Question

25. You take your lac/ strain and transform in two F' plasmids to conduct a complementation test. The first F' plasmid contains the entire lac operon from a strain that is lacz, while the second F' plasmid contains the entire lac operon from a strain that is lacY. You test the ability of the transformed bacteria to grow on media where the sole carbon source is either glucose or lactose, and obtain the following results: Lac t mutant compl lacZ lacY lemented with lact lacYLac mutant complemented with Growth Medium Glucose medium Lactose medium Medium Glucose medium Lactose medium What do you conclude? a) the lacl strain has a recessive mutation in the lacZ gene b) the lacI strain has a dominant mutation in the lacZ gene c) the lacl strain has a recessive mutation in the lacY gene d) the lac/ strain has a dominant mutation in the lacY gene e) the lacl strain has a mutation in the lac repressor that prevents it from binding lactose; the mutant repressor never stops repressing transcription of lac operon even in the presence of lactose Growth 26. What is an F' plasmid ?

Answer 1

(1) The complementation of lacI- strain with lacZ- and lacY+ shows no growth when lactose is present in the medium. However, growth is seen in the presence of glucose in the medium. This suggests that the lacI- strain has a recessive mutation in the lacZ gene.

(2) The F+ plasmid (or the fertility plasmid) is a type of plasmid through which genes to be transferred from a bacterium which possesses the plasmid to another bacterium that lacks this plasmid. The F' plasmid carries the genes that allow for this exchange. Now, once the plasmid from host bacterium (F+) gets incorporated into the recipient bacterium (F-), there are always chances of incorrect excision of the fertility factor from the recipient bacterium. The excised plasmid (known as F') will carry genes from the bacterium along with F+.