The human genome is the complete set of genetic "instructions" for human life. This information resides in genes within very large molecules of deoxyribonucleic acid (DNA). The DNA in the human genome that is passed on to offspring as information necessary for survival is called genomic DNA.

The large DNA molecules are composed of two strands twisted around each other to form a "double helix." Each strand is constructed from millions of chemical building blocks called "bases." DNA contains only four different bases (abbreviated A, T, G, C), but they can be arranged in any sequence. The order of the bases determines the message the gene contains, just as the letters of the alphabet can recombine to form new words. The base sequences of the two strands of each DNA molecule are related to each other by the following rule: A only matches with T, and C only with G. Therefore, the base sequence on one strand dictates the order of the bases on the other strand. This is called "base pairing" and enables the genome to copy itself and pass genomic DNA to the next generation.

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