DNA Replication

DNA is a twisted ladder where each rung is a pair of letters: A always pairs with T, and C always pairs with G. That strict pairing is the whole trick behind copying it. When a cell divides, an enzyme called helicase unzips the ladder down the middle, splitting each rung. Now each half-ladder is a template. Another enzyme, DNA polymerase, walks along each exposed strand and adds the only letter that fits: across from an A it places a T, across from a G it places a C. Because each old strand guides the building of a new partner, you end up with two identical ladders, each made of one old strand and one new one. That's why it's called 'semi-conservative': half of each copy is the original.