(a) Inactive repressor; (b) active activator; (c) active repressor; (d) inactive activator.
The operon will never be turned off, and transcription will take place all the time.
The result will be constitutive expression, and transcription will take place all the time.
RNA polymerase will bind the lac promoter poorly, significantly decreasing the transcription of the lac structural genes.
| Lactose absent | Lactose present | |||
|---|---|---|---|---|
| Genotype of strain | β-Galactosidase | Permease | β-Galactosidase | Permease |
| lacI+ lacP+ lacO+ lacZ+ lacY+ | − | − | + | + |
| lacI− lacP+ lacO+ lacZ+ lacY+ | + | + | + | + |
| lacI+ lacP+ lacOc lacZ+ lacY+ | + | + | + | + |
| lacI− lacP+ lacO+ lacZ+ lacY− | + | − | + | − |
| lacI− lacP– lacO+ lacZ+ lacY+ | − | − | − | − |
| lacI+ lacP+ lacO+ lacZ− lacY+/ | − | − | + | + |
| lacI− lacP+ lacO+ lacZ+ lacY− | ||||
| lacI− lacP+ lacOc lacZ+ lacY+/ | + | + | + | + |
| lacI+ lacP+ lacO+ lacZ− lacY− | ||||
| lacI− lacP+ lacO+ lacZ+ lacY−/ | − | − | + | − |
| lacI+ lacP− lacO+ lacZ− lacY+ | ||||
| lacI+ lacP− lacOc lacZ− lacY+/ | − | − | + | − |
| lacI− lacP+ lacO+ lacZ+ lacY− | ||||
| lacI+ lacP+ lacO+ lacZ+ lacY+/ | − | − | + | + |
| lacI+ lacP+ lacO+ lacZ+ lacY+ | ||||
| lacIs lacP+ lacO+ lacZ+ lacY−/ | − | − | − | − |
| lacI+ lacP+ lacO+ lacZ− lacY+ | ||||
| lacIs lacP− lacO+ lacZ− lacY+/ | − | − | − | − |
| lacI+ lacP+ lacO+ lacZ+ lacY+ | ||||
The lacI gene encodes the lac repressor protein, which can diffuse within the cell and attach to any operator. It can therefore affect the expression of genes on the same molecule or on a different molecule of DNA. The lacO gene encodes the operator. It affects the binding of RNA polymerase to DNA, and therefore affects the expression of genes only on the same molecule of DNA.
The action of an enhancer is blocked when an insulator is located between the enhancer and the promoter of the gene. It is likely that genes A, B, and C will be stimulated by the enhancer and that gene D will not be stimulated. In the example from the figure, the insulator lies between gene D and the enhancer. The enhancer’s effect on genes A, B, and C is not likely to be affected by the insulator, and these genes will be stimulated.
The phenotypic differences in traditional genetic traits such as the color and shape of peas that Mendel studied are due to differences in the DNA base sequences within the alleles. In epigenetics, the phenotypic differences are not due to changes in allele DNA base sequences, but are differences in the expression of genes that are passed on to other cells and sometimes to other generations.
We would expect to see differences in DNA methylation and histone acetylation that altered expression of genes involved in response to stress. We would also expect that the adults would show increased fear and heightened hormonal response to stress.