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Figure 2–73. Polyribosomes from rabbit reticulocytes (Courtesy of A. Rich; From Levine, 1968).
The biochemical composition of ribosomes includes mainly RNA and
proteins with very small quantities of lipids. In its essence they are
ribonucleoproteins — 60% RNA and 40% proteins, which are similar to the
nuclear histones. The proportion RNA/protein is of the same order for both
subparticles. The rRNA contained in them differ in sedimentation coefficient
and molecular weight. The small subparticle contains rRNA from 16S to
18S with molecular weight of 0.55—0.70×10⁶ daltons, and the large one —
from 23S to 28S with molecular weight of 1.2—1.7×10⁶ daltons varying in
different species. In the large subparticle one more kind of rRNA was
discovered — 5S, with a molecular weight of about 3.0×10⁴ daltons.
Still, quite a lot are the non-elucidated questions about the structural
organization of ribosomes, the presence and functions of different rRNAs in
them, etc. What is considered undoubted is, that they are the “cell plants”
for synthesizing proteins. Their key position in this process is the reason for
the great interest in them and subsequent numerous investigations.
Especially important is the question on the genesis of ribosomes in the
process of evolving the living systems and about the way of protein
synthesis in precellular forms before their appearance. There are two
possibilities: the first — they are a result of intracellular differentiation; the
second one — they have been included from outside, i.e. earlier they have
existed independently. Clarifying this question will shed light on some
aspects of the problem about the origin and evolution of life.