Ribosomes; Structure and function.

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Ribosomes; Structure and function                                                          9 April 2003

Carly Brooks

Ribosomes are cytoplasmic organelles discovered in both prokaryotic and eukaryotic organisms.  Found in great abundance up to 10,000 in bacterial cells and many times more in eukaryotic cells, they comprise of proteins and rRNA molecules known as subunits, to form a large ribosomal complex.  Both eukaryotic and prokaryotic ribosomes in association with transfer RNA (tRNA), act as a site for mRNA translation, assembling a specific sequence of amino acids into polypeptide chains, once the mRNA joins the two component subunits (large and small) of the ribosome.  The tRNA is covalently bonded to an individual amino acid and has a complimentary nucleotide sequence, an anticodon, to each mRNA codon which form base pairs, adding specificity to the selection of the corresponding amino acids.  The mRNA is linked by hydrogen bonds to the tRNA and is held in proximity to the amino acid so that a peptide bond is formed, this process occurs again and each amino acid is polymerized into a growing peptide chain.

Ribosomes exist in two distinct forms; free and bound and may be positioned in several locations throughout the cell depending on cell function.  Free ribosomes can occur individually, a monosome, or in clusters called polyribosomes or polysomes and are found in the cytosol (the fluid component of cytoplasm, excluding organelles and the insoluble, usually suspended, cytoplasmic components).  Found in greater concentrations in cells that retain proteins, they manufacture proteins that are either held in solution in the cytoplasm or those used in the formation of cytoplasmic structural and motile elements.  Bound ribosomes are situated on the outside of the endoplasmic reticulum forming rough ER (RER) and a large number are seen in cells that make proteins to be secreted out of the cell, such as pancreatic cells producing digestive proteins.  The bound ribosomes form proteins that may be utilised for the cell membrane, packaged into vesicles for storage, as well as those required for export from the cell.  

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Ribosomes are also located in mitochondria and chloroplasts within eukaryotic organisms, these are smaller than those found in the cytoplasm and can be compared to bacterial ribosomes.  The size of the ribosome denoted S (S = Svedberg unit) is derived from their rate of sedimentation relating their molecular weight, as well as their three dimensional structure.  This is not additive so the total for example in prokaryotic ribosomes is 70S consisting of 30S and 50S constituent parts, the way in which these fit together alter the Svedberg unit for the whole ribosome.

 Figure 1, Prokaryotic ribosome                Figure 2, Eukaryotic ribosome


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