STUDIES IN MICROBIOLOGY (3) 



Genetic Transformation of Bacteria ' 



(Readings: F. H. C. Crick, "The Structure of the Heriditary Material," Sci. 

 Am. 191, No. 4, 54-61, Oct. 1954, Reprint No. 5. R. D. Hotchkiss and E. Weiss, 

 "Transformed Bacteria," Sci. Am. 195, No. 5, 48-53, Nov. 1956, Reprint No. 18. 

 Further readings are suggested at the end of the exercise.) 



One of the most striking characteristics of 

 living organisms is that offspring resemble their 

 parents. This resemblance with regard to both 

 form and function is found in all forms of life 

 from bacteria to man. We tend to take it for 

 granted that human children, like their parents, 

 have five-fingered hands, and three-color vision, 

 yet these traits must just as surely be inherited 

 as such abnormalities as six fingers or color- 

 blindness. In bacteria, heredity operates equally, 

 so that in Serratia marcescens, for example, 

 daughter cells, like their parent, are rod-shaped 

 and capable of splitting hydrogen peroxide in a 

 reaction catalyzed by the enzyme catalase. 



As you know, the factors responsible for 

 heredity are called genes. All cells contain 

 deoxyribose nucleic acid or DNA, specifically 

 in the nucleus when they have nuclei. That the 

 genes are DNA molecules or portions of them 

 is demonstrated by bacterial transformation: 

 DNA isolated from one bacterial strain can 

 change the nature of a cell of another diff"erent 



*Directions for setting up these experiments will 

 be found in Appendix A. 



strain in ways that are thereafter inherited by 

 all its offspring. 



We shall study the transformation of cells of 

 a strain of Pneumococcus which is sensitive to 

 the antibiotic streptomycin, by DNA taken from 

 a strain of Pneumococcus resistant to this anti- 

 biotic. First we shall extract the DNA from 

 cells of the resistant strain by adding sodium 

 deoxycholate, which disintegrates the mem- 

 branes of the cells, releasing their contents. 

 Next we shall precipitate the DNA by adding 

 alcohol (as in the yeast analysis in Exercise III), 

 so that the molecules form long fibers which 

 can be removed. After redissolving the DNA, 

 this solution will be used to treat bacteria of the 

 sensitive strain which are in an appropriate 

 condition to take up the large molecules of 

 DNA. After allowing the freshly transformed 

 cells to develop resistance to streptomycin, we 

 shall test their ability to form colonies of re- 

 sistant offspring. 



Note on bacterial media and ecology. It may 

 be of interest to you at this stage to learn more 

 about the media used for growing bacteria. 



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