The infection of E. coli with T2 bacteriophages to prove that DNA rather than protein is the hereditary molecule

Contributed by: Drake Johnson-Scherger & Jadon Aguanno

 

Keywords

DNA, Evolution, Experimental, Fundamental research, Genes, Genetic material, Genetics, Historical figure, Lab, Macromolecules, Medicine, Molecular biology, North America, Nucleic acids, Viruses, Woman

 

Slides

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View and download in google slides here

 

Resources

Chase, M. D., & Hershey, A. D. (1952). INDEPENDENT FUNCTIONS OF VIRAL PROTEIN AND NUCLEIC ACID IN GROWTH OF BACTERIOPHAGE*. Journal of General Physiology, 39-56. doi:https://doi.org/10.1085/jgp.36.1.39

 

Notes

Slide 1: Researcher’s Background

Martha Cowles Chase worked within the field of molecular biology as a research assistant to Alfred Hershey after receiving her bachelor’s from the College of Wooster in 1950. In their work, she helped in the discovery that DNA and RNA are the molecules of information transmission, rather than proteins.   

Biography in brief

Martha Chase was born November 30th, 1927 in Cleveland, Ohio. She received her bachelor’s degree from the College of Wooster in Ohio in 1950, and went on to work alongside Alfred Hershey at Cold Spring Harbor Laboratory (CSHL) where she and Hershey studied whether proteins or DNA was the mode of information transmission for organisms. She later went on to receive her PhD in Microbiology in 1964. Later, in 1969, Hershey won the Nobel Prize in Physiology or Medicine. However, Chase failed to be recognized or mentioned in the designation of this award. While working as a geneticist in later years, Chase was diagnosed with dementia, and eventually passed away in 2003 due to a pneumonia infection.

Is (or was) their research under-valued because of their identity?

Yes

 

Slide 2: Research Overview

Take home message of study

Respectively labeled sulphur and phosphorus bacteriophages (bacteria-infecting viruses) were allowed to infect E. Coli cells before being separated from the bacteria via centrifugation. It was found that the bacteriophages incorporated their phosphorus-labeled DNA into the bacterial cells while the bacteriophages with sulphur labeled protein coats remained outside of the E. Coli in the test tube. 

Study system

This figure shows the two separate trials which were performed by Chase and Hershey. The first trial at the top shows the incorporation of T2 phages containing the incorporated S35 radioactive protein. These phages were allowed to infect the E. Coli bacterium and were later agitated in a Waring blender to remove the empty protein shell of the phage. Following this, the contents were centrifuged to separate the supernatant and bacterial pellets. The end of the flowchart summarizes the analysis done on the supernatant, and it was found to contain radioactive S35.  

Contrastingly, the bottom diagram within this figure depicts T2 phages grown with radioactive phosphorus incorporating their radioactive DNA into the E. Coli cells. Following this, the protein shells were again removed from the E. Coli membrane through the spinning of the Waring blender. The solution was then centrifuged, and upon analysis it was determined that the radioactively labeled phosphorus DNA was found within the bacterial pellets at the bottom of the centrifuge tube, rather than outside of the E. Coli cells within the mixture. This is highly suggestive that the P32 was taken up by the bacteria while the S35 was not. 

Citation for the Figure: The Hershey-Chase Experiment. (n.d.). Retrieved November 26, 2020, from https://hershey-chase.weebly.com/the-experiment.html 

 

Slide 3: Key Research Points

Main figure

This graph depicts the percent total of extracellular phosphorus and sulphur infected E. coli bacteria. The blending of the phage-bacteria solution was done to separate the phages bound to the bacterial membrane. After 8 minutes of running in the Waring blender for the trial consisting of the phages with the S35 labeled protein coat, it is evident that about 80% of the supernatant contained the radioactive sulphur. This suggests that much of the protein coat was not incorporated into the infected bacterial cells upon phage infection. Contrastingly, for the P32 labeled phage DNA trial, the extracellular percent composition for the phosphorus was relatively low (about 30%) especially in comparison to the trial containing sulphur. This suggests that much of the P32-labeled phage DNA was taken up by the E. Coli upon infection. Consequently, the highest concentration of the P32 would be found incorporated into the bacterial pellet at the bottom of the centrifuge tube, while the highest percent composition of sulphur would be found in the supernatant after blending, as it remains within the phage’s protein coat after it is unbound from the E. Coli cell, consequently ending up in solution rather than incorporated into the bacteria. The slight decrease of infected bacteria after eight minutes in the Waring blender can be attributed to the potential lysis of a small amount of the infected bacterial cells due to the applied shearing force. Additionally, the reason that the extracellular composition for phosphorus trial remained around 30% and was not fully taken up by the E. Coli could be due to the lack of extra time for the P32-labeled phages to infect more bacterial cells. As a result, this would leave some phages in solution still containing the phosphorus-labeled DNA. Potentially, due to the shear force applied from the blender, some of the existing P32-infected cells could also have been prematurely lysed, again giving rise to a higher concentration of phosphorus in the solution than expected.  

 

Societal Relevance

The knowledge that DNA is the hereditary unit of cells has pioneered further research to discover how DNA is replicated, regulated, expressed, controlled, manipulated, packaged and so much more. Projects such as the human genome project and medical techniques such as gene therapy have arisen as a result of this fundamental research. Significant scientific research has been kickstarted regarding genetics which has proven useful in many domains of social justice such as the determination of an individual present at a crime scene using DNA recognition techniques as well as in cases such as paternity testing. The role of DNA and its chemical characteristics have been used widely in medicine and in certain vaccination techniques, notably those that activate antigen-encoding sequences to produce agents that are helpful in combating viral infections.  

The Hershey-Chase experiment is one of the most classical examples used today for sex marginalization in the sciences. Recognizing the significance of excluding Martha Chase in this scientific breakthrough has highlighted one of the many previous stereotypes within the field of scientific research. By recognizing the significance of Chase’s work in one of the cornerstone scientific experiments of the past century, we begin to break the cycle of exclusion of other groups within our field. This instance is one of the many that has helped create a cascade of breaking norms and achieving social justice within the scientific field we know today. This has consequently contributed to a growingly diverse and accepting community for all to feel represented and recognized for their work.