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Mapping
of
Chromosome
by
Transduction - Transduction can be used to establish gene order and map distance. Gene order can be established by two factor transduction. For example, if gene A is co-transduced with gene Band B with gene C but A is never co-transduced with C, we have established the order ABC.
This would also apply to quantitative difference in co-transduction to detect cells that have been transduced for one, two or all the three loci. We need to eliminate the non-transduced cells. In other words after transduction, there will be k, B- ,C-cells in which no transduction event has taken place.
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There will also be seven classes of bacteria that have been transduced for one, two or all three loci (NB+C+, A'B'C-, A'B-C+, A-B'C+, A+B-C-, A-B'C- and A-B-C+). The simplest way to select for transduced bacteria is to select bacteria in which the wild type has replaced at least one of the loci.
For example if after transduction, we grow the bacteria in minimal media the requirements of B- and C- added, all the bacteria that are A+ will grow. Hence, although we lose the untransduced bacteria (A -B-C-), replicaplating allows us to determine gene type at the Band C loci for the N bacteria.
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In this example, colonies that are grown on complete medium without the requirement of the A mutant are replicaplated onto a complete medium without the requirement of the B mutant and then onto complete medium without the requirement of the C mutant. In this way, each transductant can be scored for the other two loci. Now let us take all these selected transductants in which the A' allele was incorporated.
These can be of four categories A+B'C', NB'C-, NB-C- and NB-C-. We can now compare the relative numbers of each of these four categories. The rarest category will be caused by the event that brings in the outer two markers, but not the centre one, because this event requires for crossovers.
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In recent years, we have witnessed a debate on the environmental implications of biotechnology. In this debate, risks involved in the use of biotechnological approaches have often been emphasized (or even overemphasized) and the adequate guidelines for safety have been suggested ad enforced by law.
However, there have also been rapid developments in the applications of biotechnology, which may help in controlling environment pollution, thus giving a cleaner and sustainable environment in future. According to one estimate in USA, the US market for environmental clean-up applications was expected to grow at an average rate of 17%, while that for microbes and enzymes was expected to grow by only 7% every year.
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Thus, by looking at the number of transductants in the various categories, we can determine that the gene order is ABC.In transformation, we measure the co-occurrence directly; therefore, the measure, co-transductance, is the inverse of map distance. In other words, the greater the co-transduction rate, the closer the two loci are; the more frequently two loci are transduced together, the closer they are and the higher the co-transduction value.
From these sorts of transduction experiments, it is possible to round out the details of map relations of E. coli after obtaining the overall picture by interrupted mating.
Unlike the measurements in eukaryotic mapping, prokaryotic map distances are not generally thought of in map units. Rather, the general distance between locus is determined in minutes with co transduction values used for loci that are very close to each other.
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