRNA in P site and that on the newly entered tRNA. The growing peptide chain is then transferred to the A-site. Ribosome translocates to the next codon with the help of elongation factor G and hydrolysis of GTP. The uncharged tRNA moves into the E site and it is then released. The process in translation elongation repeats until a stop codon enters the A-site. As there is no tRNA corresponding to the stop codon, the A-site is occupied by a release factor instead. When release factor binds to the complex, the polypeptide is cleaved and released. The tRNA is also released afterwards. Translation is terminated once the 30S and 50S ribosomal subunits as well as mRNA dissociate.