Figure 5.21. The basis of gene mapping in bacteria.

Figure 5.21The basis of gene mapping in bacteria

(A) Transfer of a functional gene for tryptophan biosynthesis from a wild-type bacterium (genotype described as trp +) to a recipient that lacks a functional copy of this gene (trp -). The recipient is called a tryptophan auxotroph (the word used to describe a mutant bacterium that can survive only if provided with a nutrient - in this case, tryptophan - not required by the wild type; see Section 14.1.2). After transfer, two crossovers (shown as green crosses) are needed to integrate the transferred gene into the recipient cell's chromosome, converting the recipient from trp - to trp +. (B) During conjugation, DNA is transferred from donor to recipient in the same way that a string is pulled through a tube. The relative positions of markers on the DNA molecule can therefore be mapped by determining the times at which the markers appear in the recipient cell. In the example shown, markers A, B and C are transferred 8, 20 and 30 minutes after the beginning of conjugation, respectively. The entire Escherichia coli chromosome takes approximately 100 minutes to transfer. (C) To be co-transferred during transduction and transformation, two or more markers must be closely linked, because these processes usually result in less than 50 kb of DNA being passed from donor to recipient. Transduction and transformation mapping are used to determine the relative positions of markers that are too close together to be mapped precisely by conjugation analysis. For more details on bacterial gene mapping see Freifelder (1987).

From: Chapter 5, Mapping Genomes

Cover of Genomes
Genomes. 2nd edition.
Brown TA.
Oxford: Wiley-Liss; 2002.
Copyright © 2002, Garland Science.

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