Control of gene expression Flashcards
Why is control of gene expression important?
- life cycle related changes
- responses to changes in environmental conditions
How does bacterial gene expression differ from eukaryotic gene expression?
In bacterial gene expression…
- there is no nuclear membrane
- only one cytoplasmic compartment
- no histones
- coupled transcription and translation
How are transcription and translation ‘coupled’
they occur simultaneously in the cytoplasm
Describe how transcription is initiated in bacteria
- sigma factor binds to promoter allowing RNA polymerase to bind
- RNA polymerase binds to promoter to initiate transcription
Describe how transcription is initiated in eukaryotes
- general transcription factors bind to promoter
- many promoters have TATA box, which is recognised by one of general transcription factors, allowing others transcription factors to join
- RNA polymerase then binds
What is an operon (in bacteria)
genes of related functions are clustered into ‘operons’ , it has one promoter and all genes are transcribed together within the operon
What is the promoter?
Where RNA polymerase binds to initiate transcription
What is the operator?
A DNA sequence that overlaps with the promoter and can bind to the repressor protein
What is the role of LacI
produces repressor protein
Describe how transcription occurs in the lac operon in the presence of lactose
- lactose is transported into bacterial cell by lactose permease
- inside the cell, lactose is converted into allolactose which binds to the repressor protein, preventing it from binding to the operator
- Inactive repressor allows RNA polymerase to bind to promoter and transcribe lac Z,Y,A, resulting in the production of the mRNA necessary for lactose metabolism
Describe how transcription is prevented in the absence of lactose in the lac operon
- lac repressor protein is produced by lacI and binds to the operator (which is overlapped by promoter)
- this physically blocks RNA polymerase from binding to the promoter region and so lac Y/Z/A genes cannot be transcribed and mRNA is not produced for lactose metabolism
Describe the role of glucose, when lactose is absent in the lac operon
- when glucose is present, levels of cAMP are low
- this means CAP does not activate the promoter
- CAP enhances the binding of RNA polymerase to the promoter, so the likelihood of transcription is furtherly reduced
Describe the effect on rate of transcription when glucose is in higher levels during transcription of the lac operon
- higher levels of glucose inhibit cAMP, which binds to CAP, which increases binding affinity for repressor protein
- so CAP remains inactive in higher levels of glucose, and transcription occurs at a lower rate
How do low levels/no glucose allow positive gene regulation to occur in the lac operon?
- lack of glucose enables cAMP to function effectively and bind to CAP, activating it
- CAP increases the promoters affinity for RNA polymerase, accelerating transcription
In what case would E coli use lactose as an energy source?
- when lactose is present and glucose is in short supply
Are eukaryotes controlled by combinations or a single of transcription regulator/s
combinations
How is cortisol transcription regulated by multiple genes?
- an additional transcription regulator is added
- this activates the cortisol receptor which binds to the same regulatory DNA sequence in each gene
What is the function of master regulators?
- bind to specific DNA sequences to initiate or inhibit transcription of target genes
- sensors for environmental response to mediate appropriate transcriptional response
Explain how DNA is modified using methylation in vertebrates
- occurs in the sequence CG
- Cytosine undergoes methylation to form 5-methlycytosine
- methylated DNA represses gene expression
How can methylation cause epigenetics
epigenetics- ‘the study of changes in organisms caused by modification expression rather than alteration of the genetic code itself’
- methylation patterns are inherited through cell division
How are core histones covalently modified on their N-terminal ‘tails’?
via
- acetylation
- phosphorylation
- methylation
