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DNA is not the only type of nucleic acid. Another important type is RNA. It shares the same fundamental structure as DNA, but has some crucial differences (Figure 4.14). First, the ribose sugar has a hydroxyl (–OH) group on the 2′ position on the sugar, unlike the sugar in DNA (deoxyribose), hence its name ribonucleic acid, as opposed to deoxyribonucleic acid (DNA). Second, in RNA the thymine base is replaced with a uracil (U) base. Uracil is a demethylated form of thymine (it lacks the methyl, or CH₃ group, on the ring).

Figure 4.14 The structure of RNA. (a) The schematic structure and bases of a single strand of RNA. (b) The molecular structure of the RNA molecule. The key difference with DNA is the presence of the –OH group at the 2′ position of the ribose sugar in the sugar–phosphate backbone, hence the name ribonucleic acid, in comparison to deoxyribonucleic acid (DNA). RNA also uses a uracil (U) rather than a thymine (T) base.

The synthesis of RNA is similar to DNA, but the molecule is built up using triphosphate molecules that have a ribose sugar instead of deoxyribose (i.e. ATP, CTP, GTP, and TTP).

As a sneak preview, but a point of astrobiological interest, you will meet ATP again when we discuss energy harvesting in cells (Chapter 6). It is interesting that ATP is processed to dATP to make DNA, which might suggest that RNA is more primitive and was a precursor to DNA, a point that leads to suggestions that RNA was an early form of genetic information (Chapter 12). It might also suggest a deep link between energetic processes, which use ATP to store cellular energy, and information storage, in which ATP is a building block of RNA.