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^^ändTepärät^B |5| After centrifugation, protein from cells by centrifuging.

infected bacteria form a pellet containing 32P in the bottom of the tube.

Protein E. coli protein from cells by centrifuging.

infected bacteria form a pellet containing 32P in the bottom of the tube.

10.5 Hershey and Chase demonstrated that DNA carries the genetic information in bacteriophages.

cells. Eventually, the cells burst and new phage particles emerged.

When phages labeled with 35S infected the bacteria, most of the radioactivity separated with the protein ghosts and little remained in the cells. Furthermore, when new phages emerged from the cell, they contained almost no radioactivity (see Figure 10.5). This result indicated that, although the protein component of a phage was necessary for infection, it didn't enter the cell and was not transmitted to progeny phages.

In contrast, when Hershey and Chase infected bacteria with 32P-labeled phages and removed the protein ghosts, the bacteria were still radioactive. Most significantly, after the cells lysed and new progeny phages emerged, many of these phages emitted radioactivity from 32P, demonstrating that DNA from the infecting phages had been passed on to the progeny (see Figure 10.5). These results confirmed that DNA, not protein, is the genetic material of phages. __

Concepts 9

Using radioactive isotopes, Hershey and Chase traced the movement of DNA and protein during phage infection. They demonstrated that DNA, not protein, enters the bacterial cell during phage reproduction and that only DNA is passed on to progeny phages.

www.whfreeman.com/pierce A discussion of the requirements of the genetic material and the history of our understanding of DNA structure and function.

Crystal sample

Crystal sample

X-ray source

^ .which are diffracted (bounce off).

^ The spacing of the atoms within the crystal determines the diffraction pattern, which appears as spots on a photographic film.

^ Interpretation of the diffraction pattern produced by DNA provides information about the structure of the molecule.

X-ray source

^ .which are diffracted (bounce off).

^ The spacing of the atoms within the crystal determines the diffraction pattern, which appears as spots on a photographic film.

Lead screen

Beam of X-rays

Diffraction pattern

Lead screen

Beam of X-rays

Detector (photographic plate)

10.6 X-ray diffraction provides information about the structures of molecules. (Photo from M. H. F. Wilkins, Department of Biophysics, King's College, University of London.)

Diffraction pattern

Watson and Crick's Discovery of the Three-Dimensional Structure of DNA

The experiments on the nature of the genetic material set the stage for one of the most important advances in the history of biology — the discovery of the three-dimensional structure of DNA by James Watson and Francis Crick in 1953.

Watson had studied bacteriophage for his Ph.D.; he was familiar with Avery's work and thus understood the tremendous importance of DNA to genetics. Shortly after receiving his Ph.D., Watson went to the Cavendish Laboratory at Cambridge University in England, where a number of researchers were studying the three-dimensional structure of large molecules. Among these researchers was Francis Crick, who was still working on his Ph.D. Watson and Crick immediately became friends and colleagues.

Much of the basic chemistry of DNA had already been determined by Miescher, Kossel, Levene, Chargaff, and others, who had established that DNA consisted of nucleotides, and that each nucleotide contained a sugar, base, and phosphate group. However, how the nucleotides fit together in the three-dimensional structure of the molecule was not at all clear.

In 1947, William Ashbury began studying the three-dimensional structure of DNA by using a technique called X-ray diffraction ( FIGURE 10.6), but his diffraction pictures did not provide enough resolution to reveal the structure. A research group at King's College in London, led by Maurice Wilkins and Rosalind Franklin, also was studying the structure of DNA by using X-ray diffraction and obtained strikingly better pictures of the molecule. Wilkins and Franklin, however, were unable to develop a complete structure of the molecule; their progress was impeded by personal discord that existed between them.

Watson and Crick investigated the structure of DNA, not by collecting new data but by using all available information about the chemistry of DNA to construct molecular models ( FIGURE 10.7). By applying the laws of structural chemistry, they were able to limit the number of possible structures that DNA could assume. Watson and Crick tested various structures by building models made of wire and metal plates. With their models, they were able to see whether a structure was compatible with chemical principles and with the X-ray images.

The key to solving the structure came when Watson recognized that an adenine base could bond with a thymine base and that a guanine base could bond with a cytosine base; these pairings accounted for the base ratios that Chargaff had discovered earlier. The model developed by Watson and Crick showed that DNA consists of two strands of nucleotides wound around each other to form a right-

10.7 Watson and Crick provided a three-dimensional model of the structure of DNA.

(A. Barrington Brown/Science Photo Library/Photo Researchers.)

handed helix, with the sugars and phosphates on the outside and the bases in the interior. They published an electrifying description of their model in Nature in 1953. At the same time, Wilkins and Franklin published their X-ray diffraction data, which demonstrated experimentally the theory that DNA was helical in structure.

Many have called the solving of DNA's structure the most important biological discovery of the twentieth century. For their discovery, Watson and Crick, along with Maurice Wilkins, were awarded a Nobel Prize in 1962. (Rosalind Franklin had died of cancer in 1957 and, thus, could not be considered a candidate for the shared prize.)

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