The DNA stored in the nucleus of a single human cell spans over six feet in length if stretched from end to end. Made up of four chemical building blocks called A, C, T and G, for short, DNA contains the instructions for making all living things. The building blocks link to form the molecule's famous "double helix" structure, which allows genetic information to be copied and passed down from one generation to the next. Faithful copying of DNA information accounts for why we look like our parents and why dogs don't give birth to cats, for example. Occasionally exposure to toxins or malfunction of cellular processes, among other things, does cause copying mistakes. Such changes over long time periods provide opportunities for organisms to adapt to new surroundings--or, cause them to die out.

Discrete segments of DNA, called genes, encode the instructions for making proteins. Work horses of the cell, proteins, serve as structural material, hormones, enzymes and neurotransmitters as well as play many other roles. Efforts are underway to decipher the complete set of genetic information, known as the genome, in a variety of agriculturally--and--environmentally important plants and other organisms and to learn the function of the proteins those genes encode.