Here is the same section of DNA but from another cell. If you can imagine that
DNA is a twisted ladder, then each rung of the ladder is a pair of joined
molecules, or a base pair. With this section of DNA, one of the base pairs is
different from the original.
This DNA has suffered a mutation, either through mis-copying (when its parent
cell divided), or through the damaging effects of exposure to radiation or a
chemical carcinogen.
