What Is DNA?

Deoxyribonucleic acid (DNA) is the molecule that encodes the genetic instructions governing the development, function, and reproduction of all known living organisms and many viruses. Structured as a double helix, it consists of two complementary strands wound around a shared axis, with each strand built from a backbone of alternating deoxyribose sugars and phosphate groups. Attached to each sugar is one of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), or thymine (T). Base-pairing rules, where adenine binds thymine and cytosine binds guanine, hold the two strands together and give DNA its characteristic ladder-like geometry.

DNA carries its information in the sequence of those bases. A contiguous stretch of base pairs that encodes a functional product, typically a protein, constitutes a gene. The full set of DNA in an organism is its genome; in humans, that genome spans roughly 3.2 billion base pairs distributed across 23 chromosome pairs. Understanding how DNA is organized, replicated, and expressed sits at the intersection of molecular biology, biochemistry, and, increasingly, electrical engineering and computing.

Structure and Replication

The double-helix model, determined by James Watson and Francis Crick in 1953 using X-ray crystallography data from Rosalind Franklin and Maurice Wilkins, established that the two strands run antiparallel. During cell division, the strands separate and each serves as a template for synthesizing a new complementary strand, a process called semiconservative replication. Specialized enzymes including DNA polymerase, helicase, and ligase coordinate the unwinding, synthesis, and rejoining of strands. Fidelity mechanisms embedded in these enzymes reduce replication errors to roughly one mistake per billion base pairs, a precision that enables reliable inheritance. Detailed accounts of DNA replication fidelity and proofreading can be found in resources maintained by the National Center for Biotechnology Information.

DNA Sequencing

DNA sequencing is the determination of the precise order of bases along a DNA strand or genome. The first practical method, Sanger sequencing, was developed in 1977 and dominated the field through the 1990s. Massively parallel platforms emerged in the mid-2000s and reduced the cost of sequencing a full human genome from billions of dollars to hundreds. Current technologies include short-read platforms such as Illumina's sequencing-by-synthesis chemistry and long-read platforms from Pacific Biosciences and Oxford Nanopore, the latter of which threads individual DNA strands through protein nanopores and detects base identity by changes in ionic current. A review of sequencing platform generations appears in PMC research on sequencing technologies and advancements.

Epigenetics

Epigenetics addresses heritable changes in gene expression that do not involve alterations to the underlying base sequence. The primary chemical mechanism is DNA methylation, which typically occurs at cytosine residues in CpG dinucleotides and, when concentrated near gene promoters, silences transcription by recruiting repressor proteins and restricting access for transcriptional machinery. A second major mechanism involves histone modification: the proteins around which DNA is coiled can be acetylated, methylated, or phosphorylated, altering how tightly DNA is wound and therefore how accessible its genes are. Environmental factors including age, diet, and stress can shift methylation patterns, creating phenotypic variation without any change in the genetic sequence itself. Reviews of these mechanisms are detailed in NIH-funded work on epigenetic regulation and measurement.

Applications

DNA has applications in a wide range of fields, including:

• Clinical genomics and personalized medicine, using sequencing to guide drug selection
• Forensic identification through DNA fingerprinting and profiling
• Agricultural biotechnology, including genetically modified crops with disease resistance
• Synthetic biology and DNA computing, encoding information and logic in molecular circuits
• Gene therapy, correcting disease-causing mutations in somatic cells