Cells in a multicellular organism must duplicate many times, and even single cells have to duplicate when they divide. Whatever molecule carries genetic information must be capable of copying itself in an almost error-free manner. DNA is made of four bases, abbreviated A, C, G and T. DNA is a double-stranded molecule, where an A in one strand always matches with a T in another strand, and a C always matches with a G. If the two strands of DNA are separated and a new strand is built, it will use the pattern of the existing strand to construct an exact copy, because where there was an A across from the T in the original, there will be an A across from the T in the copy.