EVOLUTION OF RIBOSOMAL-RNA

The guanine and cytocine content of rRNA of animals seems correlated with the length of periods required for maturation of those organisms. In protostomes the size of the 28S rRNA molecule does not seem to correlate with the evolutionary stage of the organism. Aphids, water-fleas and some protozoa have 18S rRNA from MW 0.9 .times. 106 against an overwhelming pressure of evolution to conserve the rRNA molecule of 0.7 .times. 106 daltons. All the deuterostomes examined were distinguished from protostomes by having the 28S rRNA without a hidden break at the central point. Aphids and nematodes are exceptional protostomes in that they have 28S rRNA without the hidden break. This was discussed in the light of the evolutionary stage of these organisms. Molecular properties of chloroplast rRNA seem to provide evidence for an endosymbiotic origin of this organelle. Mitochondrial rRNA differs considerably from prokaryotic rRNA with respect to molecular size and base composition.