Genetic Material: Protein or DNA?

Thomas Hunt Morgan, the fly guy at Columbia, showed that genes are on chromosomes in the nuclei of cells.
Two main chemicals became candidates for the genetic material:
- Proteins
- Deoxyribonucleic acids (DNA)
  - Note: this molecule was called “nucleic acid,” because it was first discovered in the nucleus of the cell and it was an acid!
It wasn't a trivial question as to which chemical carried the genetic material.
It took a series of experiments over several decades to prove that it was DNA…

Griffith's Transformation Experiment

In the 1920s, Frederick Griffith (an English physician) made a serendipitous discovery.
He was trying to develop a vaccine against a bacteria, Streptococcus pneumoniae, which causes pneumonia in humans.
- This was before antibiotics had been discovered, so this was a deadly disease.
He was working with two strains of the bacteria, S and R.
- S strain
  - Smooth-looking colonies
  - These bacteria had a polysaccharide coat.
  - When injected into mice, this strain would kill them within a day.
    - It was virulent (disease causing).
    - Since it had a polysaccharide coat, it was protected from the immune defenses of the host.
- R strain
  - Rough-looking colonies
  - These bacteria lacked a protective polysaccharide coat.
  - When injected into mice, this strain did not cause disease.
    - It was nonvirulent.
    - Since it had no polysaccharide coat, it is vulnerable to the immune defenses of the host.

Figure 11.1, Purves's Life: The Science of Biology, 7th Edition

Griffith heat-killed some of the virulent S-strain bacteria.
- The mice lived—which made sense.
He also mixed heat-killed, virulent S-strain bacteria and with living, nonvirulent R-strain bacteria.
- To his astonishment, the mice died of pneumonia!
- Living S-strain bacteria were found in the mice's blood.
- Explanation: some of the nonvirulent R-strain bacteria had been transformed into virulent S-strain bacteria.

Figure 11.1, Purves's Life: The Science of Biology, 7th Edition

Griffith realized that a chemical from one cell was capable of genetically transforming another cell.
Transformation is now defined as a change in genotype and phenotype due to the assimilation of external DNA by a cell.
However, Griffith didn't know the substance was DNA.
- So, the next 30 years were an attempt to discover the chemical nature of this “transforming agent.”
- Let's see how they finally figured it out…

In 1944, a famous experiment was published by Oswald Avery, Colin McLeod, and Maclyn McCarty.
Their basic experimental procedure:
- They treated samples of the transforming agent in a variety of ways.
  - They destroyed different types of substances—proteins, nucleic acids, carbohydrates, lipids.
  - Then, they tested the treated samples to see if they retained the transforming activity.
- The result was always the same:
  - If the DNA in the sample was destroyed, the transforming activity was lost.
  - That is, the only chemical that could transform the bacteria was DNA.
- Conclusion: The transforming agent, and hence the genetic material, must be DNA!
The good news: the Avery, McLeod, and McCarty experiment established DNA as the transforming principle.
The bad news: many scientists didn't believe them!

A 1952 experiment by Alfred Hershey and Martha Chase convinced the scientific community that the genetic material was DNA.
They used the T2 bacteriophage, or phage (virus that attacks bacteria).
- Derived from phago, to eat.
- “Bacteria-eater”
What was known at the time:
- T2, like most viruses, is basically made of protein and DNA.
- T2 turns E. coli bacteria into T2-making factories.
The question:
- Which chemical caused the bacteria to become a T2-producing factory: protein or DNA?
- Which chemical provided the blueprint for making new viruses?
- Which component of the bacteriophage is the hereditary material that enters a bacteria to direct the assembly of new viruses?

Figure 11.2, Purves's Life: The Science of Biology, 7th Edition

Use radioactive ³⁵S to label proteins, because protein contains sulfur and DNA does not.
Use radioactive ³²P to label DNA, because DNA contains phosphorus and protein does not.
Radioactivity could then determine whether DNA or protein entered the bacteria.

Grow T2 phages in a medium containing only radioactively labeled ³²P.
- Remember: P is only in DNA.
These phages with labeled DNA infect the bacteria.
A blender detaches the viruses from the bacteria.
The blended material is centrifuged.
- Bacterial material goes to the bottom (pellet).
- Virus material is in the liquid on top (supernatant).
³²P (viral DNA) is found in the pellet, with the bacteria.

Grow T2 phages in a medium containing only radioactively labeled ³⁵S.
- Remember: S is only in protein.
These phages with labeled protein infect the bacteria.
The same blending and centrifuging process from experiment #1 is followed.
³⁵S (viral protein) is found in the supernatant, with the virus.

DNA, not protein, enters bacterial cells and directs the assembly of new virus particles.

Figure 11.3, Purves's Life: The Science of Biology, 7th Edition