23. Two lac auxotrophs, lacA1 and lacB1, have mutations in different genes withi

Question

23. Two lac auxotrophs, lacA1 and lacB1, have mutations in different genes within the lac operon. Consider the experiments below: Experiment #1 : lacA1 strain-lactose minimal media : no growth Experiment #2 : /acB1 strain-lactose minimal media : no growth Experiment #3: Dead lacAl cells + Live /acB1 cells + lactose minimal media : growth 23a) What are the phenotypes for the bacteria in each experiment? (5 points) 24b) How is it possible that two lac auxotrophs could produce a lac prototroph? Explain. (7 points).

Explanation / Answer

23a) Phenotype in case of bacteria is described in terms of cell structure, cellular metabolism, and different cell components. According the the inforamtion given in the question, phenotype is described in terms of metabolism.

In experiment 1, lacA1 strain is allowed to grow on lactose minimal media. After incubation there was no growth found on the culture plate. Since the lacA1 strain has mutation in a gene of lac operon, the corresponding protein is non-functional. As a result, lactose metabolism in the cell won’t occur.

Similarly, in experiment 2 the lacB1 strain won’t synthesise of functional protein and the lactose metabolism will be hindered.

In experiment 3, the lacA1 cells are dead and lacB1 cells are living. Together they are cultured on lactose minimal media. In this case, the lacA1 DNA will be passed to lacB1. Since both the strains poses mutation in different genes, all the proteins of lac operon will be functional. Hence lactose metabolism will occur.

24b) Two lac auxotrophs can produce a lac prototroph. Both the auxotrophs have mutation in different lac genes. Suppose lacA1 has mutation in LacZ and lacB1 in LacY. Both the strains will have non-functional beta-galactosidase and beta-galactosidase permease. However, lacA1 will have functional beta-galactosidase permease and lacB1 will have functional beta-galactosidase. Thus, if the genome of both auxotrophs come together in a cell, then all the functional proteins of lac operon will be formed resulting in lac prototroph.

There are two ways by which the genomes can unite in a cell.

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