Question

The following shows the genotype of a partial diploid bacterial cell - where one chromosomal region containing the lac operon in E,coli is given, and the other fragment is from a plasmid carrying another lac operon from another source. The two are separated by a slash (/).

The possible answers indicate with a ʺ+ʺ or a ʺ-ʺ whether β-galactosidase would be expected to be produced at induced levels under two circumstances: 1) first in the absence of lactose and 2) second in the presence of lactose. (Assume that glucose is not present in the medium.)

Genotype B: I- Oc Z- / Fʹ I- O+ Z-

Genotype C: Is Oc Z-/ Fʹ I + Oc Z-

Genotype D: I- O+ Z+/ Fʹ Is O+ Z+
Genotype F: I+ Oc Z-/ Fʹ I- O+ Z+

KEY:
I+ = wild-type repressor
I- = mutant repressor (unable to bind to the operator)
Is = mutant repressor (insensitive to lactose)
O+ = wild-type operator
Oc = constitutive operator (insensitive to repressor)

a. +,+ d. -,+

b.-,- e. cannot tell

c.+,-

(genotype B , C ,D are not +,+ )(genotype F is not -,- )

Answer 1

Ans_ A mutant, in which Structural genes of an operon are always transcribed, even in the absence of inducer, is called a constitutive mutant.

lacI-mutation either do not synthesize repressor or synthesize repressor that are not capable of binding to the operator.

LacI_s mutants have a mutation that changes the repressor so that it can no longer bind the inducer .

LacOc mutation is dominant and cis acting

When operator mutant then phenotype occurs constitutive

When lac repressor mutant then inducible phenotypes occurs

When lacZ expression mutant then constitutive phenotype occurs

A lac operon in E.coli with a mutation in the operator; the operator cannot bind with wild type lac repressor protein and transcription is constitutive, even in the absence of