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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.
RNA is a single-stranded nucleic acid.
RNA has a ribose sugar instead of a deoxyribose sugar like DNA.
RNA nucleotides have a uracil base instead of thymine.
Other than these differences, DNA and RNA are the same. Their phosphates,
sugars, and bases show the same bonding patterns to form nucleotides and their
nucleotides bind to form nucleic acids in the same way.
The Uracil Base
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 ().
Figure %: Adenine:Uracil Base Pair
The Ribose Sugar
The ribose sugar is structurally identical to the deoxyribose sugar, with the
addition of a hydroxyl group at the 2' position ().
Figure %: Ribose Sugar
The Three-Dimensional Structure of RNA
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 ().
Figure %: Common Secondary Structures of RNA
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.
