Molecular Biology
STUDIES ON THE CHEMICAL NATURE OF THE SUBSTANCE INDUCING TRANSFORMATION OF PNEUMOCOCCAL TYPES
Landmark 1944 study by Avery, MacLeod, and McCarty that first identified DNA as the transforming principle responsible for hereditary changes in bacteria, fundamentally establishing DNA as the molecular basis of heredity.
Imagine you have two types of bacteria - one harmless and one deadly. Scientists found they could take a mysterious substance from the deadly bacteria and use it to transform the harmless bacteria into the deadly type. It was like giving the harmless bacteria a "recipe" that completely changed what they were. The big question was: what was this transforming substance? Most scientists thought it had to be protein (the body's workhorses), but Avery and his team proved it was actually DNA - the molecule we now know carries all genetic instructions for life. Think of it like discovering that the "instruction manual" for life was written in a completely different language than everyone expected.
- 01
Isolated a biologically active fraction from Type III pneumococci that can induce transformation of pneumococcal types
- 02
Demonstrated that the transforming principle is a desoxyribonucleic acid (DNA) fraction
- 03
Showed that minute amounts of the purified DNA fraction are sufficient to induce bacterial transformation
- 04
Established DNA as the chemical basis for hereditary transformation in bacteria
- 05
Provided evidence that DNA, not protein, is the substance responsible for genetic transformation
1. From Type III pneumococci a biologically active fraction has been isolated in highly purified form which in exceedingly minute amounts is capable under appropriate cultural conditions of inducing the transformation of unencapsulated R variants of Pneumococcus Type II into fully encapsulated cells of the same specific type as that of the heat-killed microorganisms from which the inducing material was recovered. 2. Methods for the isolation and purification of the active transforming material are described. 3. The data obtained by chemical, enzymatic, and serological analyses together with the results of preliminary studies by electrophoresis, ultracentrifugation, and ultraviolet spectroscopy indicate that, within the limits of the methods, the active fraction contains no demonstrable protein, unbound lipid, or serologically reactive polysaccharide and consists principally, if not solely, of a highly polymerized, viscous form of desoxyribonucleic acid. 4. Evidence is presented that the chemically induced alterations in cellular structure and function are predictable, type-specific, and transmissible in series. The various hypotheses that have been advanced concerning the nature of these changes are reviewed.