Ribosomes

Ribosomes

	Ribosomes - A bacterial cell contains about 10,000 ribosomes which co up to 30% of the weight of the bacterium. In contrast to most eukaryote ribosomes, the bacterial ribosomes do not appear to be attached to internal cell membranes, but are found free in the cytoplasm. In functionally, active bacteria the cytoplasm is packed -with free and randomly distributed ribosomes. This dense packing may mask any attachment to internal membranes. Strings of ribosomes have been found loosely attached to the inside of the plasma membrane.
Prokaryote ribosomes are known as 70S ribosomes, although the sedimentation coefficient may vary from 64S to 728, the average being 69S. The 70S ribosomes consists of a larger 50S subunit and a smaller 30S subunit. The E. coli ribosome -consists of three types of RNA (5S, 16Sllnd 238) and 53 proteins the 30S subunit consists of 16S RNA and 21 proteins (SI-S21). The 50S subunit contains 58 and 23S RNA and 34 proteins (Ll-L34). X-ray diffraction studies reveal the dimensions of the E. coli ribosome to be 170 x 230 x 250A (Vasiliev, 1971).
The 30S subunits are oblate ellipsoids. Hydrodynamic and X-ray scattering techniques reveal their dimensions to be 220 x 2Z0 X 55A while according to electron microscope examination the dimensions are 80 x 100 x 190A (Kurland, 1977). The same two techniques give the dimensions of the 50S subunit as 130 x 170 x260A and 160 x 200x 230A,respectively.