Lecture 11 Transcription 1 Flashcards
Transcription : RNA
Message from DNA to protein it encodes
Temporary and unstable due to secondary structure and OH group
Most control over when/which/how many proteins produced is decided at transcription level
Transcription is essential
inhibit it and life ceases e.g. Amanita fungus produces a toxin that inhibits eukaryotic RNA polymerase - if you eat it you die.
Transcription - transcripts
One strand of dsDNA is copied by transcription to form ssRNA template 3’5’ RNA 5’3’
OH group on ribose makes it very reactive and less stable
In RNA there is uracil instead of thymine
Not all RNA molecules are transcripts
Total RNA
4% of RNA is coding mRNA
The other 96% is non coding
In bacteria this is rRNA and tRNA
Other organisms also have snRNA, snoRNA, scRNA,miRNA and siRNA that act as regulators
RNA primary and secondary structure
Primary: base sequence
Secondary: folding, H bonds and hairpin loops
Differences between DNA and RNA
(DNA/RNA)
Type of sugar: deoxyribose/ribose
Presence of 2’OH group: no/yes
Bases: ACTG/ACUG
Phosphodiester bonds: yes/yes
Ds or ss: usually ds/usually ss
Secondary structure: double helix/many
Stability: stable/ easily degraded
Mechanism of transcription
Open ds DNA to expose template strand for RNA synthesis - transcription bubble
RNA polymerase travels along DNA transcribing it
No primer needed
RNA grows in 5’3’ direction
DNA rewinds as bubble moves along strand
Transcription occurs at many points on a chromosome at the same time which is much faster than if whole genome was transcribed at a time
Three stages: initiation, elongation and termination
RNA polymerase performs transcription
DNA double helix enters right and is clamped in RNA polymerase
ribonucleoside triphosphates (building blocks) enter transport tunnel and are added to RNA chain at active site.
A short region of DNA RNA helix occurs in the RNA polymerase a heteroduplex
Then newly synthesised RNA strand splits off and leaves via exit channel
DNA rewinds and exits left
Bacterial RNA polymerase
Made up of several protein subunits
Core RNA polymerase:
2 beta
one bonds nucleosides and other (beta’) binds template - acting as a rudder guiding DNA into active site
2 alpha - bind promotor
and one omega - stabilises
Sigma factor added to form RNA polymerase holoenzyme
Initiation
Begins at the promotor - the upstream region of genes - on 5’ side of region encoding gene.
Draw gene in the direction it’s transcribed.
Bacterial promotor structure - has 2 consensus regions these regions are in many genes. Hexameric (6 base sequences) nearly always at -10 (pribnow box) and -35 bases before transcription region site. Recognised by RNA polymerase to initiate transcription and open bubble.
Sigma factor aids binding and opening then detaches and core enzyme continues transcription.
Elongation
Adding bases, can proof read but is less accurate - mistakes are not as dangerous as RNA is a temporary messenger.
Phosphodiester bonds link new bases on
Termination
To stop in the right place a termination sequence - a non coding regions transcribed but not translated idea for a loop.
Hairpin bends/loops form in RNA which leverages the transcript from the template until transcript RNA and DNA released
Two types of terminator
1) intrinsic terminator
A) intrinsic factor coded for hairpin
B) followed by a run of A’s on template strand bonds to run of U’s
Strong loop followed by weak A-U section allows detachment
2) Rho dependent terminator
Rho helicase enzyme, RNA polymerase stalls at hairpin loop and Rho breaks RNA/DNA base pairs
Controlling levels of transcripion
Some genes are transcribed more often than others some genes are almost ‘switched off’ never transcribed
Summary so far
RNA polymerase creates SS complementary RNA copies of dsDNA strand
Transcription controlled: starts and stops at appropriate points on DNA
3 stages: initiation, elongation, termination (2 types)
Transcripts are messages instructing the cell to make proteins
Levels of transcription can be controlled
