DNA

DNA, short for deoxyribonucleic acid, is the molecule that carries the genetic instructions used to build and operate living things. It stores information in a long chemical sequence that is copied when cells divide and passed from parents to offspring. Almost every cell in a plant, animal or bacterium contains DNA, and slight differences in its sequence help explain why individuals differ.

Understanding DNA is central to biology because it links chemistry to inheritance. It explains how traits are transmitted, how cells follow instructions, and why some diseases run in families.

Structure of the double helix

DNA is a long chain built from smaller units called nucleotides. Each nucleotide contains a sugar, a phosphate group and one of four bases: adenine (A), thymine (T), guanine (G) and cytosine (C). Two strands wind around each other in the famous double helix, held together by bonds between paired bases. The pairing is strict: A always pairs with T, and G always pairs with C. This complementary structure means each strand can act as a template to rebuild the other, which is the chemical basis of copying.

The order of the bases along the chain is the actual information. A gene is a stretch of DNA whose sequence specifies the instructions for making a particular protein or functional molecule.

How it was discovered

The molecule was first isolated in the late nineteenth century by Friedrich Miescher, who called it "nuclein" without knowing its role. For decades, many scientists assumed proteins carried heredity. Experiments in the 1940s, notably by Oswald Avery and colleagues, showed that DNA could transfer genetic traits, and the Hershey–Chase experiments in 1952 confirmed DNA as the genetic material. In 1953 James Watson and Francis Crick, drawing on X-ray images made by Rosalind Franklin and Maurice Wilkins, proposed the double-helix model with complementary base pairing. Their insight explained both the structure and how it could be copied.

Replication, genes and the genetic code

When a cell divides, the two strands of DNA separate and each is used to build a new partner, producing two identical copies. The information in a gene is read in a process that transcribes the DNA sequence into a related molecule, RNA, which is then translated into chains of amino acids that fold into proteins. The genetic code specifies which triplets of bases correspond to which amino acids, and the code is remarkably similar across nearly all life.

Errors that arise during copying, or damage from factors such as radiation and certain chemicals, produce mutations. Some are harmless, some cause disease, and some create the variation on which evolution depends.

Applications and why it matters

Knowledge of DNA transformed many fields. In medicine, genetic testing can reveal inherited risks, and techniques such as gene editing aim to correct faulty sequences. In forensic science, DNA profiling can link a suspect to a crime scene or exonerate the wrongly accused. In agriculture, genetic modification and selective breeding shape crops and livestock. Researchers have sequenced the full genomes of humans and countless other species, opening the door to personalised medicine and studies of ancestry and evolutionary history.

Common misconceptions

DNA does not by itself determine everything about a person; environment and development matter greatly. Genes do not each control a single trait in a neat one-to-one way — most traits involve many genes and their interactions. Nor is human DNA "mostly junk" in the dismissive sense once claimed; much of the non-coding sequence has regulatory or structural roles. Finally, cloning an organism from its DNA does not reproduce an identical personality or life history.

Key points