RNA Processing During RNA Transcription Flashcards
How are RNA transcripts that are destined to become mRNA molecules processed?
(3 ways)
RNA capping (5’ end)
Polyadenylation (throughout synthesis)
RNA splicing (3’ end)
When does RNA processing happen?
During transcription
How does RNA capping work?
3
The modification of the 5’ end of the mRNA transcript
RNA is capped by addition of an atypical nucleotide
Occurs after RNA pol has produced 25 nts of RNA (long before it has completed transcribing the whole gene)
What is RNA capped with?
A guanine nucleotide with a methyl group attached
At what point does RNA capping occur?
At the 25th nucleotide
How does polyadenylation work?
4
Newly transcribes mRNA is provided with a specialised structure at the 3’ end (tail)
First the tail is trimmed by an enzyme which cuts mRNA at a particular sequence of nucleotides
A second enzyme then adds a series of repeated adenine nts onto the cut end (Poly-A tail)
Poly A tail is a few 100 nts long
What does the first enzyme do during polyadenylation?
It trims the tail (3’ end) of RNA at a particular sequence of nucleotides
What does the second enzyme do during polyadenylation?
It adds a series of repeated adenine nucleotides onto the cut end
Why is capping and polyadenylation done?
4
It increases the stability of the mRNA molecule
It aids the export of mRNA from the nucleus to cytoplasm
It identifies mRNA molecule as mRNA
It indicates to the protein synthesis machinery that both ends of mRNA are present and the message is complete
What percentage of RNA that is transcribed reaches the cytoplasm?
Only 5%
Why does only 5% of RNA transcribed reached the cytoplasm?
Eukaryotic genes have their protein coding sequences (exons) interrupted by non-coding sequences (introns)
What is splicing?
Removing non protein coding sequences from the transcribed mRNA
What happens after splicing?
The exon sequences are ‘stitched’ together
List the order of events in RNA processing.
3
Capping occurs on 5’ end first
Then splicing occurs
Then as mRNA is released and the 3’ end is exposed a Poly-A tail is added
How does the cell determine which parts of the RNA transcript to remove during splicing?
Each intron contains a few short nucleotide sequences that act as cues for its own removal
Where are the nucleotide sequences that act as cues for an intron’s removal found?
They are found at or near each end of the intron
Name the two splice sites on an intron.
5’ splice donor site
3’ splice acceptor site
What are the three types of short sequences involved in splices?
Splice donor (5’)
Splice acceptor (3’)
Branch site
What is a branch site?
The AT rich region with at least one A that’s about 30 nucleotides upstream from the splice acceptor
What molecule is mainly in charge of splicing?
Spliceosome
What is the spliceosome?
2
Small nuclear RNAs (snRNA)
Combined with proteins (snRNP)
How does the spliceosome work?
2
It recognises the beginning and end of introns
It brings the two ends of the intron together so that splicing can take place
Explain how RNA splicing works, explaining how the adenine branch site works
(6)
A particular adenine (A) branch site in intron sequence attacks the 5’ splice donor site
The sugar-phosphate backbone of RNA is cut at the 5’ site
The cut 5’ end becomes covalently linked to adenine to form a branched structure
Free 3’-OH end of exon reacts with start of next exon sequence
2 exons join together into a continuous coding sequence
Intron is then released and eventually degrades
What first happens in splicing?
3
A particular adenine (A) branch site in intron sequence attacks the 5’ splice donor site
The sugar-phosphate backbone of RNA is cut at the 5’ site
The cut 5’ end becomes covalently linked to adenine to form a branched structure
What happens after the 5’ splice donor site is cut off from the sequence during splicing?
(2)
Free 3’-OH end of exon reacts with start of next exon sequence
2 exons join together into a continuous coding sequence
What is alternative splicing?
The different ways exons can be reconnected to produce different mRNAs which can be translated into different proteins
How does mRNA leave the nucleus?
Leaves through the nuclear pore complex
What does mRNA need to be bound to in order to be transported?
(3)
Poly-A-binding protein
Cap-binding complex
Nuclear transport receptor
Where does Poly-A-binding protein bind to mRNA?
Binds to the poly-A tail
Where does Cap-binding protein bind to on mRNA?
Binds to the 5’ cap
How does nuclear transport receptors work?
2
It only associates with mature mRNAs
It actively guides them through nuclear pores
What eventually happens to all mRNA molecules even if they are translated many times?
All mRNA degrades into nucleotides by cellular RNases
What in part controls the lifetime of mRNA?
2
The nucleotide sequences contained in the mRNA itself
Especially those in the 3’ untranslated region (area between 3’ end of coding sequence and poly-A tail)
Where is the 3’ untranslated region found?
Between 3’ end of coding sequence and poly-A tail
