From long ago, it had been known from experience that organisms inherit their characteristics and features through generations. However, it was a mystery, what substance actually did that work of heredity.
In 1928, scientist Dr. Griffith proved that heredity was determined by a chemical substance. However he was not able to find out what that chemical substance was. Many scientists argued over this problem, and came to the conclusion that it was either DNA or protein. Protien was more powerful of the two at that time though, since they were more complex and had various kinds, compared with the simple structured DNA, composed only from phosophate molecules, sugar, and four nitogenous bases. It seemed more reasonable for the more complex proteins to determin the various features and characteristics of humans and other organisms.
However in 1944, Dr. Oswald Avery, Dr. Colin MacLeod, and Dr. Maclyn McCarty of Rockfeller Institute proved that DNA was actually the hereditary substance.
What experiments did these scientists do?
First let's look at Dr. Griffith's experiments.
Dr. Griffith used two strains of pneumoccocous bacteria; a smooth (S) strain and a rough (R) strain.
1) When the lethal S strain was heated and killed, the cells of the bacteria was destroyed and the genetic material released.
2) By mixing the bacteria in 1) and a nonlethal R strain, the genes of the lethal S strain went into the cells of the R strain, and into the genes. This mixing up of genes is called transformation.
3) The mouse injected by the pnuemoccocous bacteria formed by transformation died.
The function that the S strain and R strain posess is called transformation.
Up to here is what Dr. Griffith experimented on. However, the researchers at Rockfeller Institute went on to further studies- determining what genetic material makes R strains become S strains. The results were...,
A)The digits expressing how much oxygen, nitrogen, and carbon are included in the genetic material, matched each amount calculated from DNA's chemical composition.
B)Both the genetic material and DNA absorbed light, and the lights were both ultraviolet rays.
C)The same experiment was performed with a pneumoccocous bacteria without protein or fat, but no changes appeared in the function of transformation.
D)When enzymes that decomposes protein were added to the bacteria, there were no changes in the function of transformation.
ĖThis proved that protein was not the genetic material.
E)The same experiment was done, this time using an enzyme that decomposes RNA, but still transformation did not occur.
ĖThis proved that RNA was not the genetic material.
F)When enzymes for decomposing DNA was added to the pneumoccocous bacteria, the function of transformation was completely lost.
ĖThis proved that DNA was the genetic material.
So after 16 years since Dr. Griffith's theory was proposed, it was finally proven that the genetic material was DNA.
