The information in DNA remaining stable is essential. You can think of DNA as the “master copy” of a computer program. When you get a new program, you first copy it from the purchased disk onto the hard disk of the computer and use that copy as the source of the program for use. You store the original copy of the program away and only use it if the first one crashes. If DNA information were to be used regularly in the cell, it could accumulate errors, which would be passed on from one generation to the next. Before too long, the DNA would have so many errors that it would lose important functions and couldn't support the organism. Just as a clever computer user avoids this problem by making copies of the programs before using them, cells use copies of their genomic information for the working processes of information flow (essentially protein synthesis) in the cell. These copies are made of a related nucleic acid, RNA.
RNA molecules are structurally and metabolically distinct from DNA molecules. First of all, the sugar present on the nucleotide is ribose and not deoxyribose. This has the consequence that an RNA chain is less stable than the corresponding DNA because the 2′‐OH group makes the 3′,5′ phosphodiester chain of RNA hydrolyzable in alkaline solution. In an alkaline solution, OH ‐ ions in the solution remove a proton from the 2′‐OH of ribose; the 2′‐O ‐ then is attracted to the central, relatively positive, phosphorus. The resulting inter‐mediate can be resolved by the cleavage of the phosphodiester bond and the breaking off of the next nucleotide in the chain as shown in Figure 1.
Figure 1
DNA doesn't undergo this sort of cleavage because it doesn't have a 2′‐OH group to be ionized in the presence of hydroxide ion. As discussed previously, when DNA is exposed to base, the two strands of the double helix separate but otherwise aren't altered. Secondly, RNAs are almost always single‐stranded. This means that RNA information can't be repaired, at least in the way that DNA is repaired (by the use of the information on one strand as a template to direct the synthesis of the complementary strand). Again, this property is consistent with the instability of RNA during cell metabolism. The fact that RNA is single‐stranded means that intramolecular base‐pairing determines its structure. This property gives RNA chains a compact secondary structure, in contrast to the extended structure of a DNA double helix. See Figure 2 .
Figure 2
Not all the base‐pairing is of the Watson‐Crick variety, either. For example, G‐U, U‐U, and A‐G “base pairs” are relatively common. These interactions contribute to the wide variety of structures that RNA can assume. RNA molecules also contain modified nucleosides, and in some cases, quite complicated ones. These are synthesized post‐transcriptionally as part of the maturation process of RNA and serve to “fine‐tune” RNA functions.