Nucleic acids are naturally occurring chemical compounds that often carry cellular information throughout the cell. Nucleic acids help direct protein synthesis which determines the inherited characteristics of every living thing. There are two main classes of nucleic acids: DNA and RNA. DNA (deoxyribonucleic acid) is a double stranded molecule that encodes the information cells need to make proteins, and is present in the nucleus, mitochondria, and chloroplast. RNA (ribonucleic acid), comparatively, is single stranded, relatively short lived, and plays an essential role in making proteins. Nucleic acids across the board, however, have very diverse functions in cell creation, in the storage and processing of genetic information, and energy generation.

Nucleic acids are polynucleotides which are long chainlike molecules composed of a series of nearly identical building blocks, termed nucleotides. Each nucleotide consists of at least three parts; a nitrogen-containing base connected to a sugar backbone attached to a phosphate group.

The nitrogen-containing base can be adenine (A) or guanine (G), cytosine (C), thymine (T), or uracil (U). A/G are termed purines for their double ringed structure, and C/T/U are termed pyrimidines for their single ring. Notably, only U is found in RNA which often replaces T molecules. These bases are connected to a pentose sugar that contains 5 carbons numbered 1' to 5'. In DNA this sugar is deoxyribose and is located on the second carbon of the pentose ring (2'). In RNA this sugar is ribose which lacks a 2'-hydroxyl (-OH) group. Phosphate groups connect successive sugar residues by bridging the 5'-hydroxyl group on one sugar to the 3'-hydroxyl group of the next.

Each DNA and RNA strand has a sense of direction, where the 3' carbon of the deoxyribose of one nucleotide will always organically link to the 5' carbon of the next. Chemically, RNA is similar to DNA however it is much more labile. This is because most RNA molecules are only single stranded, and most do not form stable secondary structures like DNA. Together, these building blocks form the double- or single-helical structure, characteristic to DNA and RNA respectively.

Many other types of nucleic acids exist, and each have varying structural dynamics. These nucleic acids are involved in various regulatory processes and can have diverse functions in both normal cellular processes and disease.

Today there exist many types of artificial nucleic acid analogues, also termed xenonucleic acids (XNAs), which are distinguished from naturally occurring nucleic acids due to alterations in their backbones and/or nucleobases. XNAs have great potential in biomedicine for use as potential therapeutic agents and/or fluorescent probes for medical research. XNAs mimic naturally occurring nucleic acids but have several advantages including facile chemical synthesis and resistance to nuclease-mediated cleavage. For example, peptide nucleic acid (PNA) is a non-charged oligonucleotide analogue that contains an N-aminoethyl glycine-based polyamide structure instead of a sugar backbone. In contrast to natural DNA, PNA binds in antiparallel and parallel orientations to complementary nucleic acids.

Locked nucleic acids (LNA) are another form of synthetic oligonucleotides. LNAs have one or more nucleotides in which an extra methylene bridge can fix the ribose backbone into two separate conformations. LNA is particularly attractive for in vivo applications. In the literature, LNA has shown to have increased affinity over other RNA, can exhibit improved mismatch discrimination, has low toxicity, and has increased metabolic stability.

Currently, dozens of other XNAs exist with various conformations. Some other common XNAs include threose nucleic acid (TNA, with a simplified four-carbon threose backbone), glycol nucleic acid (GNA, based on a glycol monomer), and hexitol nucleic acid (HNA, that has a 1',5'-anhydrohexitol sugar).

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