Ribosomal RNA See ribonucleic acid and ribosomes

ribosomal scanning model The hypothesis that initiation of protein synthesis involves scanning of the 40S ribosomal subunit along the mRNA molecule until it encounters the first AUG from the 5' terminus in order to begin protein synthesis with a methionine.

ribosome binding site A sequence of 4-7 nucleotides in mRNA to which ribosomes bind. In prokaryotes this is termed a 'Shine-Dalgarno sequence' and is a sequence complementary to the 3' end of the 16S rRNA. In eukaryotes there is considerable sequence variation at the 5' end of the mRNA, and a consensus ribosomal binding sequence is not found.

Shine J and Dalgarno L (1974) Proc Natl Acad Sci 71, 4734

ribosomes Small, round, electron-dense particles, 10-20nm in diameter, found on the outer surface of the limiting membrane of the rough-surfaced endoplasmic reticulum. Also found free in the cytoplasm, sometimes in the nucleus and mitochondria, and present in all types of living cells. Name introduced in 1957 to distinguish the particulate ribosomes from membrane-associated ribosomes known as 'microsomes', obtained on cell fractionation. They contain 40% protein and 60% RNA, and play a vital part in protein synthesis, attaching to mRNA to form polysomes. In mammalian cells the ribosome consists of two subunits that sediment at 40S and 60S, respectively.

ribovirus RNA-containing virus.

ribozyme An RNA molecule with catalytic activity.

Rida virus An Icelandic strain of the prion disease scrapie, probably imported from Scotland, UK.

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