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Functions of Ribosomes

Page history last edited by Devi Singh 13 years, 11 months ago





                                                                                                                       X-ray structure of a 30S ribosome  [4] 





Functions of Ribosomes

Ribosomes play an important part in protein synthesis. It is the site where cells assemble proteins according to genetic information. Ribosomes help to read the message for protein synthesis. There are two cytoplasmic regions where the ribosomes function. There are free ribosomes in the cytosol and bound ribosomes attached to the membrane of the endoplasmic reticulum.The bound ribosomes make proteins that are used for the insertion into membranes ,for packaging within certain organelles,or for export from the cell.. Free ribosomes remain in the cytosol and synthesize cytosolic  proteins  and most extrinisic membrane proteins.


  • Ribosomes act as a template bringing together  the different components in protein synthesis[8]


  • Ribosomes also have a protective function , it protects the mRNA from the action of the nucleus enzymes(nucleases) [8]




Ribosomes and Protein synthesis

   Firstly, an initiation factor captures the tRNA which is carrying an amino acid. Binding occurs to a small ribosomal subunit, which occupies an active site on the ribosome the P (protein) site. This initiation complex recognized and binds to the 5' end of an mRNA molecule and slides down to the initiation codon, which is always an AUG sequence of amino acids. The complex is now joined by the large subunit of the ribosome. An elongation factor brings a second tRNA. Once the anticodon of the tRNA pairs with the next codon of the message the tRNA occupies the A (acceptor) site on the ribosome. The second amino acid is arranged adjacent to the initiation methionine. Part of the large ribosomal subunit peptidyl transferase mediates the separation of the first amino acid from its tRNA. A peptide bond is formed between initial methionine and the amino acid. An uncharged tRNA occupies the P site. Translocation now takes place which is a process whereby the ribosome moves down the mRNA by one codon. The movement causes the growing polypeptide chain to shift to the P position leaving the A site empty. A new charged tRNA can now enter and pair by forming a Hydrogen bond between the codon and anticodon. The bond holds the tRNA in place for an even more stable binding to occur. The tRNA that occupied the P site is removed from the ribosome and is recharged and recycled. The process continues along the mRNA length until the first stop codon is encountered. The termination factor then releases the complete protein from the last tRNA and the ribosome separates into it component parts.[8]



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