Transcription Flashcards
What is gene expression
The machinery to use genetic information.
If, when, where and how much a gene should be turned into a protein
What was the problem scientists had with genes and the production of proteins
DNA is in the nucleus but protein synthesis is in ribosomes which have RNA.
How does info travel, what does RNA do.
The genetic code hadn’t been determined.
What did Brenner, Jacob and Meselson discover about RNA
It travelled to ribosomes to produce proteins, they aren’t structural components. They’re messengers to instruct existing ribosomes.
How did Brenner, Jacob and Meselson work out that RNA was a messenger and not a structural component
They used T4/E.Coli, labelled RNA resulting from viral infection and bacterial ribosomes.
- Grew uninflected e.coli with N15 and c13 isotopes (make rna heavier)
- Changed medium to N14 and C12, added P32 (to track where RNA went)
- Infected dna with T4 and removed heavy isotopes to produce new radioactive viral RNA
- Sedimented purified ribosomes in conc gradient
- Found 2 new populations of ribosomes (heavy and light ones being assembled after phage entered) new goes to pre-existing ribosomes
Where does prokaryotic transcription start
At the TSS (transcription start site) a specific nucleotide position
What 3 regions does the prokaryotic transcription RNA sequence have
Promoter region - for initiation
Elongation region - between initiation and termination
Terminator region - cause RNA polymerase to detach
How does initiation work
- Promoter binds to RNA
- Opening a small bubble of unwound DNA
- Starts copying
- Polymerase moves with bubble to add nucleotides
- Can be transcribed by several RNA polymerase
How does termination work
- RNA polymerase arrives at terminator
- Detaches from DNA
- MRNA ready for translation
Why and how would you turn genes off in a prokaryote
Not all proteins are necessary all the time.
Positive and negative regulation.
Operon.
What is operon used as
A regulatory unit, regulated through an off-on switch. Allowing protein synthesis to be controlled coordinately in response to need of the cell. Directs expression of polycystronic mRNA
What happens when the lac operon is turned off
No lactose is present. Turns off lacI gene, prevents RNA polymerase transcribing the operon
What happens when lac operon is turned on
Lactose is present. LacI binds to lactose, repressing it - changing lacI structure and prevents it from binding DNA - lacI promoter is unblocked - RNA polymerase transcribes polycystronic rna
What happens, concerning lac operons, when glucose is scarce
CRP is activated and binds to lac operon, increasing transcription.
Only works if lactose is available and no better energy source is present.
Key difference between prokaryotic and eukaryotic transcription
- physical separation between DNA and ribosomes
- termination occurs by polyadenylation signal in eu
- ribosomes binds to translate mRNA
- different RNA polymerase transcribes different genes (RNA pol3 for mRNA pol1+2 for tRNA, rRNA)
Give an example of multicellularity
In the lens of your eye - contains crystallin, special property of being transparent. All cells contain genes for crystallin but it’s restricted to production in the eye lens.
