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How much of the total RNAs is tRNA? Some say 15% and some 20%. Those percentages came from my different teachers. Which is correct? And what are the functions of the extra arm (variable loop) of tRNA? My teacher said it was just for classification but an international textbook says its function is unknown.

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  • $\begingroup$ could you post sources for the claims of 20 and 15% $\endgroup$ – rg255 Feb 8 '16 at 15:57
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    $\begingroup$ Total RNA of what? The species? What species? The cell? What cell? Total in terms of what? The number of genes coding for it? Molecular mass? RNA moieties? Please edit your question and clarify. $\endgroup$ – terdon Feb 8 '16 at 17:24
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With respect to at least two sources I could find, tRNA is commonly reported as encompassing 15% total RNA in both rapidly-dividing mammalian cells such as HeLa, and in S. cerevisiae (1, 2). To give you an interesting tool to look it up in the same way I did, I used Harvards' Bionumbers webpage to search tRNA.

As for the function of the variable loop, it would seem that it serves to stabilize the tRNA overall structure through three-base interactions, and somehow interfaces with the type II tRNA synthetase (3). I'll try to find more on this interaction.

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