The complementary base pairing rule states that in DNA, adenine always pairs with thymine and guanine pairs with cytosine. This rule ensures that DNA is replicated faithfully and mutations are minimal occurrences.
The DNA molecule looks like a twisted ladder with a backbone made of alternating sugar and phosphate molecules and rungs made of the nucleotide bases adenine, thymine, cytosine and guanine. The structure of DNA only allows adenine to pair with thymine and cytosine to complement guanine. If one strand or side of the DNA ladder is known, then the complementary strand can be deduced. Complementary base pairing keeps the amount of thymine equal to the amount of adenine and the amount of cytosine the same as the amount of guanine in the DNA of organisms.