<P> Ribosomes are the workplaces of protein biosynthesis, the process of translating mRNA into protein . The mRNA comprises a series of codons that dictate to the ribosome the sequence of the amino acids needed to make the protein . Using the mRNA as a template, the ribosome traverses each codon (3 nucleotides) of the mRNA, pairing it with the appropriate amino acid provided by an aminoacyl - tRNA . Aminoacyl - tRNA contains a complementary anticodon on one end and the appropriate amino acid on the other . For fast and accurate recognition of the appropriate tRNA, the ribosome utilizes large conformational changes (conformational proofreading). The small ribosomal subunit, typically bound to an aminoacyl - tRNA containing the amino acid methionine, binds to an AUG codon on the mRNA and recruits the large ribosomal subunit . The ribosome contains three RNA binding sites, designated A, P and E. The A site binds an aminoacyl - tRNA; the P site binds a peptidyl - tRNA (a tRNA bound to the peptide being synthesized); and the E site binds a free tRNA before it exits the ribosome . Protein synthesis begins at a start codon AUG near the 5' end of the mRNA . mRNA binds to the P site of the ribosome first . The ribosome is able to identify the start codon by use of the Shine - Dalgarno sequence of the mRNA in prokaryotes and Kozak box in eukaryotes . </P> <P> Although catalysis of the peptide bond involves the C2 hydroxyl of RNA's P - site adenosine in a proton shuttle mechanism, other steps in protein synthesis (such as translocation) are caused by changes in protein conformations . Since their catalytic core is made of RNA, ribosomes are classified as "ribozymes," and it is thought that they might be remnants of the RNA world . </P> <P> In Figure 5, both ribosomal subunits (small and large) assemble at the start codon (towards the 5' end of the RNA). The ribosome uses RNA that matches the current codon (triplet) on the mRNA to append an amino acid to the polypeptide chain . This is done for each triplet on the RNA, while the ribosome moves towards the 3' end of the mRNA . Usually in bacterial cells, several ribosomes are working parallel on a single RNA, forming what is called a polyribosome or polysome . </P> <P> Presence of a ribosome quality control protein Rqc2 is associated with mRNA - independent protein elongation . This elongation is a result of ribosomal addition (via tRNAs brought by Rqc2) of CAT tails: ribosomes extend the C - terminus of a stalled protein with random, translation - independent sequences of alanines and threonines . </P>

Where is the small subunit of the ribosome assembled