Question

4. (3) Suppose that you mate a lac+ Hfr strain with the genotype str S i+ o+ z+ y+ (that you grew in the absence of lactose) with a lac - F - strain with the genotype str R i- o+ z - y. You allow sufficient time for the transfer of the genes indicated, with the exception of str S , and plate the bacteria on minimal medium supplemented with arabinose and streptomycin but no lactose. You observe transient beta-galactosidase activity in some of the colonies.
A) How can beta-galactosidase be made by some of the str R colonies?
B) Why is beta-galactosidase made for only a short time?
C) Suppose that you observe some colonies that have persistent beta-galactosidase activity. How do you explain this?

Answer 1

Ans A) Some of the str R colonies can make beta-galactosidase because of the absence of lacI gene (as shown in genotype i- ) that codes for repressor. Hence, the gene of beta-galactosisdase is expressed.  

Ans B) The beta-galactosidase is made for only a short time because later the lacI gene from str S strain (genotype shows i+) is expressed producing repressor that suppresses the beta-galactosidase expression.

Ans C) The colonies with persistent beta-galactosidase activity will be the ones, str R  that have not mated with str S and are i- as shown in genotype, hence, have no repressor therefore, express beta-galactosidase continuosly.