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Figure 2–72. Ribosomes in a bacterial cell. x 300 000 (After Oparin, 1966).
As structural units containing RNA and proteins, for the first time they
have been separated in homogenized cytoplasm of animal cells by Claude
(1943) and called microsomes. Under electron microscope they are
observed by Palade (1955). As early as next year the presence of a great
quantity of RNA was proven in them (Palade, Siekevitz, 1956 a, b).
By using the methods of gradient centrifugation and labelled atoms
many questions about ribosome structure and functions were clarified. It was
established that the ribosomes of prokaryotic cells (diameter 15—20 nm)
have a sedimentation coefficient about 70S and molecular weight of 2.5—
3.0×10⁶ daltons, and these of eukaryotic cells (diameter 20—30 nm) have a
sedimentation coefficient about 80S and molecular weight of 4—5×10⁶
daltons. The two types of ribosomes (prokaryotic and eukaryotic) have a
groove dividing them into two different subparticles — small and large. For
example 70S ribosomes consist of 30S and 50S subparticles, and 80S — of
40S and 60S. In a presence of Mg again they can form the initial 70S and
80S structures.
Sometimes ribosomes are observed in the form of small groups or
spiral structures, which have been called polyribosomes or polysomes
(Fig. 2–73). It is considered that in this way the effectiveness of using
mRNA is increased, since on them several polypeptide chains can be
synthesized at the same time. But, each one of them is an independently
functioning unit. The building of ribosomal subparticles is realized by a self-assembly
of RNA and protein molecules, at that corresponding rRNA
serves as skeleton of their structure.