Life Cycle Of T2 Bacteriophage

Living S strain (virulent)

Living S strain (virulent)

Living S strain cells found in heart

Living S strain cells found in heart

The virulent S strain bacteria are killed by heating.

Dead S strain cells are mixed with living, nonvirulent R strain bacteria.

The virulent S strain bacteria are killed by heating.

Dead S strain cells are mixed with living, nonvirulent R strain bacteria.

Nonvirulent Strain

Mouse healthy

No bacterial cells found in heart

Mouse healthy

No bacterial cells found in heart

Mouse dies (so)

Living S strain cells found in heart

Mouse healthy

No bacterial cells found in heart

Mouse healthy

No bacterial cells found in heart

Mouse dies (so)

Living S strain cells found in heart

Conclusion: A chemical component from one cell is capable of genetically transforming another cell.

Did this transformation of the bacteria depend on something that happened in the mouse's body? No. It was shown that simply incubating living R and heat-killed S bacteria together in a test tube yielded the same transformation. Years later, another group of scientists discovered that a cell-free extract of heat-killed S cells also transformed R cells. (A cellfree extract contains all the contents of ruptured cells, but no intact cells.) This result demonstrated that some substance— called at the time a chemical transforming principle—from the dead S pneumococci could cause a heritable change in the affected R cells. This was an extraordinary discovery: Treatment with a substance permanently changed an inherited characteristic. Now it remained to identify the chemical structure of this substance.

were designed with these relatively simple systems to discover the nature of the genetic material.

In 1952, Alfred Hershey and Martha Chase of the Carnegie Laboratory of Genetics published a paper that had a much greater immediate impact than Avery's 1944 paper. The Her-shey-Chase experiment, which sought to determine whether DNA or protein was the hereditary material, was carried out with a virus that infects bacteria. This virus, called the T2 bac-teriophage, consists of little more than a DNA core packed inside a protein coat (Figure 11.2a). The virus is thus made of the two materials that were, at the time, the leading candidates for the genetic material.

The transforming principle is DNA

Identifying the transforming principle was a crucial step in the history of biology. It was accomplished over a period of years by Oswald Avery and his colleagues at what is now Rockefeller University. They treated samples known to contain the pneumococcal transforming principle in a variety of ways to destroy different types of molecules—proteins, nucleic acids, carbohydrates, and lipids—and tested the treated samples to see if they had retained transforming activity.

The answer was always the same: If the DNA in the sample was destroyed, transforming activity was lost, but there was no loss of activity when proteins, carbohydrates, or lipids were destroyed. As a final step, Avery, with Colin MacLeod and Maclyn McCarty, isolated virtually pure DNA from a sample containing pneumococcal transforming principle and showed that it caused bacterial transformation. We now know that the gene encoding the enzyme that catalyzes the synthesis of the pathogenic polysaccharide capsule was transferred during transformation.

The work of Avery, MacLeod, and McCarty, published in 1944, was a milestone in establishing that DNA is the genetic material in cells. However, it had little impact at the time, for two reasons. First, most scientists did not believe that DNA was chemically complex enough to be the genetic material, especially given the much greater chemical complexity of proteins. Second, and perhaps more important, bacterial genetics was a new field of study—it was not yet clear that bacteria even had genes.

Viral replication experiments confirm that DNA is the genetic material

The questions about bacteria were soon resolved as researchers identified genes and mutations in these organisms. Bacteria and viruses seemed to undergo genetic processes similar to those in fruit flies and pea plants. Experiments

(a) The virus: T2 bacteriophage

Neck Collar . Sheath -

Tail fiber

(a) The virus: T2 bacteriophage

Neck Collar . Sheath -

Tail fiber

Collar Sheath Viral

ll T2 bacteriophage attaches to the surface of E. coliand injects its DNA.

(b) Life cycle of the T2 bacteriophage

(b) Life cycle of the T2 bacteriophage

Life Cycle Bacteriophage

Viral genes take over the host's synthetic machinery.

The bacterium breaks open, releasing about 200 viruses.

11.2 T2 and the Bacteriophage Reproduction Cycle (a) The external structures of T2 bacteriophage consist entirely of protein.This cutaway view shows a strand of DNA within the head. (b) T2 is parasitic on Eco//,depending on the bacterium to produce new viruses.

ll T2 bacteriophage attaches to the surface of E. coliand injects its DNA.

Viral genes take over the host's synthetic machinery.

The bacterium breaks open, releasing about 200 viruses.

11.2 T2 and the Bacteriophage Reproduction Cycle (a) The external structures of T2 bacteriophage consist entirely of protein.This cutaway view shows a strand of DNA within the head. (b) T2 is parasitic on Eco//,depending on the bacterium to produce new viruses.

When a T2 bacteriophage attacks a bacterium, part (but not all) of the virus enters the bacterial cell. About 20 minutes later, the cell bursts, releasing dozens of viruses. The entry of a viral component changes the genetic program of the host bacterial cell: it is converted from a bacterium into a bacteriophage factory. Hershey and Chase set out to determine which part of the virus— protein or DNA—enters the bacterial cell. To trace the two components of the virus over its life cycle (Figure 11.2b), Hershey and Chase labeled each with a specific radioactive tracer:

► Viral proteins contain some sulfur (in the amino acids cysteine and methionine), an element not present in DNA, and sulfur has a radioactive isotope, 35S. So Hershey and Chase grew a batch of T2 bacteriophage in a bacterial culture in the presence of 35S; the resulting viruses had their proteins labeled with this isotope.

► The deoxyribose-phosphate "backbone" of DNA, on the other hand, is rich in phosphorus (see Chapter 3), an element that is not present in most proteins, and phosphorus also has a radioactive isotope, 32P. The researchers grew another batch of T2 in a bacterial culture in the presence of 32P, so that all the viral DNA was labeled with 32P.

With these radioactively labeled viruses, Hershey and Chase performed their revealing experiments. They allowed bacteriophage containing either 32P or 35S to attach to bacteria. After a few minutes, they agitated the mixtures vigorously in a kitchen blender, which (without bursting the bacteria) stripped away the parts of the virus that had not penetrated the bacteria. Then Hershey and Chase separated the bacteria from the rest of the material. They found that most of the 35S (and thus the protein) had separated from the bacteria, and that most of the 32P (and thus the DNA) had stayed with the bacteria. These results suggested that the DNA was transferred to the bacteria, whereas the protein remained outside, and thus that it was DNA that redirected the genetic program of the bacterial cell (Figure 11.3).

Hershey and Chase performed other similar but more long-range experiments, allowing a progeny (offspring) generation of viruses to be collected. The resulting viruses contained almost none of the original 35S and none of the parental viral protein. However, they contained about one-

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Responses

  • Maura
    How much of the 35S label is stripped from the cells by the blender?
    7 years ago

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