Chromosome mapping using transformation

Transformation can be used to provide information on bacterial gene linkage. 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, there is a good chance that sometimes they will be carried on the same piece of transforming DNA. Hence both will be taken up, causing a double transformation. Conversely, if genes are widely separated on the chromosome, they will be carried on separate transforming segments. Any double transformants will most likely arise from separate independent transformations. Hence in the case of widely separated genes,

Free DNA

Cell wall

Free DNA

Cell wall

DNA-binding complex

Nucleotide

Transformation Bacteria Free Dna

DNA-degrading enzyme

Transformed Transferred bacterium DNA

Figure 5-19 Transformation. 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; a derivative of the other strand may integrate into the bacterium's chromosome (b). [After R. V. Miller, "Bacterial Gene Swapping In Nature." Copyright 1998 by Scientific American, Inc. All rights reserved.]

DNA-binding complex

Nucleotide

DNA-degrading enzyme

Transformed Transferred bacterium DNA

Figure 5-19 Transformation. 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; a derivative of the other strand may integrate into the bacterium's chromosome (b). [After R. V. Miller, "Bacterial Gene Swapping In Nature." Copyright 1998 by Scientific American, Inc. All rights reserved.]

the frequency of double transformants will equal the product of the single-transformation frequencies. Therefore it should be possible to test for close linkage by testing for a departure from the product rule. In other words, if genes are linked, then the proportion of double transformants will be greater than the product of single transformants.

Unfortunately, the situation is made more complex by several factors—the most important is that not all cells in a population of bacteria are competent to be transformed. Nevertheless you can sharpen your skills in transformation analysis in one of the problems at the end of the chapter, which assumes that 100 percent of the recipient cells are competent.

MESSAGE Bacteria can take up DNA fragments from the medium, which once inside the cell can integrate into the chromosome.

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