Expression Regulation Flashcards
In what cells of your body is the lac operon?
None. It is in the E. Coli bacterias of your intestine only.
Why is the lac operon gene usually turned off?
It is more efficient to break down glucose than to break down lactose. However, when lactose is present and no glucose is there, the lac operon is activated to break down the lactose into glucose and galactose.
What are the two conditions for the lac operon gene to be expressed?
Glucose must be low or absent and lactose must be present
What is the inducer of the lac operon gene?
Allolactose
How can lactose induce the lac operon gene?
The lactose will bind to the repressor to change its shape and make it fall of the operator. Hence, RNA polymerase can bind to the promoter of the lac operon and transcription can proceed for the expression of the lac operon gene.
If the lac Operon is induced, does it necessarily proceed to expressing the lac operon gene?
No. Before it is induced, the lac Operon also needs to be activated by the CAP protein, which binds to the DNA to enhance transcription as if it was a transcription factor. CAP (Catabolite activator protein) is activated by attachment of the cAMP. Concentration of cAMP increases when glucose is lower. All of this is needed above from the presence of lactose, which induces the expression of lac operon by having the repressor fall of the operator.
What are the structural genes of the lac operon?
lacZ, lacY, lacA
What are the regulatory elements of the lac operon?
Plac, O, lacI
What is the lacZ structural gene?
It codes for beta-galactosidase, the enzyme required to break down lactose into glucose + galactose
What is the lacY structural gene?
It codes for permease, the enzyme that is required to bridge the environment with the cell by transporting the lactose molecules inside the cell for them to be broken down.
What is the lacA structural gene?
(Not evaluated in the course) It codes for transacetylase, which catalyzes the addition of acetyl group to many galactosides
What is the Plac regulatory element?
It is the promoter of the lac operon gene. It is the place where RNA polymerase needs to bind just before the operator.
What is O, the operator, regulatory element?
It is a sequence of DNA where the repressor protein binds to prevent the transcription of the lac operon gene. Attachment site of repressor.
What is the lacI regulatory element?
It is the gene that codes for the repressor protein, which binds to the operator and can only be removed in the presence of lactose. It is a constitutive gene, which is always expressed. The only way for it to have no effect is if lactose is present enough to have the repressor fall off.
What is the Pi regulatory element?
It is the promoter of the lacI gene. In short, it initiates the transcription of the repressor protein.
Is beta-galactosidase present if lactose is unavailable?
No. Without lactose, the repressor cannot fall off the operator and transcription is blocked.
Is beta-galactosidase present if lactose is available but not glucose?
Yes. CAP can act as help the RNA polymerase start transcription at the Plac site and lactose induces gene expression by modifying the shape of the repressor to make it fall of the operator.
Is beta-galactosidase present if lactose and glucose are available? Whether there is a mutation or not.
No. Glucose being present decreases the concentration of cAMP, which cannot activate CAP, which cannot help RNA polymerase to start transcription of the gene. Regardless of if the other genes of the operon are functional or not.
Is beta-galactosidase present if the I gene promoter is non-functional and lactose is not available?
Yes. The I gene cannot code for the repressor, so it cannot block transcription even if lactose is not available.
Is beta-galactosidase present if the I gene promoter is non-functional and lactose is available?
Yes. Everything happens like usual except that repressor does not fall off the operator… it was never produced!
Is beta-galactosidase present if the repressor protein is not functional?
Yes. Whether lactose is there or not, if the repressor cannot block transcription of the lac operon gene, then it will be produced!
Is beta-galactosidase present if the operon promoter is not functional?
NO. Whether lactose is there or not, if the promoter does not work, RNA polymerase cannot initiate the transcription of the lac operon.
Is beta-galactosidase present if the operator is mutated?
Yes. Whether lactose is present of not, if the operator is not functional, the repressor cannot block transcription when lactose is not there.
Is beta-galactosidase present if the Z gene is mutated?
No. The function of this gene is to produce beta-galactosidase.
