Ribonucleic acid (RNA) is a family of biomolecules which perform several essential functions within all cells. As a type of nucleic acid, RNA is structurally and chemically very similar to DNA; the two largest differences being that RNA contains ribose instead of 2'-deoxyribose and uses uracil instead of thymine as one of its four bases.
Unlike DNA, which is primarily used as a template for transcription, RNA molecules perform a diverse set of functions within the cell. Subsets of RNA are generally classified by their function, which can range from acting as the template for protein synthesis ("messenger RNA" or "mRNA") to performing enzymatic functions. Additionally, many viruses use RNA instead of DNA as their genetic material (e.g. retroviruses).
As with DNA, RNAs can exist in single-stranded (annotated "ssRNA") or double-stranded ("dsRNA") forms; however, the vast majority of cellular RNAs are single-stranded. dsRNAs generally function in post-transcriptional gene regulation or as viral genetic material. Targeted degradation of dsRNAs is a major intracellular defence mechanism against viruses.
Every nucleotide of RNA consists of a ribose molecule with a nitrogenous base attached to its 1' carbon and a phosphate group attached to its 5' carbon. Individual RNA nucleotides are linked by a phosphodiester bond between the 3' carbon of one nucleotide's ribose and the 5' carbon on the next.
The four nitrogenous bases used in RNA are adenine, guanine, cytosine, and uracil. Adenine and guanine are the purine bases, cytosine and uracil are the pyrimidine bases. Like in DNA, these bases can hybridize through hydrogen bonds; adenine hybridizes with uracil and guanine hybridizes cytosine.
Hybridization of nucleotides within RNA molecules allows for the formation of secondary structures such as RNA hairpins, these secondary structures give RNA molecules an overall tertiary structure which is often essential for the RNA to perform its function. Additionally, RNAs can hybridize through base pairing with other RNAs or with complementary DNA sequences.