Gene expression Flashcards
The production if RNA from DNA is carried out by
DNA-dependant RNA polymerases (RNAP)
Prokaryotic transcription
Expression involves small single strand genome portions
No primer
And DNA template fully conserved
Direction of coding/non-coding strand
Coding strand is 5’ to 3’
Non-coding strand is 3’ to 5’
Main difference between prokaryotic and eukaryotic transcription
Genes transcribed individually in eukaryotes
Tandem layout in prokaryotes ; several genes transcribed together (these are called operons)
What are promoters?
Base sequences that allow the RNAP to bind and imitate transcription
They are found on the 5’ side, upstream of the starting nucleotide
Mutation of the promoter regions can
Regulate transcription
What direction does RNA synthesis occur in
5’ to 3’
RNAP is processive
explain what this means
RNAP does not dissociate from the DNA template
In prokaryotes , protein synthesis can begin before …..
RNA synthesis is completely synthesised
Transcription termination
Termination sites contain series of 4-10 AT bases and a G+C rich region with a palindromic sequence
this leads to the RNA formed forming a self complementary hairpin (folds in on itself)
this permits RNAP to make a conformational change that allows termination
How many RNAPs involved in prokaryotic/eukaryotic transcription ?
Eukaryotes have multiple RNAPs
prokaryotes have only one RNAP
Eukaryotic RNAPs are larger and have greater subunit complexity
Eukaryotic RNAPs
how do they work
Highly complex
can require the action of hundreds of proteins
promoters can be complex and diverse if selectively expressed (to allow specialised function of cells)
What are enhancers
Sequences upstream or downstream of the promoter region
they are recognised by TFs which can stimulate RNAP II binding
this allows selective gene expression to occur
they can have a much larger effect on transcription rate than just promoters
General transcription factors
Permit basal expression by combining with RNAP and DNA to form a pre-initiation complex
other gene-specific factors needed to further enhance transcription
often targeted to TATA box
different TFs needed once elongation phase begins
What is the TATA box
TATA box is a sequence of DNA found in the core promoter region of genes in archaea and eukaryotes
What is hnRNA
Pre-mRNA
hnRNA processing
Capping → cleavage →polyadenylation → splicing
what is RNA capping?
what is the purpose of it?
the 5’ end is modified to a 7-methylguanosine by 5’-5’ triphosphate bridge
this is called a type O cap
involves several different enzymes
provides resistance to degradation and identification of the translation start site
Polyadenylation
definition and purpose
Most eukaryotic mRNAs have defined 3’ ends terminating in 250 adenosines
added by poly(A)polymerase and not encoded by sequence
Part of transcription termination
complex formed with poly(A)-binding protein - this prevents degradation
What is alternative splicing
Differentials spliced transcripts may lead to proteins with differing catalytic properties
this means 1 gene can encode several different proteins
source of complexity in organism
Mutatations in the splice sites of intron sequences can ____
Lead to disease
