Information AboutRibosome |
| CATEGORIES ABOUT RIBOSOME | |
| ribozymes | |
| protein biosynthesis | |
| organelles | |
| SHOPPER'S DELIGHT | |
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A ribosome is an Organelle composed of RNA and Ribosomal Protein s (known as a Ribonucleoprotein or RNP). It Translates MRNA into a Polypeptide chain (e.g., a Protein ). It can be thought of as a factory that builds a protein from a set of genetic instructions. Ribosomes can float freely in the Cytoplasm (the internal fluid of the cell) or bind to the Endoplasmic Reticulum , or to the Nuclear Envelope . Since ribosomes are Ribozyme s, it is thought that they might be remnants of the RNA World . The structure and function of the ribosomes and associated molecules, known as the ''translational apparatus'', has been of research interest since the mid 20th century and is a very active field of study today. OVERVIEW Ribosomes consist of two subunits (Figure 1) that fit together (Figure 2) and work as one to translate the ) RIBOSOME LOCATIONS Free ribosomes Free ribosomes occur in all cells, and also in Mitochondria and Chloroplast s of eukaryotic cells. Free ribosomes usually produce proteins used in the Cytosol or Organelle in which they occur. As the name implies, they are free in solution and not bound to anything within the cell. Membrane bound ribosomes When certain proteins are synthesized by a ribosome they can become "membrane-bound". The newly produced polypeptide chains are inserted directly into the Endoplasmic Reticulum by the ribosome and are then transported to their destinations. Bound ribosomes usually produce proteins that are used within the cell membrane or are expelled from the cell via '' Exocytosis ''. STRUCTURE AND FUNCTION The ribosomal subunits of prokaryotes and eukaryotes are quite similar. However, prokaryotes have 70S ribosomes, each consisting of a (small) 30S and a (large) 50S subunit, whereas eukaryotes have 80S ribosomes, each consisting of a (small) 40S and a bound (large) 60S subunit. However, the ribosomes found in chloroplasts and mitochondria of eukaryotes are 70S, this being but one of the observations supporting the Endosymbiotic Theory . The unit "S" means Svedberg units, a measure of the rate of sedimentation of a particle in a centrifuge, where the sedimentation rate is associated with the size of the particle. It is important to note that Svedberg units are not additive - two subunits together can have Svedberg values that do not add up to that of the entire ribosome. The differences between the prokaryotic and eukaryotic ribosomes are exploited by humans since the 70S ribosomes are vulnerable to some Antibiotic s that the 80S ribosomes are not. This helps pharmaceutical companies create drugs that can destroy a bacterial infection without harming the animal/human host's cells. (AUG) and ends with a stop codon (UAG).]] In Figure 3, both ribosomal subunits (small and large) assemble at the start codon (the 5' end of the mRNA). The ribosome uses TRNA (transfer RNAs which are RNA molecules that carry an amino acid and present the matching anti-codon, according to the genetic code, to the ribosome) which 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 mRNA, while the ribosome moves towards the 3' end of the mRNA. Usually in bacterial cells, several ribosomes are working parallel on a single mRNA, forming what we call a ''polyribosome'' or '' Polysome ''. Atomic structure The general molecular structure of the ribosome has been known for several decades, but recently its structure has been achieved at novel resolutions, in the order of a few Angstrom s. The atomic structure of the 50S large subunit ribosome from the Archea l, ''Haloarcula marismortui'' was published in ''Science'' on August 11, 2000 by N. Ban, ''et al.''Ban N, Nissen P, Hansen J, Moore PB, Steitz TA. The complete atomic structure of the large ribosomal subunit at 2.4 Å resolution. ''Science.'' 2000 Aug 11;289(5481):905-20.. PMID 10937989 Soon after the structure of the 30S from ''Thermus thermophilus'' was published in ''Cell'' on September 1, 2000, by F. Schluenzen ''et. al.''.Schluenzen F, Tocilj A, Zarivach R, Harms J, Gluehmann M, Janell D, Bashan A, Bartels H, Agmon I, Franceschi F, Yonath A. Structure of functionally activated small ribosomal subunit at 3.3 angstroms resolution. ''Cell.'' 2000 Sep 1;102(5):615-23. PMID 11007480 Shortly after a more detailed structure was published in ''Nature'' on September 21, 2000 by B. T. Wimberly, ''et al.''.Wimberly BT, Brodersen DE, Clemons WM Jr, Morgan-Warren RJ, Carter AP, Vonrhein C, Hartsch T, Ramakrishnan V. Structure of the 30S ribosomal subunit. ''Nature.'' 2000 Sep 21;407(6802):327-39. PMID 11014182 Using these coordinates, M. M. Yusupov, ''et al.''Yusupov MM, Yusupova GZ, Baucom A, Lieberman K, Earnest TN, Cate JH, Noller HF. Crystal structure of the ribosome at 5.5 A resolution. ''Science.'' 2001 May 4;292(5518):883-96. Epub 2001 Mar 29. PMID 11283358 were able to reconstruct the entire ''Thermus thermophilus'' 70S particle at low resolution, which was published in ''Science'' on May 4 2001. More recently the structure of the ''E. coli'' 70S ribosome was determined at 3.5 angstroms by Schuwirth ''et al.'' (Science, 2005)Schuwirth BS, Borovinskaya MA, Hau CW, Zhang W, Vila-Sanjurjo A, Holton JM, Cate JH. Structures of the bacterial ribosome at 3.5 A resolution. ''Science.'' 2005 Nov 4;310(5749):827-34. PMID 16272117. Also an EM structure was recently published by Mitra ''et al.'' (Nature, 2005)Mitra K, Schaffitzel C, Shaikh T, Tama F, Jenni S, Brooks CL 3rd, Ban N, Frank J. Structure of the E. coli protein-conducting channel bound to a translating ribosome. ''Nature.'' 2005 Nov 17;438(7066):318-24. PMID 16292303 which depicts a ribosome at 11-15 angstroms in the act of passing a newly synthesized protein strand into a translocation channel. SEE ALSO # Translation # Prokaryotic Translation # Eukaryotic Translation # Organelle REFERENCES EXTERNAL LINK
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