Ever since the Greek philosopher and mathematician Pythagoras surmised in 500 B.C. that all hereditary material came from a child's father, great thinkers have pondered the mechanism of heredity. Just how are our traits passed from one generation to the next? Even nineteenth-century English naturalist Charles Darwin, who developed the theory of evolution, wasn't quite sure. He theorized that heredity occurred by means of small particles called pangenes, which were produced in every organ and tissue of the body and flowed freely in the blood. While these notions proved to be false, they did point toward the real explanation. Other scientists of the nineteenth and twentieth centuries -- including Gregor Mendel, with his research on pea plants; Thomas Hunt Morgan, who studied fruit flies; and Barbara McClintock, who studied pigmentation in corn kernels -- further advanced the search for the key to heredity.

In the mid-1950s, James Watson, Francis Crick, Rosalind Franklin, Maurice Wilkins, and several others finally found the key and unlocked the door. Using X-rays and molecular models, Watson and Crick, with help from their colleagues' research results, determined the shape of the DNA molecule. The twisted, ladder-like structure, which Watson and Crick called a "double helix," revolutionized the study of genetics. Suddenly scientists could begin to explain how the DNA molecule reproduces, or replicates, itself.

First, the double-stranded DNA molecule "unzips" down the middle. Then, as the two strands separate, new strands form along each original one, with the original strands serving as guides for the formation of the new ones. Nucleotide bases, the building blocks of DNA, move into place to line up with their complements: A's fit with T's and G's fit with C's. In this way, each strand forms a copy of its original partner strand, and, in the end, two exact replicas of the complete DNA molecule are produced. Watson and Crick's discovery was groundbreaking, and it paved the way for all of the major genetic discoveries of the last half century.

Most recently, DNA research has culminated in the successful sequencing of the entire human genome -- all three billion letters that make up our genetic code. This enormous undertaking, called the Human Genome Project, is bringing scientists and doctors closer than ever before to understanding the significance of the structure of DNA. They know, for instance, that tiny mutations in the sequence of bases -- a spelling difference of even a single letter of DNA code -- can cause debilitating disease. Armed with this information, researchers are discovering that it is possible not only to understand the causes of genetic diseases, but also to do something to cure them.