Let’s consider the general features of RNA. Although both RNA and DNA are nucleic acids, RNA differs from DNA in several important ways:
1. RNA is usually a single-stranded nucleotide chain, not a double helix like DNA. A consequence is that RNA is more flexible and can form a much greater variety of complex three-dimensional molecular shapes than can double-stranded DNA. An RNA strand can bend in such a way that some of its own bases pair with each other. Such intranwkcular base pairing is an important determinate of RNA shape.
2. RNA has ribose sugar in its nucleotides, rather than the deoxyribose found in DNA. As the names suggest, the two sugars differ in the presence or absence of just one oxygen atom. The RNA sugar contains a hydroxyl group (OH) bound to the 2′-carbon atom, whereas the DNA sugar has only a hydrogen atom bound to the 2′-carbon atom. As you will see later in this chapter, the presence of the hydroxyl group at the 2′-carbon atom facilitates the action of RNA in many important cellular processes. Like an individual DNA strand, a strand of RNA is formed of a sugar-phosphate backbone, with a base covalently linked at the 1′ position on each ribose. The sugar-phosphate linkages are made at the 5′ and 3′ positions of the sugar, just as in DNA; so an RNA chain will have a 5′ end and a 3′ end.
3. RNA nucleotides (called ribonucleotide,) contain the bases adenine, guanine, and cytosine, but the pyrimidine base uracil (abbreviated U) is present instead of thymine. Uracil forms hydrogen bonds with adenine just as thymine does. In addition, uracil is capable of base pairing with G. The bases U and G form base pairs only during RNA folding and not during transcription. The two hydrogen bonds that can form between U and G are weaker than the two that form between U and A. The ability of U to pair with both A and G is a major reason why RNA can form extensive and complicated structures, many of which are important in biological processes.
4. RNA—like protein, but unlike DNA—can catalyze biological reactions. The name ribozyme was coined for the RNA molecules that function like protein enzymes.