DNA is the abbreviation for deoxyribonucleic acid, the principal hereditary material of cellular organisms and many viruses. It is a biological macromolecule composed of nucleotide units, with each nucleotide containing a deoxyribose sugar, a phosphate group and one nitrogen containing base. The four principal bases in cellular DNA are adenine, thymine, guanine and cytosine, commonly abbreviated A, T, G and C. DNA usually exists as two complementary strands arranged in an antiparallel double helix.
The sugar and phosphate components form the external backbone, while bases project inward and form specific hydrogen bonded pairs, with adenine pairing with thymine and guanine pairing with cytosine. The sequence of bases stores biological information because different sequences can encode different RNA molecules and proteins or regulate when and where genes are active. DNA therefore functions both as an information storage system and as a chemically structured molecule whose physical properties permit accurate copying, repair and transmission.
The double helical structure of DNA was famously described by James Watson and Francis Crick in 1953, building on crucial experimental evidence from Rosalind Franklin, Maurice Wilkins, Erwin Chargaff and others. The structure explained how complementary base pairing could provide a mechanism for copying hereditary information. During DNA replication, the two strands separate and each serves as a template for synthesis of a new complementary strand.
DNA polymerases add nucleotides to a growing strand in a defined chemical direction, producing daughter molecules that normally contain one parental strand and one newly synthesised strand. This process is called semiconservative replication. Replication is coordinated with many additional proteins that unwind DNA, stabilise separated strands, initiate synthesis and remove or repair errors. Although replication is highly accurate, occasional mutations arise through replication errors, chemical damage, environmental agents or imperfect repair. Such changes can be harmful, neutral or beneficial depending on their location and biological consequences, and inherited variation provides one of the molecular foundations of evolution.
DNA is organised differently at different biological scales. In bacteria and archaea, the principal chromosome is generally a circular DNA molecule, although additional DNA molecules called plasmids are common. In eukaryotic cells, most DNA is contained within the nucleus and is divided among linear chromosomes. Eukaryotic DNA is extensively associated with proteins, especially histones, forming chromatin. Wrapping DNA around histone proteins helps package very long molecules into compact structures while retaining regulated access to selected regions.
Chromatin can adopt different functional states, with relatively accessible regions generally associated with active transcription and more compact regions often associated with reduced transcriptional activity. DNA is also present in mitochondria in eukaryotic organisms, and plant cells additionally contain chloroplast DNA. The organisation of DNA is therefore not merely a packaging problem. Its three dimensional arrangement, chemical modification and interaction with proteins influence replication, repair, recombination and gene expression.
Genes are functional regions of DNA that contribute to the production of functional biological products or regulatory outcomes. Protein coding genes are transcribed into RNA, and messenger RNA can subsequently be translated into polypeptide chains. DNA also contains regulatory sequences that influence transcription, replication and chromosome organisation. Promoters are important transcriptional control regions, while enhancers and other regulatory elements can influence gene activity over considerable genomic distances through three dimensional interactions.
Not all DNA directly encodes proteins, and a large fraction of genomes consists of regulatory sequences, repeated sequences, structural regions, noncoding genes and other elements. Introns occur within many eukaryotic genes and are removed from precursor messenger RNA during RNA processing. Exons that remain in mature messenger RNA may be joined in different combinations through alternative splicing, allowing one genomic region to contribute to multiple RNA or protein products. Consequently, understanding DNA requires distinguishing the physical molecule from genes, chromosomes, genomes and individual coding sequences, because these terms describe different levels of biological organisation.
DNA is chemically stable enough to preserve hereditary information over long periods, but it is not chemically inert. Bases can undergo spontaneous reactions, DNA strands can break, and ultraviolet radiation, ionising radiation, reactive chemicals and other factors can damage DNA. Cells therefore possess extensive DNA repair systems. Different repair pathways recognise and correct different forms of damage, including mismatched bases, chemically modified bases, bulky lesions and breaks in DNA strands.
Homologous recombination can use a related DNA molecule as a template for accurate repair, whereas other pathways can join broken DNA ends with greater risk of introducing sequence changes. DNA recombination also occurs normally during meiosis in sexually reproducing organisms, where homologous chromosomes exchange corresponding regions. This generates new combinations of inherited variants. Mutation and recombination are therefore distinct processes, although both contribute to genetic diversity. DNA damage and repair also have medical importance because defects in repair mechanisms can increase susceptibility to particular diseases, including some forms of cancer.
DNA analysis has become central to biology, medicine, forensic science, archaeology, evolutionary research and biotechnology. Polymerase chain reaction can selectively amplify particular DNA sequences, allowing very small quantities of starting material to be analysed. DNA sequencing determines the order of nucleotides and can be used to identify variants, investigate biological relationships or characterise genomes. Modern sequencing technologies have made whole genome and large scale population studies practical, while targeted sequencing remains useful when specific genes or regions are of interest.
DNA profiling can distinguish individuals by examining selected variable regions, although interpretation requires appropriate statistical and legal frameworks. In medicine, DNA analysis can help identify inherited variants associated with disease risk, diagnose some genetic disorders and guide selected treatments. In research, genetically modified DNA can be introduced into cells to study gene function or produce useful biological products. These applications depend on careful experimental design because detecting a DNA difference does not automatically establish that the difference causes a particular biological outcome.
DNA is frequently confused with genes, chromosomes and genomes, but these concepts are related rather than interchangeable. A gene is a functional unit encoded by DNA, whereas a chromosome is a large organised DNA molecule together with associated proteins in organisms that package DNA in this way. A genome is the complete set of genetic material associated with an organism or biological system.
DNA itself refers to the chemical polymer regardless of whether a particular region is classified as a gene. DNA also differs from RNA in both chemical composition and typical biological roles. DNA contains deoxyribose and thymine, whereas RNA contains ribose and usually uracil. DNA is generally more stable and is commonly used for long term information storage, while RNA participates extensively in information transfer, regulation and molecular catalysis. Nevertheless, these distinctions are not absolute across all biological systems because some viruses use RNA rather than DNA as their hereditary material. The diversity of nucleic acid biology demonstrates that information storage and expression can be implemented through related but chemically distinct molecules.
The biological importance of DNA extends from individual molecules to populations and ecosystems because hereditary information influences traits and can be transmitted between generations. DNA sequences vary among individuals and populations, and these variants can be used to reconstruct ancestry, population history and evolutionary relationships. Comparative DNA analysis can reveal conserved sequences whose functions are important because natural selection tends to preserve advantageous or essential features.
Other regions evolve more rapidly and can provide useful markers for distinguishing closely related populations. Horizontal transfer of DNA is particularly important in microorganisms because genetic material can sometimes move between organisms outside ordinary parent to offspring inheritance. Such transfer can spread useful traits, including antibiotic resistance. DNA therefore participates in both stable inheritance and dynamic biological change. Its capacity to encode information, replicate with high fidelity, undergo controlled recombination, acquire mutations and interact with complex regulatory machinery makes it the fundamental hereditary molecule for cellular life as currently understood.
