RNA, or ribonucleic acid, has ribose as its sugar. Ribose has five carbon atoms and is called a pentose sugar. DNA, or deoxyribonucleic acid, also contains a pentose sugar, deoxyribose, but its sugar has one less carbon atom than ribose.
The chemical properties of ribose lend shape to the RNA molecule. The phosphate group (PO4) has a negative charge. Ribose has an extra oxygen atom bonded to a hydrogen atom, yielding a hydroxyl group. Hydroxyl groups also have a negative charge. The negative charge on the phosphate group and the negative charge on the hydroxyl group repel each other, forcing the RNA molecule to have less attraction and less coiling in the molecule. DNA, on the other hand, lacks this hydroxyl group, so there is no repelling of different groups; therefore, DNA has more attraction between its molecules and appears tightly coiled.