BIOL 114
Biology Dept
Kenyon College

The Lac Operon

After your response, look above for reply.

 
1 How could a bacterial cell have two different versions of the lac operon?
The two lac operons are on homologous chromosomes inherited from the mother and father.
An F' plasmid or a specialized transducing phage could add a second copy of the lac operon from a previous host.
Generalized transduction or Hfr conjugation could bring in a second copy of the operon.
Bacteria actually replicate two or four copies of their own chromosome before the cell divides.
2 Which operon will produce B-galactosidase even if lactose is absent?
p +   o-c   lacZ-    lacI +
p +   o +   lacZ +   lacI +
p -   o-c   lacZ +   lacI -
p +   o +   lacZ +   lacI -
3 Will this genotype produce B-galactosidase?
p +   o-c   lacZ +        lacI + 
------------------------    -------- 
p +   o +   lacZ -        lacI +
B-galactosidase will only be expressed in the presence of lactose.
The lacI repressor is wild-type, so it will repress expression of B-galactosidase.
B-galactosidase will be made continuously.
B-galactosidase will never be made, because the lacZ- coding region is defective.
4 Will B-galactosidase be expressed?

p +   o-c   lacZ -        lacI + 
------------------------   -------- 
p +   o +   lacZ +       lacI +
Yes, with or without lactose, because the operator is constitutive.
B-galactosidase will not be made, because the lacZ- is nonfunctional.
The lacZ will express B-galactosidase only if lactose is present to remove repressor and induce the operon.
The repressor made by lacI will prevent transcription.
5 Will B-galactosidase be produced?
p +   o +   lacZ -      lacI - 
-----------------------   -------- 
p +   o +   lacZ +     lacI +
Yes, B-galactosidase will be produced when induced by lactose.
Yes, constitutively, because of lacI - (a non-functional repressor gene).
No, because of the non-functional lacZ - .
No, because the repressor is nonfunctional.
6 Will this genotype produce B-galactosidase?
p +    o +   lacZ +       lacI - 
-----------------------    -------- 
p -    o -c   lacZ +       lacI -
Yes, constitutively, because of o -c.
Yes, constitutively, because no repressor is made.
Yes, only with lactose inducer, since the functional promoter has a wild-type operator.
No, because there is a non-functional promoter p - .
7 Are constitutive operator alleles dominant or recessive?
Constitutive alleles of the operator are always dominant to the wild-type inducible operator.
Constitutive operator mutations are always recessive.
Constitutive operator alleles are dominant if the other cis-acting elements of the operon are functional.
Constitutive operator alleles are dominant only if the repressor is nonfunctional.
8 Are non-functional repressor alleles dominant or recessive?
Non-functional repressor alleles are recessive, because the repressor made by the functional lacI can act on any copy of the operon.
Non-functional lacI is dominant because it prevents binding of functional lacI.
Non-functional lacI is recessive because non-functional alleles are always recessive.
Non-functional lacI is dominant because it eliminates the entire function of the lac operon.
9 Could a mutation in the coding region of lacZ prevent expression of a gene downstream, such as lacY?
No, because lacY has its own ribosome binding site.
No, because lacY has its own promoter.
No, because although lacY is part of the lac operon, it has its own operator site.
Yes, if a lacZ mutation interrupts transcription, then lacY downstream will not be expressed.
10 Suppose a "superrepressor" mutation of lacI causes the repressor to bind operator whether or not lactose is present. Will this allele be dominant or recessive to wildtype lacI?
Dominant, because expression of all operators will be repressed.
Recessive, because lacI- non-functional alleles are always recessive.
Recessive, because lacI+ on the other operon will still be inducible.
Dominant, so long as it is linked to a functional Lac operon in cis.