DNA from a bacterial strain that is his leu lac is used to t

DNA from a bacterial strain that is his\' leu\' lac\' is used to transform a strain that is his\' leu\' lac\'. The following percentages of cells are transformed: his leu lac % + + + 0.02 + + - 0.00 + - + 2.00 + - - 4.00 - + + 0.10 - - + 3.00 - + - 1.50 What is the order of these three genes? Which genes are closest? A transduction experiment is performed, where the donor strain of bacteria with the genotype leu\' gal\' pro\' is infected with a bacteriophage and allowed to lyse. The progeny phage are used to infect a second strain that is leu\' gal\' pro\'. Surviving cells from this infection are selected for leu\' by growing on medium that lacks leucine. 47% of these cells are also pro\', while 26% of these cells are gal. Which genes are closest, leu and gal, or leu and pro? Among leu\' recombinants, no cells were found that were both pro\' and gal. What is the order of these three genes in the chromosome?

Solution

In transformation, the genetic material is picked up from the environment or the surrounding and inserted in the host bacterial plasmid or chromosome. The process of transformation has been used to know the order of genes. The genes which are located closer to each other have been found to show high frequency of simultaneous transformation or cotransformation generating large number of transformants.

The percentage of his⁺leu^-lac⁺ cells is 2% whereas the percentage of his⁺leu⁺lac^- is 0, indicating that the cotransformation frequency is more in case of his and lac genes compared to his and leu genes. This in turn indicates that his and lac genes lie closer to each other compared to his and leu genes. This in turn implies that the order of genes is such that the lac gene lies between the his and leu genes.