One danger in any information-storage system is that the information will become faulty or garbled. Some of this danger is lessened in DNA by having the "message" protected by a second strand running in the opposite direction, with the bases on the two strands paired in a complementary manner. Each purine on one strand is paired with a pyrimidine on the complementary strand in a highly specific way: A only with T, and G only with C. The result is a ladder molecule, as shown on the right, with the 5`-to-3` direction different in the two uprights of the ladder, and with purine-pyrimidine rungs. Because of the specific A-T and G-C base pairing, each strand has exactly the same information, although in a slightly different language. This is what is meant by saying that the two strands are complementary.
This duplication of information is a protective device, since mismatchings caused by chemical mutation or radiation can be recognized by repair enzymes and corrected. Either strand is sufficient to make an intact duplicate of the original DNA again. (In some primitive societies, accounts are kept by notches on sticks, which then are split down the middle with one half going to the debtor, the other to the creditor. Tampering with the records is instantly recognizable by matching the halves of the stick. This is not a bad analogy for the double-stranded information storage in DNA.)

Right: The four bases of DNA, paired as shown on the next page, are the four letters in the alphabet of the genetic code. The paired bases are the rungs of a DNA ladder, with a 5`-to-3` chain arrangement of the two sides of the ladder running in opposite directions.