Control of gene expression Flashcards
How is gene expression regulated?
Environmental factors through (‘induction’ or ‘repression’).
Transcription factors- activators and repressors/suppressors
what are the different ways genes can be controlled? (what are the 3 ways to genes can be turned on).
Some genes are turned on only in a specific organ or cell type.
Some genes are turned on in multiple cells/organs, but expressed at different levels.
Some genes are turned on during a specific stage in development
why does a cell change the expression of a gene in response to an external signals.
Because lots of energy is needed for an organism to express all gene at all times SO It is more energy efficient to turn on the genes only when they are required= when they receive an external signal.
what is the effect when transcription factors bind to the regulatory sequence?
Binding of a transcription factor to a regulatory DNA sequence acts as the switch to control gene transcription.
Transcription factors turn gene expression ON (activators) or OFF (repressors).
Negative Regulation and the effect of ligand.
- what does the bound repressor protein bind to?
- what does the protein have?
- absence of ligand
- addition of ligand
The bound repressor protein binds to the regulatory DNA sequence on the promoter region.
The protein has a binding pocket for the ligand.
- Absence of ligand= the protein binds to the DNA regulatory sequence preventing transcription.(switches OFF).
- Addition of ligand= the ligand binds to the repressor, changing its conformation that it can no longer bind or recognise the regulatory DNA sequence= Repressor protein is removed from the DNA= preventing transcription (Switches OFF)
In some cases, transcription repressors (protein
) CAN bind to the regularity DNA sequence in the presence of ligand. However if the ligand is removed, the transcription repressor becomes inactive and allows transcription of the gene (switches ON).
Positive Regulation and the effect of ligand.
Absence of ligand= transcription activator binds to the regulatory DNA sequence on the promotor region, promoting transcription (Switches ON)
Addition of ligand= The ligand binds to the activator protein, causing a conformational change. This change enhances the activator’s ability to bind to the regulatory DNA sequence, further promoting transcription (Switch ON).
However when the ligand is removed, the activator becomes inactive and is removed from the DNA- switches OFF gene transcription.
Transcription factor (protein) involved in Negative regulation?
Transcription factor (protein) involved in Positive regulation?
summary of negative and positive regulation?
Transcription repressor protein
Transcription activator protein
negative regulation prevents transcription in the absence of a ligand
positive regulation promotes transcription in the presence of a ligand
trp operon in E.coli. what does it encode for?
trp operon in E. coli encodes 5 structural genes
necessary for tryptophan biosynthesis in the absence of tryptophan in the environment.
what is trp operon regulated by?
trp operon is regulated by trp repressor:
In the presence of trp operon is OFF
In the absence of trp the operon is ON
what is the uptake of lactose mediated by?
what is lactose hydrolysed by?
by lactose permease
Hydrolysis of lactose to galactose and glucose by β-galactosidase
Regulation of the Lac operon
- No lactose
- what happens to the lac operon?
- what does the lac repressor bind to?
- Lactose present
- what does it convert to?
- what does the lac repressor bind to?
- what happens when repressor is removed?
No Lactose Present
- The lac operon constitutively expresses the lac repressor protein.
- The lac repressor binds to the lac operator sequence, inhibiting transcription by blocking RNA polymerase from accessing the promoter.
Lactose Present
Allolactose: When lactose is present, it is converted into allolactose, which acts as an inducer.
Repressor Displacement: Allolactose binds to the lac repressor, causing a conformational change that prevents the repressor from binding to the operator.
Transcription Activation: With the repressor removed, RNA polymerase can bind to the promoter and transcribe the genes of the lac operon, allowing the cell to metabolize lactose.
what does E.coli prefer to use over lactose if BOTH availble?
Glucose
Sufficient glucose → no expression of the Lac operon
(Uses glucose for energy)
Not enough glucose → expression of the Lac
operon
levels of Cyclic AMP (secondary Messenger) for E.coli to use when glucose:
- is present
- is absent
Glucose present= [cAMP] ~ 10-7 M (lower)
Glucose absent= [cAMP] ~ 10-4 M (higher).
Higher cAMP levels signal the cell to use alternative carbon sources, such as lactose
what is the role of AMP?
what does this complex bind to? what is the effect.?
AMP acts as a ligand to bind to the transcription activator called cAMP receptor protein (CRP )(also known as CAP)- forming cAMP-CRP complex.
The complex binds to the activator site of the lac operon .
The binding of the cAMP-CRP complex enhances the binding of RNA polymerase to the promoter (Plac), increasing the rate of transcription of the lac operon genes.
what happens if glucose runs out?
if glucose runs out, intracellular [cAMP] level rises – signal for mobilisation of alternative carbon
source.
