Twentieth century biology began with the rediscovery of Gregor Mendel's work on peas and ended with the sequencing of the human genome. In between came Thomas Morgan's studies of fruit flies, the grand synthesis between genetics and evolutionary biology in the 1930s, and Watson and Crick's publication in 1953 of the double helix structure of deoxyribose nucleic acid (“DNA”), the substance in the nucleus of cells, which carries the genetic code of all eukaryotic life.
The genetics of the second half of the century focused on learning how DNA coded for proteins, how to splice, clone, and recombine pieces of DNA, and how genetic mutations caused disease. In the late 1980s, a project to identify the actual sequence of all 3.2 billion base pairs of the human genome began. In June 1999, President Clinton and Prime Minister Blair announced that a working draft of the human genome was complete, with the final completed draft to 99.9% accuracy expected in May 2003, fifty years after the publication of Watson and Crick's landmark paper.