MadSci Network: Cell Biology

Re: What is Ribosomal RNA's purpose???

Area: Cell Biology
Posted By: Michael Onken, WashU
Date: Tue Nov 26 10:33:47 1996
Message ID: 847240135.Cb

What is Ribosomal RNA's purpose???

Ribosomes are the cytoplasmic particles which convert the base codes of all of the cell's mRNA's (the messages transcribed from genes) into protein sequences, to produce all of the proteins in the cell. Each ribosome is composed of two parts: a smaller 40S (MW 1,400,000) subunit composed of the 18S rRNA species and 33 proteins; and a larger 60S (MW 2,800,000) subunit composed of three rRNA species - 28S, 5S, and 5.8S - and 49 proteins. Each subunit serves specific functions of translating mRNA into protein, and both are required for a ribosome to function.

For many years, the rRNA has been regarded as a scaffold upon which the proteins assemble to form the ribosome. In higher organisms (eukaryotes), the 82 ribosomal proteins are required for every part of translation, however in bacteria, which have much fewer ribosomal proteins, some of the functions - like start-site recognition and tRNA binding - are done by the rRNA itself. In fact, recent experiments have shown that there are several cases in which RNA molecules have catalytic activity and can act as enzymes. There has even been work suggesting that most of the functions of the ribosomes can still be carried out (though very slowly) by naked rRNA.

In fact, some rRNA's can catalyze much of translation, and some tRNA's can catalyze their own amino acylation (addition of an amino acid), and several introns can splice themselves in and out of other RNA molecules, so it is logical that a cell could exist which relied on RNA enzymes instead of proteins to function. This has prompted scientists to consider RNA an important part of the beginning of life.

So originally, the rRNA was the ribosome. Over time, certain small proteins bound to the rRNA in a way that enhanced its activity. Since proteins are much better enzymes than RNA, the addition of helper proteins at important sites makes a huge difference in the ribosomes ability to translate. Over billions of years, all of the active sites of the rRNA have been replaced by proteins in higher organisms, such that now the rRNA acts more as a scaffold than as an enzyme.

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