By the early 1940s, scientists knew that chromosomes existed and that they were composed of smaller units called genes. Chemical analysis had revealed that the eucaryotic chromosome consists of about 50 percent protein and 50 percent deoxyribonucleic acid ( DNA). There was no particular interest in DNA for several previous decades because no role had been assigned to it. This changed when a Canadian-born American named Oswald Avery showed that DNA is responsible for the transmission of heritable characteristics.

Avery moved from Canada to New York City in 1887. He attended Colgate University and in 1904 received his medical degree from the College of Physicians and Surgeons at Columbia University. He practiced medicine for several months before he became more interested in the transmission of infectious diseases. In 1913, Avery arrived at Rockefeller Institute, where he worked as a bacteriologist for over 43 years.

Avery and his co-workers studied the life cycle and chemical make-up...