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.
