Structurally, DNA and RNA are nearly identical. As mentioned earlier, however, there are three fundamental differences that account for the very different functions of the two molecules.
The uracil base replaces thymine in RNA. Thymine and uracil are structurally very similar. Uracil has fundamentally the same structure as thymine, with the deletion of a methyl group at the 5' position. Uracil will base pair with adenine in the same way as thymine pairs with adenine ().
The ribose sugar is structurally identical to the deoxyribose sugar, with the addition of a hydroxyl group at the 2' position ().
Unlike DNA, RNA cannot adopt the B-form helix because the additional 2' hydroxyl interferes with the arrangement of the sugars in the phosphate backbone. Although RNA does not adopt the highly ordered B-form of helix, it can be found in the A-form and does base pair to form complex secondary and tertiary structures. The primary structure of a nucleic acid refers to its sequence of base pairs. In RNA, the secondary structures are the two- dimensional base-pair foldings in which local sequences have regions of self- complementarity, giving rise to base pairs and turns. Common secondary structural motifs include hairpins, bulges, and loops ().
The main difference between the three-dimensional structures of DNA and RNA is that in RNA the three-dimensional structure is single-stranded. The base- pairing that occurs in RNA is all through regions of self-complementarity. This three-dimensional arrangement is called the tertiary structure of RNA and it can be very complex.