A nucleic acid is a biological polymer composed of linked nucleotides. Its two principal forms are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These molecules are essential components of living cells and provide the genetic material of viruses. DNA chiefly stores hereditary information, whereas RNA participates in the expression of that information and performs numerous structural, regulatory, and catalytic functions. The order of nucleotides, rather than simply their chemical composition, determines much of a nucleic acid’s biological significance. (genome.gov)
Chemical composition
Each nucleotide contains a nitrogen-containing base, a five-carbon sugar, and one or more phosphate groups. DNA contains the sugar deoxyribose; RNA contains ribose. The principal DNA bases are adenine, guanine, cytosine, and thymine, conventionally abbreviated A, G, C, and T. RNA generally uses uracil, abbreviated U, instead of thymine. Both nucleic acids are therefore built from a small alphabet of repeating chemical units, arranged in sequences of potentially great length. (genome.gov)
Adjacent nucleotides are connected by phosphodiester bonds, producing an alternating sugar–phosphate backbone. Each linkage connects the 3′ position of one sugar with the 5′ position of the next through phosphate. This arrangement gives a strand chemical directionality: nucleotide sequences are conventionally written from the 5′ end toward the 3′ end. The prime marks distinguish positions in the sugar from positions in the base. In the DNA double helix, the phosphate-containing backbones lie outside the paired bases and carry negative charges. (nature.com)
An important chemical distinction occurs at the sugar’s 2′ position. RNA has a hydroxyl group there, whereas DNA has a hydrogen atom. This hydroxyl group makes RNA susceptible to backbone cleavage, contributing to its lower chemical stability relative to DNA. Nevertheless, RNA stability also depends on molecular structure and environmental conditions; RNA is not uniformly short-lived. (nature.com)
Structure and complementary pairing
Cellular DNA typically forms a double helix consisting of two antiparallel strands: one runs 5′ to 3′ while the other runs in the opposite direction. The bases face inward, with adenine pairing with thymine and guanine pairing with cytosine. These complementary pairs are connected by hydrogen bonds. Complementarity allows the sequence of one strand to specify that of the other, providing a molecular basis for accurate copying of genetic information. (nature.com)
RNA is often single-stranded, but this does not mean that it lacks organized structure. Complementary regions within one molecule can pair, creating stems, loops, and more complex folds. These structures allow RNA molecules to recognize other molecules and perform specialized functions. DNA and RNA also have exceptions to their usual strand arrangements: RNA can be double-stranded, and some biological DNA molecules are single-stranded. (genome.gov)
Genetic storage and expression
DNA contains genes and regulatory sequences that influence when and where genes are used. In eukaryotes, most DNA is located in the cell nucleus, where it is organized into chromosomes. Additional DNA occurs in mitochondria. The complete genetic complement of an organism is its genome. Thus, despite the historical name “nucleic acid,” these molecules are not restricted to nuclei. (genome.gov)
During DNA replication, existing DNA provides templates for new DNA. DNA polymerase constructs complementary strands by incorporating nucleotides. During transcription, a DNA sequence instead serves as the template for RNA production. Transcription is a central step in gene expression, but the RNA produced need not encode a protein: many RNA molecules function directly. (genome.gov)
Messenger RNA carries information used in translation. A ribosome reads this information in three-nucleotide units, or codons. Transfer RNA molecules match codons with the appropriate amino acids, which are incorporated into a growing protein. Ribosomal RNA contributes both to ribosome structure and to the chemical reactions that assemble proteins. (genome.gov)
RNA beyond protein synthesis
RNA has functions extending well beyond its role as an intermediate between DNA and protein. Some RNAs regulate gene activity by interacting with DNA, other RNAs, or proteins. Others contribute to the processing of RNA molecules. In RNA splicing, selected regions are removed from a transcript; different combinations of retained regions can produce distinct RNAs from the same gene. (genome.gov)
A catalytically active RNA is called a ribozyme. RNA’s capacity to carry information and facilitate chemical reactions motivates the RNA-world hypothesis, which proposes that RNA played a central role in an early stage of life’s evolution. This is a hypothesis about biological origins, not an established description of how life began. (genome.gov)
Discovery and experimental study
In 1869, Friedrich Miescher isolated a previously unrecognized substance from white blood cell nuclei and named it “nuclein.” Its properties differed from those of proteins; the material was subsequently identified as DNA. In 1953, James Watson and Francis Crick published their double-helical model, whose complementary base pairing suggested a mechanism for copying genetic material. (fmi.ch)
Several laboratory methods exploit nucleic-acid structure. Polymerase chain reaction amplifies a selected DNA segment through repeated rounds of synthesis. DNA sequencing determines the order of its bases, allowing researchers to identify genes, regulatory regions, and sequence differences. Gel electrophoresis separates nucleic-acid molecules using an electric field and a porous matrix; comparison with size standards helps characterize the fragments present in a sample. (genome.gov)