The primary structure of RNA is similar to that of DNA, but with a few notable exceptions. First, in RNA, instead of thymine, the pyrimidine base uracil occurs, forming a complementary base pair with adenine in regions of double-stranded RNA. Also, a wide variety of ribonucleotides having modified or minor bases are found in naturally occurring RNA, one of the commonest of which is pseudouridine. In human tRNAs, as many as 25% of the bases are nonstandard. Although the role of base modification is not clear, it may be important for biological recognition. The other important feature of the primary structure of RNA is the presence of the 2'-hydroxyl group in ribose. Although this hydroxyl group is never involved in phosphodiester linkages, it does impose restriction on the helical conformations accessible to double-stranded RNA.

RNAs are usually single-stranded molecules that are stable allowing different regions of the ribonucleotide to form distinct secondary structural elements. When self-complementary regions of the RNA strand are aligned, duplex regions, which may have Watson-Crick base pairs, are formed.

RNA has a variety of functions within a cell; for each function, a specific type of RNA is required. Messenger

Thymine h

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The Mediterranean Diet Meltdown

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