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I) Evidence for DNA as the Hereditary Molecule

A) Griffiths Experiment (1928)

Pneumococcus; Griffith was an MD in the Navy

(1)R-typerough colony

(a)

Non-virulent(b)Non-encapsulated

(2)S-typesmooth colony

(a) Virulent

(b)Encapsulated (polysaccharide shell)

Only living virulent pneumococcal cells would produce pneumonia in mice

Heat-killed virulent injected would not cause pneumonia

Living virulent cells did not cause the disease

Griffith injected heat-killed and non-virulent and mice surprisingly died

The Transforming Principlewhen non-virulent transforms to virulent It was thought that the non-virulent strain picked up the DNA of the virulent strain thereby

conferring the ability to produce a polysaccharide shell, become virulent itself

B) Avery

Oswald Avery was an American physician and medical researcher

Injection into mice was not necessary for transformation to occur

Prevailing belief that protein was the hereditary material

Protein was thought to be the hereditary molecule because there was more variation

Extracted and purified various macromolecules from bacteria to see if they induced

transformation

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Treated extract with RNase and protease, transforming activity was still present

Treated extract with DNase, no transforming activity

Avery, MacLeod, and McCarty (1944)Strong suggestion that DNA was genetic material

Received a lot of scrutiny, some believed that DNA was contaminated with protein

C) Chase and Hershey (1953)

Published article in the early 1950s

T2 phage (bacteriophage) consists 50% protein and 50% DNA

(1)First Experiment

DNA of phages was marked with and allowed to inject its genetic material into

the bacterial cells Sought to prove that DNA was the hereditary molecule

Marked DNA and then blenderized the ghost phages off

Centrifuged and whatever was injected into the cells went with it

Measured the radioactivity of both the supernatant and the pellet was the only thing thathad the radioactive material

Discarded the supernatant and resuspended the pellet in a new buffer solution. Waiting for

the phage to go through its life cycle and centrifuged again

Found that the radioactive material was in the supernatant. This indicated that whatever

was injected into the cell must have been DNA as a result of the cell lysis and the 2nd

generation phages were in the supernatant

(2)Second Experiment

Same process was used but with and protein was found in the supernatant that

was discarded and not in the pellet. Therefore, it was concluded that DNA wasinjected into bacterial cells during the life cycle of a bacteriophage and that DNA was

the genetic material, not protein as previously thought

The production of new viruses occurs within the bacterial cell

Some molecular component of the phage-DNA or protein or both enter the bacterial cell

and directs viral reproduction