5.02:_The_Hershey_-_Chase_Experiments

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<p class="lt-bio-4740" style="background-color: unset;">In 1952 (seven years after Avery's demonstration that genes were DNA), two geneticists: A. D. Hershey and Martha Chase, provided further proof. They worked with a DNA virus, called <strong>T2</strong>, which infects <em>E. coli</em> (and so is a bacteriophage). Figure 5.2.1 shows the essential elements of the infective cycle of DNA bacteriophages like T2. The virions attach to the surface of their host cell <strong>(a)</strong>. The proteins of the capsid inject the <strong>DNA core</strong> into the cell <strong>(b)</strong>. Once within the cell, some of the bacteriophage genes (the &quot;early&quot; genes) are transcribed (by the host's RNA polymerase) and translated (by the host's ribosomes, tRNA, etc.) to produce enzymes that will make many copies of the phage DNA and will turn off (even destroy) the host's DNA.</p>

<figure><img class="internal" alt="Diagram of the lytic cycle of a T2 bacteriophage infecting E. coli, showing five stages labeled (a) through (e)." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6521/T4.gif?revision=1" aria-describedby="remedy-description-0469a4837535b2dd" /><span id="remedy-description-0469a4837535b2dd" data-remedy-image-description="0469a4837535b2dd" style="display: block; white-space: pre-line;">Circular diagram illustrating the infective cycle of a DNA bacteriophage (such as T2) on a host bacterium (E. coli), depicted as a blue rod-shaped cell. A central legend identifies two components: 'Virus particle' (a red hexagonal head with a tail structure) and 'Virus DNA' (a small squiggly line). The cycle proceeds clockwise through five labeled stages: (a) A single phage particle sits on the outer surface of an intact bacterium, its tail attached to the cell wall. (b) The phage remains attached while its DNA (shown as a squiggly line) has been injected through the tail into the interior of the bacterium, leaving the empty capsid outside. (c) Inside the bacterium, multiple squiggly DNA strands and several partially assembled phage particles (red heads with tails) are present, representing replication of phage DNA and synthesis of capsid proteins. (d) The bacterium now contains many complete phage particles assembled within it. (e) The bacterial cell wall is ruptured (shown with a jagged break), and numerous new phage particles burst out and disperse, ready to infect new host cells. An arrow from stage (e) curves back up to stage (a), indicating the cycle repeats. Arrows connect each stage to the next in clockwise order: (a) to (b) to (c) to (d) to (e) and back to (a).</span><figcaption>Figure 5.2.1: The Hershey - Chase Experiment</figcaption></figure>

<p class="lt-bio-4740" style="background-color: unset;">As fresh copies of phage DNA accumulate, other genes (the &quot;late&quot; genes) are transcribed and translated to form the proteins of the capsid <strong>(c)</strong>. The stockpile of DNA cores and capsid proteins are assembled into complete virions <strong>(d)</strong>. Another &quot;late&quot; gene is transcribed and translated into molecules of lysozyme. The lysozyme attacks the peptidoglycan wall (from the inside, of course). Eventually the cell ruptures and releases its content of virions ready to spread the infection to new host cells <strong>(e)</strong>.</p>

<p class="lt-bio-4740" style="background-color: unset;">Bacteriophages produced within bacteria growing in radioactive culture medium will themselves be radioactive. If radioactive sulfur atoms (<sup><small><font size="1">35</font></small></sup>S) are present, they will be incorporated into the protein coats of the bacteriophages since two of the amino acids &mdash; cysteine and methionine &mdash; contain sulfur (Figure 5.2.2). However, the DNA will be nonradioactive because there are no sulfur atoms in DNA. If radioactive phosphorus (<sup><small><font size="1">32</font></small></sup>P) is used instead, the DNA become radioactive &mdash; because of its many phosphorus atoms &mdash; but not the proteins.</p>

<figure><img height="408" width="516" class="internal" alt="Hershey-Chase experiment diagram: sulfur-labeled protein coats stay outside cells while phosphorus-labeled DNA enters during bacteriophage infection." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/6517/Hershey_Chase_experiment.png?revision=1" aria-describedby="remedy-description-b020b688541e80d2" /><span id="remedy-description-b020b688541e80d2" data-remedy-image-description="b020b688541e80d2" style="display: block; white-space: pre-line;">Diagram of the Hershey-Chase experiment, titled 'Bacteriophages,' split by a dotted vertical line into two parallel three-step sequences. Left column (red): bacteriophages with 'sulfur labeled protein capsule (red)' ??? red hexagonal heads with tail fibers and black DNA inside. Step 1, Infection: two red phages attach to an oval 'cell'; their DNA (black squiggles) is injected inside while capsids remain outside. Step 2, Blending: arrow down to a cell containing only black DNA squiggles, with empty red capsids floating outside. Step 3, Centrifugation: arrow down to a cell still containing black DNA, with paired empty red capsids clustered outside. Caption: 'After centrifugation no sulfur in cells.' Right column (black/green): bacteriophages with 'phosphorus labeled DNA (green)' ??? black capsids containing green zigzag DNA. Step 1, Infection: two black phages inject green DNA into an oval 'cell'. Step 2, Blending: cell contains green DNA; empty black capsids outside. Step 3, Centrifugation: cell retains green DNA; paired empty black capsids outside. Caption: 'After centrifugation phosphorus in cells.' Together the panels show that labeled protein remains outside the infected cell while labeled DNA enters it, demonstrating DNA is the genetic material.</span><figcaption>Figure 5.2.2: Diagram of the Hershey Chase experiment.  (GFDL; Thomasione )</figcaption></figure>

<p class="lt-bio-4740" style="background-color: unset;">Hershey and Chase found that when bacteriophages containing <sup><small><font size="1">32</font></small></sup>P (radioactive), were allowed to infect nonradioactive bacteria, all the infected cells became radioactive and, in fact, much of the radioactivity was passed on to the next generation of bacteriophages. However, when the bacteria were infected with bacteriophages labeled with <sup><small><font size="1">35</font></small></sup>S, and then the virus coats removed (by whirling them in an electric blender), practically no radioactivity could be detected in the infected cells. From these experiments, it was clear that the DNA component of the bacteriophages is injected into the bacterial cell while the protein component remains outside. However, it is the injected component &mdash; DNA &mdash; that is able to direct the formation of new virus particles <strong>complete</strong> with protein coats. So here is further proof that <strong>genes are DNA</strong>.</p>

<hr class="autoattribution-divider" /><div class="autoattribution"><p>This page titled <a target="_blank" class="internal mt-self-link" href="/Sandboxes/johnnyphung/biology/05:_DNA/5.02:_The_Hershey_-_Chase_Experiments">5.2: The Hershey - Chase Experiments</a> is shared under a <a rel="nofollow" href="https://creativecommons.org/licenses/by/3.0" target="_blank">CC BY  3.0 (creativecommons.org)</a>  license and was authored, remixed, and/or curated by <a rel="nofollow" target="_blank" href="http://www.biology-pages.info/">John W. Kimball (biology-pages.info)</a> via <a rel="nofollow" href="https://www.biology-pages.info/" target="_blank">source content (biology-pages.info)</a> that was edited to the style and standards of the LibreTexts platform.</p></div>

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