RNA is instable, because

RNA is more instable than DNA. The difference between DNA and RNA is the component of them. A nucleotide consists of a nucleic base, a five-carbon sugar and a phosphate group. The sugar of DNA is the deoxyribose, whereas that of RNA is the ribose. The oxygen of 2’-OH at the ribose has two lone pairs. In a basic state, the neighboring phosphate group receives a nucleophilic attack by the hydroxyl group of a ribose. The nucleophilic attack is a chemical reaction that the phosphorus of a phosphate group bonds to the oxygen of a hydroxyl group for filling up the instable electrons mutually. As the result, the phosphodiester bond between nucleotides is cut. In contrast, deoxyribose doesn’t have the 2’-OH and its lone pair. Hence, such a reaction doesn’t occur, therefore DNA is more stable than RNA.