L

RNase

RNase 1 (destroys RNA)

Protease 1 (destroys proteins)

2 Treat samples with enzymes that destroy proteins, RNA, or DNA.

DNase

(destroys

DNA)

Type IIR bacteria

Type IIR bacteria

Type IIR bacteria

Type IIR bacteria

Type IIR bacteria

Type IIIS and Type IIIS and type IIR bacteria type IIR bacteria

4 Cultures treated with protease or RNase contain transformed type IIIS bacteria,...

Type IIR bacteria

Type IIIS and Type IIIS and type IIR bacteria type IIR bacteria

4 Cultures treated with protease or RNase contain transformed type IIIS bacteria,...

Type IIR bacteria but the culture treated with DNase does not.

Conclusion: Because only DNase destroyed the transforming substance, the transforming principle is DNA.

Concepts]"

The process of transformation indicates that some substance—the transforming principle — is capable of genetically altering bacteria. Avery, MacLeod, and McCarty demonstrated that the transforming principle is DNA, providing the first evidence that DNA is the genetic material.

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The Hershey-Chase experiment A second piece of evidence implicating DNA as the genetic material resulted from a study of the T2 virus conducted by Alfred Hershey and Martha Chase. T2 is a bacteriophage (phage) that infects the bacterium Escherichia coli ( FIGURE 10.4a). As stated in Chapter 8, a phage reproduces by attaching to the outer wall of a bacterial cell and injecting its DNA into the cell, where it replicates and directs the cell to synthesize phage protein. The phage DNA becomes encapsulated within the proteins, producing progeny phages that lyse (break open) the cell and escape ( FIGURE 10.4b).

At the time of the Hershey-Chase study (their paper was published in 1952), biologists did not understand exactly how phages reproduce. What they did know was that the T2 phage consists of approximately 50% protein and 50% nucleic acid, that a phage infects a cell by first attaching to the cell wall, and that progeny phages are ultimately produced within the cell. Because the progeny carried the same traits as the infecting phage, genetic material from the infecting phage must be transmitted to the progeny, but how this occurs was unknown.

Hershey and Chase designed a series of experiments to determine whether the phage protein or the phage DNA was transmitted in phage reproduction. To follow the fate of protein and DNA, they used radioactive forms (isotopes) of phosphorus and sulfur. A radioactive isotope can be used as a tracer to identify the location of a specific molecule, because any molecule containing the isotope will be radioactive and therefore easily detected. DNA contains phosphorus but not sulfur; so Hershey and Chase used 32P to follow phage DNA during reproduction. Protein contains sulfur but not phosphorus; so they used 35S to follow the protein.

First, Hershey and Chase grew E. coli in a medium containing 32P and infected the bacteria with T2 so that all the new phages would have DNA labeled with 32P (< Figure 10.5). They grew a second batch of E. coli in a medium containing 35S and infected these bacteria with T2 so that all these new phages would have protein labeled with 35S. Hershey and Chase then infected separate batches of unlabeled E. coli with the 35S- and 32P-labeled phages. After allowing time for the phages to infect the cells, they placed the E. coli cells in a blender and sheared off the now-empty protein coats (ghosts) from the cell walls. They separated out the protein coats and cultured the infected bacterial

Phage E. coli

Phage chromosome

Phage genome is DNA.

Phage genome is DNA.

All other parts of the bacteriophage are protein.

All other parts of the bacteriophage are protein.

Phage attaches to E. coli and injects its chromosome.

Bacterial chromosome

Expression of phage genes produces phage structural components.

Progeny phage particles assemble.

Bacterial wall lyses, releasing progeny phages.

Phage attaches to E. coli and injects its chromosome.

Bacterial chromosome

^ Bacterial chromosome breaks down and the phage chromosome replicates.

Expression of phage genes produces phage structural components.

Progeny phage particles assemble.

Bacterial wall lyses, releasing progeny phages.

10.4 T2 is a bacteriophage that infects E. coli.

(a) T2 phage. (b) Its life cycle. (Photo, Harold W. Fisher/Visuals Unlimited.)

Question: Which part of the phage—its DNA or its protein—serves as the genetic material and is transmitted to phage progeny? [Experiment il Infect E. coli grown In T medium containing 35S.

^ST..and separate protein from cells

Protein E coli

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