By Aunice Reed • Updated Aug 30, 2022
Deoxyribonucleic acid (DNA) is the molecular carrier of genetic information in every cell. Each DNA strand is composed of repeating units called nucleotides.
A nucleotide consists of a five‑carbon sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), or guanine (G). These units link together through phosphodiester bonds to form a long, continuous DNA chain.
Hydrogen bonds hold complementary bases together: adenine pairs exclusively with thymine, and guanine pairs exclusively with cytosine. This complementary pairing creates the rungs of the DNA double‑helix ladder and ensures each strand is a precise copy of its partner.
The precise sequence of bases constitutes the genetic instruction set—essentially a blueprint that dictates how an organism is built, maintained, and repaired. Individual segments of this sequence are called genes, and genes are organized into structures known as chromosomes located in the cell nucleus.
Although DNA stores the instructions, the cell reads them via messenger RNA (mRNA). The process of copying DNA into RNA is called transcription. Once an mRNA strand is produced, ribosomes translate its code into proteins—a complex cascade that ultimately yields functional biomolecules.
Johann Friedrich Miescher first isolated DNA in the 1860s, naming it “nuclein.” In 1944, Oswald Avery and colleagues proved that DNA carries genetic information. Erwin Chargaff later discovered that the amounts of A equal T and G equal C, a finding that guided the understanding of DNA’s structure. Rosalind Franklin’s X‑ray diffraction work revealed the double‑helix geometry, which James Watson and Francis Crick modeled in 1953, completing the picture of DNA’s architecture.
Gene expression shapes more than just physical traits; it determines cellular function and is the basis for inherited conditions such as sickle‑cell anemia. Human cells contain roughly 30,000–40,000 genes, with individual genes ranging from a few hundred to several hundred thousand base pairs. The entire human genome consists of about three billion base pairs.
