lecture 8: operons Flashcards
what type of regulation do operons conduct?
regulation at the gene level
what type of cells do operons exist in?
prokaryotic cells
what are prokaryotic genes organized into?
organized into operons (the genes are the operons)
what are the 2 elements to operons?
1) promoter: contains an on/off switch called operator
2) genes that will be under control of the operator (code for proteins involved in the same metabolic pathway)
what is a repressible operon?
-operon for synthesizing (anabolic)
-repressor protein is made in an inactive form (will be activated by presence of metabolite)
-trasncription is ON by default
what acts as a co-repressor for repressible operons?
metabolite that will bind to repressor and put it in its active form
are repressible operons on or off by default?
on by default and repressor proteins deactivates them
what is an inducible operon?
-operon for breaking something down
-repressor is made in an active form (will be deactivated by present of metabolite)
-transcription is off by default
how are the genes of the operon transcribed?
as one single mRNA molecule
what happens when an operon is turned off?
-repressor protein binds to the operator (thats why its off)
-Rna polymerase is prevented from binding to the promoter and transcription is blocked
what happens when an operon is turned on?
-RNA polymerase binds to the promoter
-all genes of the operon are transcribed as one long mRNA
-each gene on the mRNA is translated into a polypeptide
-repressor protein is off
true or false, a repressor protein is always transcribed and translated.
true
what is a repressor protein?
-product of a regulatory gene
-gene is always being transcribed
-binding of repressor protein is reversible
-contains an allosteric site: allosteric regulators are metabolites in the cell
how do repressor proteins function?
they have allosteric regulators, which are metabolites in the cell, and make them vary between inactive and active repressors.
what are the 2 types of operons?
repressible and inducible
what is a repressible operon? give an example.
transcription is turned off by the product of a metabolic pathway. An example of this would be the trp operon. this type of operon is made for synthesis
what is an inducible operon? give an example.
transcription is turned ON by the presence of a reactant in a metabolic pathway. an example of this is the lac operon
what is the tryptophan operon?
-produces enzymes involved in the production of tryptophan
-repressor protein is active when bound to tryptophan
what happens if there’s low tryptophan?
-repressor protein is inactive and does not bind to operator
-operon is turned on and mRNA is produced
what happens if there’s high tryptophan?
-repressor protein is active and binds to operator
-operon is turned off and mRNA is not produced
what is lactose metabolism?
the breakdown of lactose to make ATP
what happens to the repressor protein when lactose is present?
-repressor protein is inactive when lactose is present
-allolactose acts as an inducer
-lactose is converted into allolactose in the cell
what happens to lac operon when lactose is not present?
-repressor protein is active and binds to operator
-operon is turned off
what happens to lac operon when lactose is present?
-allolactose binds to receptor and inactivates it
-repressor protein is inactive and does not bind to operator
-operon is turned on and mRNA is produced
what is allolactose?
-it acts as an inducer (induces activity of metabolic pathway)
-its present when lactose is present, cause lactose is converted into allolactose in the cell
what is negative control of gene expression? give an example.
-operon can be turned off by repressor
-repressible and inducible operons
example: lac and trp operon (repressor)
what are the two types of gene expression controls?
negative and positive control
what is the positive control of gene expression? give an example.
transcription of operon can be increased by an activator
-example: lac operon (CAP-cAMP)
what is the difference between positive and negative control of lac operon?
positive control adjusts levels of transcription (doesn’t turn it on/off), while negative controls turns it on/off via a repressor protein
if the glucose concentration level is high, why are the transcription levels of the lac operon very low even when lactose is present?
-glucose determines the transcription levels (positive control)
-glucose is a better/more efficient source of energy (dont need to break down lactose to form energy)
for the lac operon, if glucose is low, do we want high levels of transcription?
yes
explain what CAP/cAMP is
-in addition to an operator, the lac operon promoter also had a CAP-binding site
-when CAP binds to the promoter, it stimulates transcription
-CAP is only in its active form when bound by cyclic-AMP (cAMP)
-cAMP levels are high when glucose concentration is low
-
why are cAMP levels high when glucose concentration is low?
glucose inactivates the enzyme adenylyl cyclase, which is the enzymes that turns ATP into cAMP
what happens when an active cAMP binds to the CAP-binding site?
transcription levels increase
will transcription be turned on if there is no glucose and no lactose in the cell?
look at lactose first: because only lactose tells us if transcription is on/off, then glucose tells us about how fast transcription is.
what will the level of transcription be like if lactose and glucose are present? what type of regulation is happening?
transcription is on, but because of the presence of glucose, we have low level of transcription. we have both positive and negative regulation occurring at the same time.
