The graphic above accurately depicts the process of the Hershey-Chase experiment which provided us the information and knowledge that DNA is the primary genetic material of life, not proteins. Below is an explanation of the experimental design and overall explanation of what the end result of the experiment meant.
"The work of Doermaml (1948), Doermann and Dissosway (1949), and Anderson and Doermann (1952) has shown that bacteriophages T2, T3, and T4 multiply in the bacterial cell in a non-infective form. The same is true of the phage carried by certain lysogenic bacteria (Lwoff and Gutmann, 1950). Little else is known about the vegetative phase of these viruses. The experiments reported in this paper show that one of the first steps in the growth of T2 is the release from its protein coat of the nucleic acid of the virus particle, after which the bulk of the sulfur-containing protein has no further function," (Hershey 39).
This quote from the first page of the publication written by Hershey and Chase shows their usage of previous, known information from experiments in the past, like Doermaml, Lwoff and Gutmann, among others before the 1960's. They were able to use this information to reduce the amount of work they needed to do, making sure they didn't experiment to find anything that hadn't been done before, which would have wasted time. They were then able to use this previously known information about the phages as well as other things, to better understand what they needed to find out and work towards finding out whether proteins or DNA is the genetic material of life.
At the bottom of the page is an animation that shows the previous information that Hershey and Chase had before they did their experiment and the procedure they followed in their experimentation.
"The experiments reported in this paper show that one of the first steps in the growth of T2 is the release from its protein coat of the nucleic acid of the virus particle, after which the bulk of the sulfur-containing protein has no further function," Hershey and Chase wrote in their 1952 publication.
The four steps outlined in the graphic above are a representation of the procedure Chase and Hershey went through to uncover the truth about DNA.
Excerpts from the experiment truly show how complex and well-controlled this experiment is:
"Adsorption of isotope to bacteria was usually measured by mixing the sample in adsorption medium with bacteria from 18 hour broth cultures previously heated to 70°C. for 10 minutes and washed with adsorption medium. The mixtures were warmed for 5 minutes at 37°C., diluted with water, and centrifuged. Assays were made of both sediment and supernatant fractions," (Hershey 40).
In conclusion, it was the phage DNA that entered the bacterial cells but the phage proteins DID NOT! The phage DNA was found inside the cell and the bacterial cells with the radioactive labeled phage DNA released new phages through the Lysogenic Cycle and those new phages contained radioactive phosphorous. The infected bacterial cells reproduced and because their DNA was taken over by the phage DNA labeled with radioactive phosphorous, and cell reproduction involves DNA, the daughter cells had radioactive phosphorous which means DNA functions as the genetic material of life. The tracing of DNA through the use of radioactive phosphorous led Alfred Hershey and Martha Chase to the conclusion that DNA is the genetic material of life.
"Adsorption of T2 to heat-killed bacteria, and heating or alternate freezing and thawing of infected cells, sensitize the DNA of the adsorbed phage to DNase. These treatments have little or no sensitizing effect on unadsorbed phage. Neither heating nor freezing and thawing releases the phage DNA from infected cells, although other cell constituents can be extracted by these methods. These facts suggest that the phage DNA forms part of an organized intracellular structure throughout the period of phage growth," (Hershey 55).
Describing how no matter how the infected cells were altered (heating or freezing) the phage DNA from the infected cells seemed to form "part of an organized intracellular structure (DNA) throughout the period of phage growth," (Hershey 55).
For other detailed animations of the experiment refer to the Videos/Animations page.
"The work of Doermaml (1948), Doermann and Dissosway (1949), and Anderson and Doermann (1952) has shown that bacteriophages T2, T3, and T4 multiply in the bacterial cell in a non-infective form. The same is true of the phage carried by certain lysogenic bacteria (Lwoff and Gutmann, 1950). Little else is known about the vegetative phase of these viruses. The experiments reported in this paper show that one of the first steps in the growth of T2 is the release from its protein coat of the nucleic acid of the virus particle, after which the bulk of the sulfur-containing protein has no further function," (Hershey 39).
This quote from the first page of the publication written by Hershey and Chase shows their usage of previous, known information from experiments in the past, like Doermaml, Lwoff and Gutmann, among others before the 1960's. They were able to use this information to reduce the amount of work they needed to do, making sure they didn't experiment to find anything that hadn't been done before, which would have wasted time. They were then able to use this previously known information about the phages as well as other things, to better understand what they needed to find out and work towards finding out whether proteins or DNA is the genetic material of life.
At the bottom of the page is an animation that shows the previous information that Hershey and Chase had before they did their experiment and the procedure they followed in their experimentation.
"The experiments reported in this paper show that one of the first steps in the growth of T2 is the release from its protein coat of the nucleic acid of the virus particle, after which the bulk of the sulfur-containing protein has no further function," Hershey and Chase wrote in their 1952 publication.
The four steps outlined in the graphic above are a representation of the procedure Chase and Hershey went through to uncover the truth about DNA.
- There are two batches.
- One batch (Batch 1) had phages that were grown with radioactive sulfur that was incorporated into the phage protein (inside the phage head).
- The other batch (Batch 2) had phages that were grown with radioactive phosphorous that was incorporated into the phage DNA
- The difference in substance incorporation was because they wanted to determine which of the two substances is the genetic material of life.
- The steps these batches of phages went through are outlined below:
- First, the batches of radioactively labeled phages were mixed with bacteria. This caused the phages to infect the bacterial cells and enter the Lysogenic Cycle outlined on the page "The Importance of Bacteriophages."
- Second, the mixture was agitated in a blender which freed the phage parts that were outside the bacteria cells and remove them from it.
- Then, the mixture was centrifuged to separate the bacteria from the free phages and phage parts and caused the bacteria to settle in what is known as a "pellet" at the bottom of the test tube. The liquid, which consisted of free phages and phage parts, was above the pellet.
- Next, the radioactivity of the liquid and the pellet was measured. It was found that with the sulfur labeled proteins, of Batch 1, radioactivity was found in the liquid, only with the free phage and phage parts, meaning none of the sulfur had entered the cells, therefore meaning no protein entered the cell either. In Batch 2 however, it was found that the pellet, NOT the liquid above, was radioactive and contained the radioactively labeled phosphorous. This result for Batch 2 meant that the phosphorous entered the cell, which means DNA entered the cell and because the phages insert their DNA, previously labeled with radioactive phosphorous, into the bacteria cells and their DNA take over the bacteria cell's DNA, the radioactive labeled phosphorous was found inside the cell.
Excerpts from the experiment truly show how complex and well-controlled this experiment is:
"Adsorption of isotope to bacteria was usually measured by mixing the sample in adsorption medium with bacteria from 18 hour broth cultures previously heated to 70°C. for 10 minutes and washed with adsorption medium. The mixtures were warmed for 5 minutes at 37°C., diluted with water, and centrifuged. Assays were made of both sediment and supernatant fractions," (Hershey 40).
In conclusion, it was the phage DNA that entered the bacterial cells but the phage proteins DID NOT! The phage DNA was found inside the cell and the bacterial cells with the radioactive labeled phage DNA released new phages through the Lysogenic Cycle and those new phages contained radioactive phosphorous. The infected bacterial cells reproduced and because their DNA was taken over by the phage DNA labeled with radioactive phosphorous, and cell reproduction involves DNA, the daughter cells had radioactive phosphorous which means DNA functions as the genetic material of life. The tracing of DNA through the use of radioactive phosphorous led Alfred Hershey and Martha Chase to the conclusion that DNA is the genetic material of life.
"Adsorption of T2 to heat-killed bacteria, and heating or alternate freezing and thawing of infected cells, sensitize the DNA of the adsorbed phage to DNase. These treatments have little or no sensitizing effect on unadsorbed phage. Neither heating nor freezing and thawing releases the phage DNA from infected cells, although other cell constituents can be extracted by these methods. These facts suggest that the phage DNA forms part of an organized intracellular structure throughout the period of phage growth," (Hershey 55).
Describing how no matter how the infected cells were altered (heating or freezing) the phage DNA from the infected cells seemed to form "part of an organized intracellular structure (DNA) throughout the period of phage growth," (Hershey 55).
For other detailed animations of the experiment refer to the Videos/Animations page.