1) The logic of the Lac operon was deciphered by using bacterial lac-operon mutants. Crucial to the experiments was creation of partial diploid cells in which operon from a wild-type cell is introduced into the mutant cell [thus there are 2 copies of the operon], and its ability to restore repression is assessed. Three classes of mutants were found that affect expression of the lac operon. Two of these resulted in constitutive expression of the operon, and the other eliminated all expression, even in the presence of lactose. Use the information above to complete the following table indicating with a (Y) yes or (N= No) if enzymatic activity of B-Gal will be detected (protein made and functional). Assume there is no glucose. Remember: Promoter for operon controls ALL genes downstream of it, coding sequence of the genes themselves matter (so if transcription is occurring but structural gene is producing a non-functional protein there will be no activity detected) For the lac operon partial diploid genotypes the following notations are used: + = Wildtype version ; - sign = non-functional version of the gene/ mutation in sequence I = Gene for Lac I repressor protein P = Promoter for lac operon O = Operator Sequence Oc = Mutation within operator sequence, such that Lac repressor cannot bind it. The // shows the genotypes of the 2 copies within the bacterial cell. Genotype of Strain Lactose Absent B- Gal Lactose Present B-Gal I -, P +, O +, Z +// I +, P +, O +, Z+ I -, P +, Oc , Z +// I +, P +, O +, Z+ I +, P-, Oc, Z+ // I -, P +, O +, Z +
1)
The logic of the Lac operon was deciphered by using bacterial lac-operon mutants. Crucial to the experiments was creation of partial diploid cells in which operon from a wild-type cell is introduced into the mutant cell [thus there are 2 copies of the operon], and its ability to restore repression is assessed.
Three classes of mutants were found that affect expression of the lac operon. Two of these resulted in constitutive expression of the operon, and the other eliminated all expression, even in the presence of lactose.
Use the information above to complete the following table indicating with a (Y) yes or (N= No) if enzymatic activity of B-Gal will be detected (protein made and functional). Assume there is no glucose.
Remember: Promoter for operon controls ALL genes downstream of it, coding sequence of the genes themselves matter (so if transcription is occurring but structural gene is producing a non-functional protein there will be no activity detected)
For the lac operon partial diploid genotypes the following notations are used:
+ = Wildtype version ; - sign = non-functional version of the gene/ mutation in sequence
I = Gene for Lac I repressor protein
P = Promoter for lac operon
O = Operator Sequence
Oc = Mutation within operator sequence, such that Lac repressor cannot bind it.
The // shows the genotypes of the 2 copies within the bacterial cell.
|
Genotype of Strain |
Lactose Absent B- Gal |
Lactose Present B-Gal |
|
I -, P +, O +, Z +// I +, P +, O +, Z+ |
|
|
|
I -, P +, Oc , Z +// I +, P +, O +, Z+ |
|
|
|
I +, P-, Oc, Z+ // I -, P +, O +, Z + |
|
|
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