DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).
The information in DNA is stored as a code made up of four chemical bases: adenine (A), thymine (T), cytosine (C), and guanine (G). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences.
DNA was first isolated by Swiss physician Friedrich Miescher in 1869. In 1928, American geneticist Thomas Hunt Morgan showed that genes are carried on chromosomes and are passed from one generation to the next. In 1953, James Watson and Francis Crick proposed the double helix model for the structure of DNA. This model explained how information could be stored within cells using only four types of molecules. Their discovery has had a profound impact on our understanding of biology and medicine.