The discovery of DNA’s backbone began in 1867 when Friedrich Miescher noted that cells contained a high phosphorus content and a substance resistant to protein digestion. Subsequent research revealed that the sugar–phosphate backbone is composed of phosphate groups and deoxyribose sugars, forming the ladder’s structural scaffold.
James Watson and Francis Crick’s landmark 1953 model described DNA as a right‑handed double helix, with the sugar–phosphate backbones on the outside and nitrogenous bases stacked in the interior. Each nucleotide comprises one phosphate, one deoxyribose, and one base.
The rungs are built from four nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C). Erwin Chargaff’s 1950 findings showed that A pairs with T and G pairs with C in equal amounts, a principle now known as Chargaff’s rules.
Adenine forms two hydrogen bonds with thymine, while guanine forms three hydrogen bonds with cytosine. These complementary pairings keep the two strands aligned and maintain a uniform rung length, enabling precise replication.
Human DNA contains roughly 60% A–T and 40% G–C pairs, with about 3 billion base pairs per chromosome. During replication, helicase unwinds the double helix, and DNA polymerase synthesizes new strands in short 50‑nucleotide fragments. The fidelity of base pairing results in a low error rate.
During mitosis, the entire genome is duplicated, ensuring each daughter cell receives an identical DNA complement. Meiosis, on the other hand, halves the chromosome number to produce gametes; fertilization restores the full complement in the zygote.
Replication errors can lead to mutations, classified as substitutions, insertions, deletions, or frameshifts. Because the sequence of bases dictates genetic instructions, frameshift mutations can profoundly alter protein synthesis.
