Transcription & RNA Processing Flashcards
Define gene expression.
The transfer of information from DNA (where it’s stored) to RNA.
How is transcription different in eukaryotes and prokaryotes?
In prokaryotes, RNA is transcribed directly from the genome onto the ribosomes. In eukaryotes the process of transcribing RNA is much more complex and originates in the nucleus., starting with the primary transcript, then the mRNA.
What are the five stages of the transcription cycle?
- Template binding
- Chain initiation
- Promoter clearance
- Chain elongation
- Chain termination and RNAP release
How does the presence of lactose affect bacterial cells’ gene expression?
The presence of lactose in the medium induces the synthesis of the enzyme B-galactosidase. This is an example of how bacterial cells selectively express genes to use the available resources effectively.
Define bacterial operons.
An operon is a functional complex of genes containing the information for enzymes of a metabolic pathway. Operons allow a set of genes with similar functions to all be turned on or off at the same time–kind of like a master light switch.
What do the structural genes of an operon code for?
They code for enzymes and are translated from a single mRNA that is usually polycistronic (encodes for more than one protein).
Where does the RNA polymerase bind on an operon?
The promoter.
What is the operator of an operon?
The site next to the promoter where the regulatory protein can bind.
What is the repressor of a bacterial operon?
This repressor binds to a specific DNA sequence (the operator) to determine whether or not a particular gene is transcribed. RNA polymerase is unable to bind to a promoter if the repressor is bound.
In the tryptophan biosynthetic operon (which is an example of a repressible operon), when is the repressor protein active and able to bind to the operator?
The repressor protein is only active (and able to bind to the operator) when there is excess tryptophan co-repressor. When Trp is not in excess, the repressor is inactivated and synthesis is derepressed.
In the lactose catabolic operon (which is an example of an inducible operon) what activates the operon?
The inducible operon is turned on in the presence of lactose (inducer) which needs to be digested. Lactose binds to the repressor, changing its conformation and making it unable to bind to the operator. The repressor can bind to the operator and prevent transcription in the absence of lactose.
What does catabolite repression allow the cell to do?
It allows the cell to use preferred energy sources (usually glucose).
How does glucose affect cAMP levels?
When glucose falls, cAMP rises.
How does cAMP affect RNA binding to the promoter?
cAMP acts by binding to a cAMP receptor protein (CRP)
Binding of CRP-cAMP to the lac control region changes the conformation of DNA, allowing DNA polymerase to transcribe the lac operon when the repressor is no longer an operator. Without cAMP, RNA binding to the promoter is very poor/weak.
What two conditions must be met to use the lactose operon?
- The presence of lactose with the ability to bind to the lactose oppressor to remove its ability to bind to the operator.
- An adequate promoter for RNA polymerase, which only occurs when cAMP levels are high and able to bind to CRP, which binds to a site upstream from the promoter.
What is the function of RNA polymerase (RNAP)?
RNAP is responsible for incorporating ribonucleoside triphosphates into RNA strands whose sequence is complementary to the DNA template.
What direction does RNAP move?
RNAP moves in a 3’ –> 5’ direction.
What direction does the nascent (newly synthesized) RNA grow in?
5’ –> 3’.
How are the template strand and the coding strand related?
They are complementary and antiparallel. The template strand has the opposite orientation to the coding strand and the nascent RNA chain.
What is the function of the five subunits of bacterial RNAP?
Bacterial RNA polymerase has five subunits that can bind randomly to transcribe RNA. Association with the sigma factor forms a complete haloenzyme with these five core subunits to allow promoter binding and strand separation → transcription initiation.
What are the two basic elements of a bacterial promoter?
- The 35 bp upstream element.
- The 10 bp (Pribnow Box) upstream element.
How is bacterial transcription terminated when it is rho-dependent?
Rho purses the polymerase, the hairpin forms, the polymerase pauses, the rho catches up, and the rho causes termination.
What drives RNA formation of hairpin loops?
Bacterial transcription termination sequences.
Why is eukaryotic transcription more complex than prokaryotic transcription?
It is tissue/temporally specific. This means that depending on the type of cell of the organism (for example, neuron vs liver cell), different genes must be transcribed. Time and location tell the cells which genes to activate and which to repress.
