TRANSCRIPTION DNA-mRNA Flashcards
What happens in transcription and translation
Trc - make messenger RNA copy of the information in a gene that codes for a protein
Trl - conversion of RNA message into protein at ribosomes
What extra steps are present in gene expression in Eukaryotes that arent present in Prokaryotes?
RNA processing (splicing step) and RNA transport (made in nucleus so must travel to ribosome)
Differences in transcription and translation in Pro vs Euk
-Both happen close together in Pro as nucleoid is in cytoplasm
-Transcription doesnt have to be complete for translation to start in prokaryote - cant happen in eukaryotes due to compartmentalisation
What is a gene composed of?
Specific base sequences organised in a way that allows DNA to be transcribed
-Contains a promotor, terminator and other regulatory sequences
What is the regulatory sequence?
Site for the binding of regulatory proteins
-Role of regulatory proteins is to influence the rate of transcription
What attracts RNA polymerase?
-10 (pribnow box) and -35 bases before transcription start site there is a common or consensus sequence
These are known as the core promotor elements
What makes a promoter a good promoter?
The more closely the promoter resembles the consensus sequence the stronger the promotor effect and the better the promoter is.
What is the enzyme that catalyses transcription?
RNA polymerase
What are the 5 subunits of the core enzyme of bacterial RNA polymerase and what do they do?
-alpha 2- 2 alpha subunits assemble the enzyme and bind regulatory factors (influence level of trc)
-ß - has the polymerase activity (catalyses the synthesis of RNA) which includes chain initiation and elongation
-ß’ - binds to DNA (nonspecifically)
-w - restores denatured RNA polymerase to its functional form in vitro, has protective function to the ß’ subunit and promotes enzyme assembly
What is the additional factor involved in RNA polymerase holoenzyme and what is its role?
-o
Role:
–Reduce affinity of enzyme for non-specific DNA
–Increase affinity for promoter sites - directs core enzyme to where it should go
When does the sigma (o) factor dissociate from the core enzyme?
once a promoter has been bound - then goes to find another core enzyme
What does RNA polymerase 1 synthesise?
A pre-rRna 45S which matures into 28S, 18S and 5.8S rRNAs which will form the major RNA sections of the ribosome
What RNA polymerases do eukaryotes have?
RNA polymerase 1, 2 and 3
-diff types characterized by type of RNA they synthesise
What does RNA polymerase II synthesis?
Synthesises precursors of mRNAs - most studied type - a range of transcription factors are required for its binding promoters
What does RNA polymerase III synthesise?
Synthesizes tRNAs, 5S rRNA and other small RNAs found in the nucleus and cytosol
How is the initiation of transcription achieved in eukaryotes?
-Transcriptiom initiation is more complex in euk than in pro
-RNA polymerase does not directly recognise the core promoter sequences
-Transcription factors mediate the binding of RNA polymerase forming a transcription initiation complex
What happens in the next stage (stage 2) of transcription - Elongation?
-The polymerase unwinds the template DNA ahead of it and rewinds the DNA behind it, maintaining an unwound region of about 17 bps in region of transcription
-The template strand of DNA is copied to RNA using complimentary base pairing and insertion of appropriate ribonucleotides
-Multiple RNA polymerase can transcribe a single DNA molecule simultaneously
-RNA polymerase can proofread
What are the 2 processes that can take place in bacteria during termination?
-Rho-dependent
-Rho-independent
both lead to release of mRNA transcript
How does termination take place in eukaryotes?
Less understood but occurs in conjunction with polyadenylation of the transcripts
What happens in Rho-dependent termination?
-Rho protein binds the single stranded RNA as it exits from polymerase enzyme complex, it can move along the mRNA strand and uses a helicase activity to release mRNA from the enzyme complex.
What happens in Rho-independent termination?
-Terminated due to specific sequence in terminator DNA
-Terminator DNA contains invert repeat which causes complimentary pairing within transcript RNA forming hairpin structure
-PolyT in template strands leads to polyU /polyA structure - combined with hairpin leads to instability and termination
What is the template strand? What is it also referred to as?
-Serves as DNA template for transcription
-Addition of nucleotides occurs in the 5’ to 3’ direction by RNA polymerase
-Directionality of template strand should be 3’ to 5’
-Also referred to as antisense strand, non-coding strand or neg strand, transcribed strand
What is the coding strand? What direction does it run?
-Runs in the 5’ to 3’ direction
-Contains the same nucleotide sequence in the mRNA strand.
-Also referred to as sense strand, non-template strand or pos strand
