RNA, like DNA, is a polymer consisting of nucleotides joined together by phosphodiester bonds (see Chapter 8 for a discussion of DNA structure). However, there are several important differences in the structures of DNA and RNA. Whereas DNA nucleotides contain deoxyribose sugars, RNA nucleotides have ribose sugars (Figure 10.1a). With a free hydroxyl group on the 2′-­carbon atom of the ribose sugar, RNA is degraded rapidly under alkaline conditions. The deoxyribose sugar of DNA lacks this free hydroxyl group, so DNA is a more stable molecule. Another important difference is that thymine, one of the two pyrimidines found in DNA, is replaced by uracil in RNA.

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A final difference in the structures of DNA and RNA is that RNA usually consists of a single polynucleotide strand, whereas DNA normally consists of two polynucleotide strands joined by hydrogen bonding between complementary bases. Although RNA is usually single stranded, short complementary regions within a nucleotide strand can pair and form secondary structures (Figure 10.1b). For example, complementary regions within a transfer RNA molecule fold to form a cloverleaf secondary structure, which is important in the function of the tRNA. Exceptions to the rule that RNA is usually single stranded are found in a few RNA viruses that have double-stranded RNA genomes. Similarities and differences in DNA and RNA structures are summarized in Table 10.1. TRY PROBLEM 15