By the end of World War II, biologists were getting close to identifying the heritable material. However, it is much easier to disprove an incorrect hypothesis than it is for a community to agree on a single explanation. It seemed probable that DNA was the molecule that carried inherited information, but no one had provided irrefutable evidence that proteins were not the heritable material. In 1952, a pair of biologists from Cold Spring Harbor Laboratory on Long Island, New York, attempted to show beyond any reasonable doubt that DNA, and not protein, was the heritable material. Scientists often are enamored with solving problems so difficult that most others have failed. Others had supported Avery’s conclusion that DNA was the genetic information, but Alfred Hershey and his laboratory assistant, Martha Chase, accepted the challenge to produce overwhelming evidence. No one had designed an experiment to finally reject the protein inheritance hypothesis. Only 2 years out of college, Chase would take part in one of the most famous biology experiments of the twentieth century.

Hershey and Chase set out to demonstrate a negative, that protein was not the heritable material. In order to demonstrate a negative, essentially the harshest skeptic must be convinced that only one plausible answer exists, and that all other answers are highly improbable. To accomplish this very difficult task, Hershey and Chase benefited from the newly discovered atomic isotopes from which they could produce radioactive DNA and proteins. You may remember from a chemistry or physics course that many atomic isotopes (such as, uranium and plutonium) are unstable chemical elements that decay and emit radiation in the process. Any given isotope emits a characteristic energy level of radiation. The radiation can be detected with sensitive instruments as well as x-ray film. Hershey and Chase used radioactive isotopes of sulfur (written 35S, but pronounced “S 35”) and phosphorous (32P, pronounced “P 32”). Radioactive isotopes 35S and 32P emit very different levels of energy, which can be distinguished from one another.

Phosphorous was chosen for making radioactive DNA, because DNA contains many phosphates (see Figure 1.5 and Table 1.1). cuisinart smart stick powertrio high torque hand blender reviewSulfur was chosen for making radioactive proteins because, unlike DNA, proteins contain sulfur in amino acids cysteine (see Figure 1.5) and methionine with each amino acid containing one atom of sulfur. bamix hand blender indonesiaTherefore, Hershey and Chase realized that they could produce radioactive DNA or protein and determine which isotope was present based on the type of radiation each isotope emitted.hamilton beach 51103 single serve blender with travel lid reviews

Figure 1.6 Bacteriophage anatomy. Electron micrograph of about 30 phage infecting one larger E. coli bacterial cell . breville pick and mix hand blender 400 watt - vanilla creamThe round white phage head contains the DNA which travels through the skinny tail into the bacteria to start infection.  osterizer blender 4172Image by Graham Beards. russell hobbs blender yorum Hershey and Chase combined radioactive DNA and protein with a new experimental system that became popular among geneticists after World War II—phages, viruses that infect bacteria. vitamix 5028 blenderViruses are the ultimate parasites; they are dependent upon their hosts for their physiological functions, including reproduction.

Because viruses are not cells, new phage must emerge from their larger bacterial host cells. Figure 1.6 shows an electron micrograph of a type of phage similar to T2. Phage T2 was an ideal model system, because it cannot infect humans and they reproduce rapidly with one parental virus yielding over 100 progeny viruses in under an hour. T2 phage consist of a protein coat surrounding a DNA genome. Phage proteins could be labeled, or tagged with radioactivity, using 35S, and the viral DNA could be labeled with 32P. If the investigators could rule out either DNA or protein as the heritable material, then the other molecule had to be the genetic material. Hershey and Chase realized their phage system was perfect for answering a major research question. It would have been too difficult to chemically synthesize radioactive DNA or protein in vitroIn vitro literally means "in glass", but, in general, it refers to experiments performed outside live cells., so like any smart biologist, they let nature do the hard work for them.

Hershey and Chase grew T2 phage inside E. coli hosts in the presence of either radioactive amino acids or DNA nucleotides with each experiment conducted in separate tubes. Hersey and Chase produced radioactive viruses and some non-radioactive bacteria host cells (Figure 1.7A). Because radioactivity is invisible, the investigators employed a convenient way to assay whether a population of virus contained 35S viral proteins or 32P viral DNA based on the amount of energy emitted. Hershey and Chase hypothesized that if DNA were the heritable material, then the only role of the proteins was to deliver the DNA. The viral DNA would remain inside the infected bacteria, the viral protein coat would be torn off by blending, and the bacteria would remain intact. They used a common kitchen blender to separate the infecting virus from its host, but they had no idea how long to pulse the blender. Their experiments were the first time anyone had used a blender to tear off viruses from the surface of E. coli.

The data in Figure 1.8 show the results of their experiments trying various pulsing times after allowing viruses with 32P-labled DNA or 35S-labeled proteins to infect E. coli for 5 minutes. At each time point, a small portion of the mixture was removed prior to additional blending. Even with zero blending, you can see that some of the 32P-labled DNA spilled outside the bacterial cells, as did some of the 35S-labeled protein. Unlike data presented in typical textbooks, you can see that no separation process is perfect. Real research produces imperfect data. In their experiment when only the proteins were radioactive, about 80% of the radioactive protein was in the media and therefore 20% was still with the infected cells. Conversely, 70% of the radioactive DNA was in the infected bacteria with only 30% in the media. At this point, it was still possible that 20% of protein associated with the cells was the heritable material for those who preferred the protein inheritance hypothesis.

Hershey and Chase worked very hard to locate the protein not present in the media. They determined that many of radioactive protein pieces were loosely associated with the outside of infected E. coli. Hershey and Chase were surprised by their own results, because just one year before the blender experiment, Hershey and Chase had incorrectly published that radioactive viral protein had been incorporated into the second generation phage. Their reversal of conclusions is science at its best. When a better method produces results that contradict earlier conclusions, the erroneous interpretation must be modified or retracted. Hershey and Chase wanted to demonstrate that proteins were not the heritable material. A diehard protein hypothesis supporter could justifiably argue that the amount of protein necessary for inheritance was the 20% associated with infected E. coli cells. However, the next round of experiments provided more conclusive results. If biology was going to explain the mechanism of inheritance, it was essential to be clear about the heritable material.

Once biology could explain how offspring came to resemble their parents, biologists could study genetic diseases and produce improved crops to feed a growing world population. At this point in the debate, every biologist had to use evidence to determine if DNA was the heritable material, 70% of which was inside infected bacteria. When Hershey and Chase labeled phage DNA with 32P, they detected second generation phage whose DNA also contained radioactive 32P. The two meticulous investigators refined their experimental technique and produced more compelling evidence (Table 1.2). Hershey and Chase wanted to finally answer the question of whether protein or DNA was the heritable material. When confronted with data and two possible sources of genetic information, scientists typically choose the simpler answer, a principle known as Occam’s razor. After careful consideration of both possibilities, biologists reached a consensus that DNA is the source of genetic material, not protein. As cautious scientists, Hershey and Chase did leave an opening for an alternative explanation—the protein responsible for heritable material might lack sulfur and thus escape their experimental detection.