How can DNA be read and copied Ao reliable? The four chemical bases used in the DNA ladder -A, T, G and C- form the ladders individual rungs by always pairing in the same way: A with T, and G with C. If fone side of a rung is A, the other side is always T; G always meet C. Therefore if you have one side of the ladder, you know the other side of the ladder, where one side of the ladder reads GTCA, the other side must read CAGT. the partial rungs differ in lenght, but when they pair up with their complements , they make complete rungs of one uniform length.
Discovering that fact lead scientist to another breakthrough about this remarkable molecule: DNA is perfectly suited for being copied over and over. The enzyme machine that replicates DNA takes in free-floating units of those four chemicals from the environment in the nucleus. Then it uses them to complete each rung on the split DNA strand.
So a DNA molecule really is like a book that is read and copied over and over again. In the average life span of a human , DNA is copied some 10,000,000,000,000,000 times, with amazing fidelity. 
