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The bacteriophage, a vital part of this experiment, is a virus that infects and replicates inside a bacterium. Hershey and Chase used them in their experiment, along with bacterial cells, to determine whether DNA or protein was the genetic material that is required for growth. In order to better understand the experiment, it's important to understand the life cycle of bacteriophages and how it connects to the experiment. It's also important to understand the structure and function of the bacteriophages and again, how it connects to the experiment.
There are two separate cycles of the bacteriophages, both making up the life cycle of one. They are:
The Lysogenic Cycle and the Lytic Cycle.
There are three steps in the Lysogenic Cycle.
1. First, the phage attaches to a bacteria cell and inserts it's DNA into the bacteria cell.
2. Second, the phage's DNA mixes with the bacteria cell's DNA.
3. And third, the host cell reproduces, and continues to reproduce until the Lytic Cycle begins.
...
The Lytic Cycle is entered when more and more copies are made that it is triggered, by the Lysogenic Cycle.
Lytic Cycle Steps:
1. First, the phage attaches to the bacterial cells and inserts it's DNA into the bacterial cell
2. Second, the phage's DNA takes over the bacterial cell
3. Next, the phage's DNA starts to create new phages inside the bacteria cell.
4. Finally, the cell membrane breaks and releases the new phages outside the cell to go and infect neighboring cells.
There are two separate cycles of the bacteriophages, both making up the life cycle of one. They are:
The Lysogenic Cycle and the Lytic Cycle.
There are three steps in the Lysogenic Cycle.
1. First, the phage attaches to a bacteria cell and inserts it's DNA into the bacteria cell.
2. Second, the phage's DNA mixes with the bacteria cell's DNA.
3. And third, the host cell reproduces, and continues to reproduce until the Lytic Cycle begins.
...
The Lytic Cycle is entered when more and more copies are made that it is triggered, by the Lysogenic Cycle.
Lytic Cycle Steps:
1. First, the phage attaches to the bacterial cells and inserts it's DNA into the bacterial cell
2. Second, the phage's DNA takes over the bacterial cell
3. Next, the phage's DNA starts to create new phages inside the bacteria cell.
4. Finally, the cell membrane breaks and releases the new phages outside the cell to go and infect neighboring cells.
How does this connect to the experiment?...
Alfred Hershey and Martha Chase used the bacteriophages because of their connection to DNA. In one batch, the phages (short for bacteriophages) were grown with radioactive phosphorous, which means it was incorporated into phage DNA. In the other batch, the phages were grown with radioactive sulfur, which was incorporated into phage protein! It's important to know the difference between the two batches, one focused on protein, which was used in the heads of the phages and labeled with radioactive sulfur. In the other batch, DNA was focused upon, and was used in the heads of the phages similarly to the protein, but labeled with a different radioactive element, radioactive phosphorous. The phages were mixed with bacteria and because of the way they function, the phages infected the bacterial cells. The mixture of the two was then agitated in a blender to free the phage parts outside the bacteria from the cells. The mixture was then centrifuged, or separated, so that the bacteria formed a pellet at the bottom of the test tube, the free phages and the phage parts, which are lighter, remained suspended above the pellet in the liquid. The radioactivity in the pellet was measured and it was also measured in the liquid. In the first batch with the labeled protein, the free phages and phage parts were found in the liquid and radioactivity was found, while there was no radioactivity found in the pellet. In the second batch with the labeled DNA, the free phages and phage parts were found to have no radioactivity but the pellet, which consisted of phage DNA which was inserted into the cell due to the Lysogenic and or Lytic Cycle, had radioactivity because it was the DNA that was labeled with the radioactive phosphorous in the phages to begin the experiment. The phages played an important role in this experiment because the material that was labeled in the heads of the phages, the protein, which wasn't inserted into the bacteria cell because DNA is the only thing inserted, and the DNA, which again, is the only thing inserted into the bacteria cell, was tested at the end. Based off the life cycle of a bacteriophage, you can see that because DNA is the only substance that is inserted into the bacteria cells, the pellet, which was the bacteria, consisted of the radioactively labeled DNA. Its important to point out that it would not be possible for the first batch to have radioactive bacteria because the radioactively labeled substance, the protein, was not inserted into the cell and therefore stayed within the free phages and phage parts which were still suspended within the liquid of the test tube.
"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). The beginning of the publication, by Hershey and Chase, outlining the experiment shows how much their work was inspired by others who experimented before them, while also introducing their usage of the phages and the role they'll have later on in the publication.
Below is a video/animation of the cycles of bacteriophages and give a basic overview of their function:
Alfred Hershey and Martha Chase used the bacteriophages because of their connection to DNA. In one batch, the phages (short for bacteriophages) were grown with radioactive phosphorous, which means it was incorporated into phage DNA. In the other batch, the phages were grown with radioactive sulfur, which was incorporated into phage protein! It's important to know the difference between the two batches, one focused on protein, which was used in the heads of the phages and labeled with radioactive sulfur. In the other batch, DNA was focused upon, and was used in the heads of the phages similarly to the protein, but labeled with a different radioactive element, radioactive phosphorous. The phages were mixed with bacteria and because of the way they function, the phages infected the bacterial cells. The mixture of the two was then agitated in a blender to free the phage parts outside the bacteria from the cells. The mixture was then centrifuged, or separated, so that the bacteria formed a pellet at the bottom of the test tube, the free phages and the phage parts, which are lighter, remained suspended above the pellet in the liquid. The radioactivity in the pellet was measured and it was also measured in the liquid. In the first batch with the labeled protein, the free phages and phage parts were found in the liquid and radioactivity was found, while there was no radioactivity found in the pellet. In the second batch with the labeled DNA, the free phages and phage parts were found to have no radioactivity but the pellet, which consisted of phage DNA which was inserted into the cell due to the Lysogenic and or Lytic Cycle, had radioactivity because it was the DNA that was labeled with the radioactive phosphorous in the phages to begin the experiment. The phages played an important role in this experiment because the material that was labeled in the heads of the phages, the protein, which wasn't inserted into the bacteria cell because DNA is the only thing inserted, and the DNA, which again, is the only thing inserted into the bacteria cell, was tested at the end. Based off the life cycle of a bacteriophage, you can see that because DNA is the only substance that is inserted into the bacteria cells, the pellet, which was the bacteria, consisted of the radioactively labeled DNA. Its important to point out that it would not be possible for the first batch to have radioactive bacteria because the radioactively labeled substance, the protein, was not inserted into the cell and therefore stayed within the free phages and phage parts which were still suspended within the liquid of the test tube.
"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). The beginning of the publication, by Hershey and Chase, outlining the experiment shows how much their work was inspired by others who experimented before them, while also introducing their usage of the phages and the role they'll have later on in the publication.
Below is a video/animation of the cycles of bacteriophages and give a basic overview of their function: