DNA, or deoxyribonucleic acid, is the principal hereditary material of cellular organisms. It stores biological information in the sequence of its chemical building blocks and transmits that information through reproduction and cell division. Particular regions, including genes, provide instructions for producing functional RNA molecules and proteins. DNA usually consists of two complementary strands arranged in a double helix; their pairing allows each strand to serve as a template for copying the other. (genome.gov)
Chemical structure
DNA belongs to the class of biological macromolecules called nucleic acids. It is a long chain of nucleotides, each containing a deoxyribose sugar, a phosphate group, and a nitrogen-containing base. The four principal bases are adenine, thymine, guanine, and cytosine, conventionally abbreviated A, T, G, and C. Phosphodiester bonds connect neighboring sugars through phosphate groups, forming the sugar–phosphate backbone. Unlike RNA, DNA contains deoxyribose rather than ribose and generally uses thymine instead of uracil. (ncbi.nlm.nih.gov)
The two strands run in opposite directions, described as antiparallel: one extends from its 5′ end toward its 3′ end, while the other runs oppositely. In standard complementary pairing, A pairs with T and G with C. Hydrogen bonds between paired bases, together with interactions between stacked bases, stabilize the helix. The sugar–phosphate backbones lie on its outside, while the bases face inward. This arrangement produces major and minor grooves through which proteins can recognize and interact with DNA. (ncbi.nlm.nih.gov)
Cellular organization
DNA is organized into chromosomes. In eukaryotes, most DNA resides within the cell nucleus, where it associates with proteins to form chromatin. DNA wraps around histone protein complexes, creating nucleosomes that help package long molecules into a small space. This packaging is dynamic and also influences whether particular DNA regions are accessible for copying or gene activity. Chromosomes become especially condensed during cell division. (genome.gov)
DNA also occurs outside the nucleus. Mitochondria contain their own genetic material, as do chloroplasts in plants and algae. Most bacteria have one or a few circular chromosomes rather than the multiple linear nuclear chromosomes characteristic of eukaryotes. An organism’s complete genetic complement is its genome. Genome size and chromosome number vary considerably among organisms; neither is a straightforward measure of biological complexity. (ncbi.nlm.nih.gov)
Genetic information and expression
DNA’s informational content depends primarily on base order rather than on the repeating backbone. During transcription, an RNA polymerase uses one DNA strand as a template to synthesize a complementary RNA molecule. For protein-coding genes, the resulting messenger RNA carries information to ribosomes. During translation, the genetic code relates successive three-base units, called codons, to amino acids or signals that terminate protein synthesis. (genome.gov)
Not all DNA encodes proteins. Some genes produce functional noncoding RNAs, while other sequences participate in regulation or chromosome organization. Gene expression therefore depends on more than a protein-coding sequence: it also involves regulatory DNA, interacting proteins, and cellular conditions. Different cell types can use different portions of essentially the same genome. Chemical modifications such as DNA methylation can alter how DNA is used without changing its base sequence, forming part of the mechanisms studied in epigenetics. (genome.gov)
Replication, damage, and variation
Before a cell divides, its DNA must generally be copied through DNA replication. Replication is semiconservative: each resulting double-stranded molecule contains one parental strand and one newly synthesized strand. Enzymes separate the strands, and DNA polymerases extend new strands by adding complementary nucleotides. Synthesis proceeds in the 5′-to-3′ direction. Because the templates are antiparallel, one new strand is synthesized continuously at a replication fork, while the other is assembled from short fragments that are subsequently joined. (ncbi.nlm.nih.gov)
DNA is chemically stable enough to preserve information, but it is not immune to damage. Radiation, reactive chemicals, and spontaneous chemical changes can alter bases or break strands. Polymerase proofreading and DNA repair systems correct many errors and lesions. A persistent change in sequence is a mutation. Mutations can have harmful, beneficial, or negligible effects, depending on their location and biological context. Heritable sequence changes supply variation on which evolutionary processes act; changes confined to particular body cells need not pass to offspring. (ncbi.nlm.nih.gov)
Discovery and experimental significance
Friedrich Miescher first isolated the substance he called “nuclein” in 1869. In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty identified DNA as the material responsible for bacterial transformation. Alfred Hershey and Martha Chase provided further evidence for DNA’s hereditary role through bacteriophage experiments in 1952. In 1953, James Watson and Francis Crick proposed the double-helical model, drawing on chemical evidence and X-ray diffraction research, including work by Rosalind Franklin and Maurice Wilkins. The structure suggested a mechanism for accurate inheritance through complementary copying. (nlm.nih.gov)
DNA is both an object of investigation and an experimental material. DNA sequencing determines base order, allowing researchers to locate genes, examine regulatory regions, and compare genomes. The polymerase chain reaction amplifies selected DNA segments for analysis. Sequencing and amplification made it possible to investigate hereditary information at scales ranging from individual regions to whole genomes, supporting studies of biological development, variation, and relationships among organisms. (genome.gov)