Ribosomal Structure and Assembly
Ribosome are some microscopic , S , DNAse , membrain less , granular ribosome protein organelle found in all living cell . In prokaryotic cell they are found freely scattered in the cytoplasm. They occur singly (polysome). At the time of protein synthesis 6-8 ribosomes temporarily joined with a mRNA to form a cluster called poly ribosomes or polysomes. On the basis of sedimentation coefficient measured by berg units or S- units. Prokaryotic ribosomes are 70s type with molecular weight 2.7×10⁶ daltons. Its 70s ribosomes is made up of 2 sub units the r-30s subunit remains attached with larger 50s subunit like a cap. Composition :- The prokaryotic ribosome ( 70s ribosomes) contain 3 types of rRNA such as 5s, 16s & 23s . The 16s RNA occurs in a smaller 13s subunit while the 5s & 25s occur in the larger 60s subunit. The 70s bacterial ribosomes are formed of about 54 different type of proteins. 34 are present in 50s subunit & 21 in the 30s subunit. One protein is common to both the subunit & hence there are all together 54 protein. Structure :- Ribosomes are smallest & most abundant organelles of a cell. Its ribosome is porous, hydrated & composed of to unequal subunits larger one dome shaped & the smaller one oblate- ellip said. The larger subunit has a proteiberance , a ridge & a stalk . The smaller subunit has a plateform , claft, head & base. It is about half size of larger sub unit. The smaller subunit fits over the larger one at one end like a cap. The two sub units usually remain separated & come together only at the time of protein synthesis . It's ribosome has four sites for specific function in protein synthesis. They are- mRNA binding sites in smaller subunit ‘A’ site or amino acyl – tRNA site. ‘P’ site or peptidyl – tRNA site. E-site or exoit site to which uncharged tRNA come before living ribosome. |
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| Fig: Different Active site or Ribosome |