What Is DNA? The Instruction Manual Inside Every Living Cell

DNA (deoxyribonucleic acid) is often described mysteriously, but it’s really closer to an extremely long instruction manual stored inside almost every cell of a living organism. That manual is written using a code made of just four different chemical "letters," commonly abbreviated A, T, C, and G, and the specific order those letters appear in carries the actual information — similar to how the specific order of letters in a word gives it a specific meaning.

DNA is shaped like a long, twisted ladder — a structure called a double helix — with the four letters paired up in a specific way to form the ladder’s rungs (A always pairs with T, and C always pairs with G). Sections of this long DNA molecule, called genes, each contain the instructions for building one particular thing the body needs, most often a specific protein that carries out some job in the cell.

This is roughly how the instructions get followed: a cell "reads" a gene’s sequence of letters, and uses that sequence as a blueprint to build the exact protein described. Different genes get switched on or off depending on what a particular cell needs to do — a muscle cell and a skin cell contain the exact same complete DNA instruction manual, but each only actively uses (or "expresses") the specific pages relevant to being a muscle cell or a skin cell.

This also explains inherited resemblance in families: a child’s DNA is a combination of instructions copied from both parents, which is why children often resemble their parents in specific traits — the same underlying "manual pages" (genes) were passed down, just combined and expressed in a slightly new way.