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Lac Operon Self Quiz

Terms in this set (8)

1. Predict the phenotype of a lacI mutant.
a. The lac genes would be expressed efficiently only in the absence of lactose.
b.The lac genes would be expressed efficiently only in the presence of lactose.
c.The lac genes would be expressed continuously.
d.The lac genes would never be expressed efficiently.
C. In a lacI mutant, the mutant lac repressor protein cannot bind to the operator. In the absence of the repressor, RNA polymerase can bind to the lac promoter, and the lac genes will be transcribed continually regardless of whether the inducer, allolactose, is present or not. This is referred to as a constitutive phenotype. Remember, though, that the level of lac gene expression will depend on whether glucose is present in the medium.
2. If a second wild type or normal copy of the lacI gene (just lacI and not lacZ, lacY, or lacA) is introduced into the lacI mutant cell, what would be the phenotype of this partial diploid (also referred to as a merodiploid)?
a.The lac genes would be expressed efficiently only in the absence of lactose.
b.The lac genes would be expressed efficiently only in the presence of lactose.
c.The lac genes would be expressed continuously.
d.The lac genes would never be expressed efficiently.
B. Constitutive mutants in the lacI gene are recessive. The lac repressor protein is diffusible. Therefore the wild type copy of the repressor protein will be able to diffuse and bind to the operator in the chromosomal copy of the lac operon and will block expression of the chromosomal lac genes.
3. Predict the phenotype of a lacI S or "super-repressor" mutant. A lacI S mutant synthesizes a repressor that cannot bind to the inducer.
a.The lac genes would be expressed efficiently only in the absence of lactose.
b.The lac genes would be expressed efficiently only in the presence of lactose.
c.The lac genes would be expressed continuously.
d.The lac genes would never be expressed efficiently.
d. The "super repressor" has lost its binding site for the inducer (allolactose). Therefore, the inducer cannot bind to the "super repressor," which binds permanently to the operator.
4. Predict how a lacI S mutant would be affected by the construction of a merodiploid that has a second normal copy of the lacI gene.
a.The lac genes would be expressed efficiently only in the absence of lactose.
b.The lac genes would be expressed efficiently only in the presence of lactose.
c.The lac genes would be expressed continuously.
d.The lac genes would never be expressed efficiently.
D. The wild-type repressor can be bound and inactivated by the inducer (allolactose). However, the mutant "super-repressor" will bind to the lac operator and will not be inactivated because allolactose cannot bind to the mutant repressor.
5. Predict the phenotype of an operator mutant (O c) which would prevent the binding of the repressor.
a.The lac genes would be expressed efficiently only in the absence of lactose.
b.The lac genes would be expressed efficiently only in the presence of lactose.
c.The lac genes would be expressed continuously.
d.The lac genes would never be expressed efficiently.
C. Active repressor cannot bind to the mutant O c operator. Therefore, the repressor cannot prevent RNA polymerase from binding to the lac promoter, causing the lac operon to be transcribed continuously (or constitutively). Once again, the level of expression is inversely proportional to the level of glucose present.
6. Predict the phenotype of a promoter mutant (lacP) which has a mutation in the promoter for the lac operon.
a.The lac genes would be expressed efficiently only in the absence of lactose.
b.The lac genes would be expressed efficiently only in the presence of lactose.
c.The lac genes would be expressed continuously.
d.The lac genes would never be expressed efficiently.
D. If the promoter region is mutated, RNA polymerase will not be able to recognize it and bind to it. Therefore, the lac genes will not be transcribed even when the repressor protein is inactivated (in the presence of allolactose) and glucose is absent from the medium (the catabolite activator protein is bound to cyclic AMP and to the promoter).
7. Predict the phenotype of a lacZ mutant, which has a mutation in the gene for β-galactosidase.
a.The production of all protein products would be affected.
b.The production of β-galactosidase would be affected, but other protein products would be unaffected.
c.The production of β-galactosidase would be affected, and the production of some other protein products might also be affected.
d.The production of β-galactosidase would be unaffected, but other protein products would be affected.
C. If the lacZ gene carries a mutation in an essential part of the gene, the β-galactosidase protein produced will be not be functional. Other protein products may not be functional as well. Such a mutation might cause:
substitution of an essential amino acid resulting in a loss of activity due to, for example, improper folding of the β-galactosidase protein.
substitution of many amino acids (caused, for example, by a frameshift mutation) leading to complete loss of protein function.
premature termination of protein synthesis leading to a truncated, nonfunctional β-galactosidase protein and no lactose permease or transacetylase.
8. Predict the phenotype of a lacY mutant, which has a mutation in the gene for lactose permease.
a.The lac genes would be expressed efficiently only in the absence of lactose.
b.The lac genes would be expressed efficiently until the lactose supply in the cell is exhausted.
c.The lac genes would be expressed continuously.
d.Expression of the lac genes would cease immediately.
B. lactose permease is involved with the transport of lactose into the cell through the cell membrane. Therefore,

No lactose permease.→lactose cannot enter cell.

This means that when the cell's supply of lactose has been used up, no more lactose will be able to enter the cell. As a result, the lac repressor protein will remain permanently bound to the operator region in the lac promoter because there will be no allolactose available in the cell to inactivate it, even though there may be abundant lactose present in the surrounding medium.

RNA polymerase will be permanently blocked.→NO expression of lac genes.
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