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Transformation as Proof that DNA is the Genetic Material

Add kna to rouĆ¼h variant

FIGURE 18.04 Avery's experiment

Avery isolated DNA from the virulent variant and added it to the rough variant of S. pneumonia. He noticed that the virulent DNA "transformed" or changed the rough variant into a smooth variant. To confirm that the bacteria were truly transformed, he exposed mice to the newly created smooth variants, and the mice died. Thus the transformed bacteria had gained both the smooth appearance and virulence by taking up DNA from the original virulent strains.

FIGURE 18.04 Avery's experiment

Avery isolated DNA from the virulent variant and added it to the rough variant of S. pneumonia. He noticed that the virulent DNA "transformed" or changed the rough variant into a smooth variant. To confirm that the bacteria were truly transformed, he exposed mice to the newly created smooth variants, and the mice died. Thus the transformed bacteria had gained both the smooth appearance and virulence by taking up DNA from the original virulent strains.

interacted in some unknown way with the host chromosome to promote a genetic change. His experiment was therefore not absolute proof that DNA is the genetic material. Nonetheless, this is the most obvious interpretation and this observation convinced many scientists that genes were very likely made of DNA.]

Special terminology is used when scientists use naked viral DNA during transformation. In a viral infection, the virus punctures a hole in the bacterial cell wall and injects DNA from the viral particle into the cytoplasm.The viral DNA induces the host to manufacture new viral particles. When viruses infect cells naturally, they often leave their protein coats behind and only the viral genome enters (see Ch. 17). The term transfection (a hybrid of transformation with infection) refers to the use of purified viral DNA in transformation. In this case the experimenter purifies the viral genome from the virus particle and offers it to competent cells (Fig. 18.05). If taken up, purified viral DNA induces the cell to synthesize virus, illustrating that the virus coat is only necessary to protect the viral DNA outside the host cell and does not carry any of the virus genetic information. The fact that virus infection may be caused by the DNA alone is further evidence that DNA is the genetic material.

Transformation and transfection have two other meanings, also. Cancer specialists use the term "transformation" to refer to the changing of a normal cell into a cancer competent cell Cell that is capable of taking up DNA from the surrounding medium transfection Process in which purified viral DNA enters a cell by transformation. Often used to refer to entry of any DNA, even if not of viral origin, into an animal cell transformation (As used of cancer) Changing a normal cell into a cancer cell, even if no extra DNA enters the cell

P1 virus particles

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