Prokaryotic Transcription Flashcards
Is primer needed for RNA synthesis?
No.
What are the major types of RNA and what are their roles?
Messenger RNA - encode information that is translated to proteins.
Transfer RNA - translation machinery to translate RNA to protein
Ribosomal RNA - same as tRNA
Are all types of RNA made by the same RNA polymerase?
Yes, in prokaryotes.
What are the requirements for Transcription?
- DNA Template
- Ribonuclease triphospates (ATP,GTP,UTP,CTP) - building blocks of new RNA strand
- RNA polymerase
How many strands of the dsDNA template are transcribed?
One - the template strand (antisense).
What is the coding (sense) strand in RNA transcription.
Same sequence as the DNA transcript, but with uracil instead of thymine.
What are the stages of transcription?
Initiation
Elongation
Termination
How is transcription in prokaryotes initiated?
A promoter DNA sequence directs RNA polymerase to the correct site to begin transcription.
What are the two common promoter sequences in prokaryotes?
-10 and -35 elements ( located 10 and 35 nucleotides upstream - towards the 5’ end of the start site. (Physical card 18).
How are the promoter sites in bacteria recognised (their code)?
TTGAACA and TATAAT are the average sequences deduced from analysing many promoter sequences. Not the same for every promoter - just the most common.
The efficiency or strength of a promoter sequence dictates what?
The frequency of transcription - e.g once every 2 seconds to once every 10 minutes.
Strong promoters have sequences that resemble what?
The average consensus sequence - they are more likely to be transcribed.
What will happen if the distance between promoter sequences is too far away or too close?
Will impact how often the gene is transcribed. The sigma subunit may not align with the promoter sequences if they are too far apart/close.
Promoters are recognised by _______.
The RNA polymerase sigma subunit.
What is the role of the sigma subunit?
Decreases the general affinity of RNA polymerase to DNA (looser grip so it can move along the sequence quickly looking for the promoter).
What happens to the sigma subunit once transcription is initiated?
It is released from the core enzyme.
What happens during the elongation phase of transcription in prokaryotes?
Physical card 19.
Elongation proceeds within a _________.
Transcription bubble.
RNA has a proofreading activity? When?
Yes. When in the presence of accessory proteins.
Why does transcription terminate?
Signals within the newly synthesised RNA - NOT the DNA template.
What are the two types of stop signals in RNA transcription?
Hairpin loop
Rho protein binding to newly made RNA strand.
Explain how the hairpin loop signal works to terminate RNA transcription.
Formed by GC rich region with 4 or more uracil. Uracil region dissociates from DNA template strand and seeks coding strand - re-anneal with the RNA.
Explain how the Rho binding signal works to terminate RNA transcription.
Uses ATP to go along newly formed RNA strand until it collides with the transcription bubble and makes the RNA strand break free from DNA template.
How is the expression of functionally related genes controlled? Give an example.
As an operon. Bacteria use glucose as a main source of energy, but can use lactose when glucose is scarce. Bacteria will need to produce a set of enzymes that breakdown and transport lactose
What is the lac operon?
3 lactose genes have the same promoter and so are transcribed together as one mRNA.
What happens to the lac genes when lactose is absent?
Repressor binds to the operator site and blocks the expression of lac genes as there is no need to make the lac genes.
What happens to the lac genes when lactose is not present?
A lactose derivative - allolactose - binds to a repressor and it detaches from the operator site - allowing lac genes to be expressed. The shape of the promoter is also changed so it can’t bind to the sequence.
How can lac expression be regulated by glucose.
Physical card 20.
What is the trp operon?
Bacteria can make tryptophan using enzymes encoded for by 5 genes in the trp codon. All under same promoter.
What occurs at the trp operon when tryptophan is present?
Bacteria take up tryptophan and not make any so trp genes are switched off - tryptophan binds to the repressor and the two bind to the operator site to prevent transcription.
What occurs at the trp operon when tryptophan is absent?
No tryptophan, repressor cant bind to the operator and so transcription of trp genes allowed.
