transcription Flashcards
Compare and contrast transcription in prokaryotes and eukaryotes
Prokaryotic
- Cytoplasm
- Coupled transcription and translation (simultaneous)
- No definite phase of occurrence
- Single RNA polymerase
- No initiation factors
- Polycistronic
Eukaryotic
- Nucleus
- No coupling
- Occurs in G1 and G2
- 3 RNA polymerases
- Multiple General Transcription Factors (GTFs)
- Monocistronic - 1 ORF
what parts are included in the structure of a eukaryotic gene
- control elements
- promoters
-enhancers - introns
- exons
- silencers/repressor proteins
define control elements
Non-coding DNA segments regulating transcription by binding transcription factor
Define promoters
Sequences of DNA bound by first components of the pre-initiation complex
Define enhancers
Distal control elements
Short nucleotide sequences influencing rate of transcription
May be bound by silencers or repressor proteins
Define transcription
Process by which information in a DNA strand is copied into a new molecule of mRNA
Takes place within nucleus and mature mRNA is exported to cytoplasm for translation
Why is RNA processing essential before exporting mature mRNA from the nucleus?
To ensure that mRNA doesn’t degrade in the cytoplasm by enzymes
Which type of DNA in eukaryotes is transcribed like prokaryotic transcription?
Non-nuclear or extrachromosomal DNA e.g. mitochondrial DNA
Name 4 RNA polymerases
RNA polymerase I
RNA polymerase II
RNA polymerase III
Mitochondrial RNA polymerase
What are the products of each RNA polymerase?
RNA polymerase I - pre-rRNA
RNA polymerase II - pre-mRNA
RNA polymerase III - pre-tRNA
Mitochondrial RNA polymerase - all mitochondrial RNA
Name the 4 stages of transcription
Initiation
Elongation
Processing
Termination
Name the 4 stages of transcription
Initiation
Elongation
Processing
Termination
Explain the events of the initiation stage of transcription
- DNA unwound
- Chromatin remodelling exposes promoter
- Pre-initiation complex binds to recognition within promoter
- Control elements facilitate binding of complex components
- Transcription occurs within a transcription bubble opened by RNA polymerase
Name 3 additional proteins involved in the initiation stage of transcription
Mediator complex
Transcriptional regulatory proteins
Nucleosome modifying enzymes
What is the PIC made up of?
General Transcription Factors and RNA Polymerase II
explain how the pre-initiation complex is formed
- » TATA-binding protein (TBP) binds first to the TATAA box (a recognition site within the promoter with the nucleotide sequence TATAA..)
- » TBP (a subunit of TFIID) recruits General Transcription Factors made up of TAFs (TBP- Associated Factors)
○ TFIID recruits TFIIA and TFIIB
○ TFIIB binds to its B Recognition Element (BRE)
○ RNA Pol II – TFIIF complex recruited
TFIIE and TFIIH recruited to form the pre- initiation complex
Name 3 processes that pre-mRNA can undergo during elongation
Capping
Splicing
Poly-adenylation
Explain what happens during the elongation stage of transcription
- RNA transcript carries the same information as non-template (coding) DNA strand but contains uracil instead of thymine
- RNA polymerase opens DNA in transcription bubble from which the template strand is read
- RNA produced in 5’ to 3’ direction complementary to 3’ to 5’ template strand of DNA (antisense)
- Non-template (coding) strand not involved but has the same sequence as mRNA except thymine replaced by uracil (sense strand)
- Only one PIC required, it can keep recruiting a new RNA polymerase II to elongate another mRNA molecule resulting in multiple transcripts from same gene
How can pre-mRNA molecules be modified by the addition of chemical groups?
Add a 5’ cap or 3’ tail (protects transcript and helps it export from nucleus)
Introns removed and remaining exons spliced together
how does capping work
As RNA polymerase is elongating the transcript, guanyltransferase attaches a methylated GTP cap to the 5’ end of the mRNA
What is the function of a methylated GTP cap on the 5’ end of mRNA?
Regulation of nuclear export
Prevention of degradation by exonucleases (not recognised 5’ to 5’)
Promotion of translation
Promotion of 5’ proximal intron excision
Define splicing
Removing introns from the transcript to produce mature mRNA
Define proteome
Portion of DNA encoding polypeptides
What is the spliceosome?
a complex of RNA’s and proteins that remove introns
What are the specific sequences found towards the end of a gene (AAUUAA) called?
Polyadenylation signals
How is a poly-A tail added to RNA?
- polyadenylation signals bind to cleavage factors which manipulate 3’ end of RNA into correct configuration for cleavage
- 3’ end is cut and cleavage factors dissociate
- Poly-A polymerase adds 100-250 adenine nucleotides to the cut ends to form a poly-A tail
What is the function of a poly-A tail?
Stabilises transcript
Aids in transcription termination
Aids in exportation from nucleus
Describe the termination mechanism for RNA polymerase I
TTF1 (Transcription Termination Factor) causes transcript to bend
PTRF-1 (Polymerase I Transcript Release Factor) and thymine rich sequence causes dissociation
Describe the termination mechanism for RNA polymerase II
Poly-A tail doubles as TTS (Transcription Termination Signal) and 3’ end modification to stabilise pre-mRNA
Describe the termination mechanism for RNA Polymerase III
Thymine resides repeat in ‘terminator sequence’
Similar to RNA polymerase I
