Control of gene expression Flashcards
How is gene expression regulated?
Environmental factors can control gene expression through
(‘induction’ or ‘repression’).
Transcription factors- activators and
repressors/suppressors modulate gene expression
what are the different ways genes can be controlled?
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.
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= it binds to the DNA sequence.
- Addition of ligand= it binds to the transcription 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 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- ligand binds to the transcription activator= conformational changes= activator becomes inactive and is removed from the DNA- prevents transcription (Switches OFF).
In some cases, the activator is can bind to the regulatory DNA sequence in the presence of the ligand, recruiting RNA polymerase (it is switched ON). However when the ligand is removed, the activator becomes inactive and is removed from the DNA- switches OFF gene transcription.
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?
Uptake of lactose
mediated by lactose
permease
Hydrolysis of lactose to
galactose and glucose
by β-galactosidase
Regulation of the Lac operon
- No lactose
- Lactose present
No Lactose Present
Lac Repressor: The lac operon constitutively expresses the lac repressor protein.
Binding to Operator: 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 availble?
Glucose
Sufficient glucose → no expression of the Lac operon
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)
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.
Is lactose present?
→ Yes: Lac repressor falls off and Lac operon is
transcribed
→ No: Lac repressor inhibits Lac operon transcription