RNA, unlike DNA, have only one strand. The single-stranded nature of these molecules does not mean their structure is random. The single strand tend to take up a right-handed helical conformation that is dominated by base-stacking interactions, which are stronger between to purines than between a purine and a pyrimidine or between two pyrimidines.

Any self-complementary sequences in the molecule will lead more complex and specific structures. Unlike the double helix of DNA, there is no simple, regular secondary structure that forms a reference point for RNA structure.

Several classes of RNAs are found in cells, each with a distinct function. Ribosomal RNA (rRNA), are structural components of ribosomes, Messenger RNA (mRNA) are nucleic acids that carry the information form one or a few genes to the ribosome, where the corresponding protein ca be synthesized and the Transfer RNA (tRNA), which are adapter molecules that faithfully translate the information in mRNA into a specific sequence of amino acids.