Every time one of your cells divides, something in it has to copy your entire genome — around three billion base pairs — accurately enough that you're still recognisably you afterward. It does this in hours, without a single day off since you were a single cell, and it gets away with an error rate that most engineered systems can only dream of.
A four-letter alphabet with one strict rule
DNA is two strands wound around each other, each one a sequence built from four bases — A, T, C and G — and the strands are held together because each base only ever pairs with one partner: A with T, C with G. That rule is the whole trick. It means either strand, on its own, contains enough information to rebuild its missing partner, because you always know what has to go opposite each letter. A single strand isn't half the message. It's the whole message, written in a form that also happens to specify its own complement.
Unzip, then rebuild both halves at once
An enzyme called helicase unzips the double strand, and DNA polymerase moves along each of the two separated strands building a fresh partner base by base, matching A to T and C to G as it goes. The two original strands each end up paired with one brand-new strand — which is why this process is called semi-conservative: every finished copy is half old material, half new. That wasn't just assumed; it was directly confirmed in 1958 by Meselson and Stahl, who tagged the original DNA and watched, generation by generation, exactly the mixed old-and-new pattern the model predicted.
Copying that fast still needs a proofreader
DNA polymerase doesn't just write — it checks its own work as it goes, catching most mismatched bases and correcting them before moving on. Between that proofreading step and additional repair systems that patrol the finished strand afterward, the final error rate lands at roughly one mistake for every billion bases copied. For comparison, a human transcribing text by hand makes far more than one error per billion characters. A molecular machine, working in the dark, at a scale no hand can match, still comes out more accurate.
What we're still unsure about
"More accurate" doesn't mean "flawless," and that's not a flaw in the system — it's load-bearing. The rare replication errors that slip past every check become mutations, and mutations are the raw material evolution runs on. A copying process with a genuinely perfect error rate wouldn't be safer for a species over deep time; it would remove the one mechanism that lets a population change when its environment does. The proofreading is tuned low, not to zero, and the fact that it isn't zero is doing real work.