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Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.

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An Introduction to Genetic Analysis. 7th edition.

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Bacterial transformation

Some bacteria have another method of transferring DNA and producing recombinants that does not require conjugation. The conversion of one genotype into another by the introduction of exogenous DNA (that is, bits of DNA from an external source) is termed transformation. Transformation was discovered in Streptococcus pneumoniae in 1928 by Frederick Griffith; in 1944, Oswald T. Avery, Colin M. MacLeod, and Maclyn McCarty demonstrated that the “transforming principle” was DNA. Both results are milestones in the elucidation of the molecular nature of genes. We consider this work in more detail in Chapter 8.

After DNA was shown to be the agent that determines the polysaccharide character of S. pneumoniae, transformation was demonstrated for other genes, such as those for drug resistance (Figure 7-15). The transforming principle, exogenous DNA, is incorporated into the bacterial chromosome by a breakage-and-insertion process analogous to that depicted for Hfr × F crosses in Figure 7-12. Note, however, that, in conjunction, DNA is transferred from one living cell to another through close contact, whereas, in transformation, isolated pieces of external DNA are taken up by a cell. Figure 7-16 depicts this process.

Figure 7-15. The genetic transfer of streptomycin resistance (strr) to streptomycin-sensitive (strs) cells of E.

Figure 7-15

The genetic transfer of streptomycin resistance (strr) to streptomycin-sensitive (strs) cells of E. coli. The recovery of strr transformants among strs cells depends on the concentration of strr DNA. (From G. S. Stent and R. Calendar, Molecular Genetics, (more...)

Figure 7-16. Bacterium undergoing transformation (a) picks up free DNA released from a dead bacterial cell.

Figure 7-16

Bacterium undergoing transformation (a) picks up free DNA released from a dead bacterial cell. As DNA-binding complexes on the bacterial surface take up the DNA (inset), enzymes break down one strand into nucleotides; meanwhile the other strand may integrate (more...)

Linkage information from transformation

Transformation has been a very handy tool in several areas of bacterial research. We learn later how it is used in some of the modern techniques of genetic engineering. Here we examine its usefulness in providing linkage information.

When DNA (the bacterial chromosome) is extracted for transformation experiments, some breakage into smaller pieces is inevitable. If two donor genes are located close together on the chromosome, then there is a greater chance that they will be carried on the same piece of transforming DNA and hence will cause a double transformation. Conversely, if genes are widely separated on the chromosome, then they will be carried on separate transforming segments and the frequency of double transformants will equal the product of the single-transformation frequencies. Thus, it should be possible to test for close linkage by testing for a departure from the product rule.

Unfortunately, the situation is made more complex by several factors—the most important of which is that not all cells in a population of bacteria are competent, or able to be transformed. Because single transformations are expressed as proportions, the success of the product rule depends on the absolute size of these proportions. There are ways of calculating the proportion of competent cells, but we need not detour into that subject now. You can sharpen your skills in transformation analysis in one of the problems at the end of the chapter, which assumes 100 percent competence of the recipient cells.

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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21993

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