 Born: October 21, 1877  Died: 2 February 1955  Nationality: Canadian  Known for: DNA transmits heredity.

 Born: June 9, 1911  Died: January 2, 2005  Nationality: Indian  Known for: His part in the monumental discovery that DNA, rather than protein, constituted the chemical nature of a gene.

 Born: January 28, 1909  Died: February 11, 1972  Nationality: Canadian-American  Known for: DNA transmits heredity.

 The Avery-MacLeod-McCarty experiment served as the beginning of molecular genetics.  Their experiment showed that DNA is the substance that causes bacterial transformation.

The Avery-MacLeod-McCarty Experiment was presented by Oswald Avery, Colin MacLeod, and Maclyn McCarty in The three scientists devised an experiment to understand rough and smooth bacteria and their differences in virulency. This experiment would allow them to determine if rough bacteria could be transformed into smooth bacteria, hence passing along the genetic information causing the transformation. By isolating and purifying this chemical component, they could deduce if it had characteristics of a protein or DNA molecule.

 Bacteria grown in petri dishes can grow spots or colonies inside the dish multiplying under certain conditions. Virulent (deadly) colonies look smooth or like tiny droplets, where as non-deadly bacteria formed rigid, uneven edges, basically rough colonies. While analyzing a certain kind of pneumonia caused by bacteria in mice, they were able to isolate a "variant" (mutant) strain that did not kill the mice. During the experiments, Avery and MacLeod injected a mouse simultaneously with "boiled" or dead smooth bacteria and live rough bacteria. Thereafter a short while they were surprised to see that the mouse died. When they took samples from the dead mice, and cultured the samples in a petri dish, Avery and MacLeod found that what grew inside the culture was in fact the smooth deadly bacteria. This suggested that something from the "dead" bacteria somehow converted the rough bacteria into smooth bacteria. The rough bacteria had been permanently converted or transformed into the smooth dangerous bacteria. They had confirmed that they could not grow smooth bacteria from the boiled culture and cause disease if the dead smooth bacteria were injected alone. This all implied that a chemical component in the smooth bacteria survived and transformed the rough bacteria into smooth. Isolating and purifying that chemical component had shown that is was DNA, NOT proteins that transferred the genetic code from the smooth to the rough.

 Here is a simpler demonstration for this experiment by Oswald Avery, Colin MacLeod, and Maclyn McCarty. There are two sets of bacteria – one is smooth (virulent), one is rough (nonvirulent).  1) They first inject deadly encapsulated bacteria into the mouse – the mouse dies at the end.  2) They then inject non-encapsulated, nonvirulent bacteria into the mouse – the mouse lives.  3) Next, they heated the virulent bacteria at a temperature that kills them and injected these bacteria into the mouse – the mouse lives.  4) After that, they then have the denatured fatal bacteria mix into the living non-encapsulated, nonfatal bacteria. The mixture was then injected into the mouse – the mouse dies.  5) Finally, they mix the live, non-encapsulated harmless bacteria with the DNA that was extracted from the heated, lethal bacteria. These “harmless” bacteria injected to the mouse after being mixed – the mouse dies.

Results: From these experiments, Avery and his group showed that nonvirulent bacteria become deadly after mixing with the DNA of the virulent virus. Such a demonstration shows that nonvirulent bacteria became virulent because of the genetic information that originally came from the virulent virus. The protein from the virulent bacteria was already denatured during Step 3. Thus, it was DNA and not protein that transferred the genetic information to the nonvirulent bacteria.