(Dr. Heinemann) (Unit C) Topic 20 Flashcards
How must primary RNA transcript be processed in eukaryotes?
- Capping of the 5’ end
- Polyadenylation
- Splicing
What is the 5’ end capped with?
7-methylguanosine
What is added in polyadenylation?
A long A-tract to the 3’ end
What is the process for polyadenylation?
- RNA is cleaved ~30 bases following an AAUAAA sequence
- A string of A residues is then added (around 300 bases long)
Why is polyadenylation important?
- Reduces the effects of 3’ exonucleases (stability)
- Role in nuclear export
- Role in translation
What are noncoding sequences in eukaryotes known as?
Introns
What are coding sequences in eukaryotes known as?
Exons
Differential splicing
mRNA can be put together in different ways genearting functionally related but distinct gene products
What sites are important in splicing (loss of these sites will result in defects in splicing)?
- 5’ junction
- Branch point
- 3’ junction
Spliceosome (snRNPs)
Remove introns as the result of catalytic activity
What are snRNPs?
Small nuclear ribonucleoprotein particles
* Complexes of RNA (snRNA) and protein
Function of RNA component in snRNPs
Recognition function
* Acting through base pairing with sequences on the precursor mRNA
True or False:
snRNPs are critical in arranging the ends into position
True
What is a spliceosome?
The assembly of snRNPs that catalyze splicing
How does splicing occur?
2 transesterification reactions
Explain the mechanism of the first transesterification reaction
- 2’ OH group of branch point adenosine residue (intron) attacks phosphate at 5’ end of intron
- Frees the 3’ end of the exon
- Forms a lariat-shaped intemediate (loop)
Explain the mechanism of the second transesterification reaction
- Free 3’ OH on the first exon attacks the 5’ phosphate at the second exon
- Forms a phosphodiester bond
- The exon is kicked out in this reaction and degraded
What is beta-thalassemia and how does it occur?
Disease characterized by anemia
* Beta subunit of hemoglobin not properly produced, results in excess alpha subunit
* Caused by splicing mutations
When does RNA processing occur?
While transcription is ongoing
Does exporting happens at the same time as processing?
No, export happens AFTER processing
What is required for mRNA export from the nucleus?
- 5’ cap
- Poly-A binding protein
- Specific protein carriers (transport receptor)
How is mRNA transport out of the nucleus?
Through nuclear pores
* Nuclear pore components recognize proteins associated with the mRNA
What happens to mRNAs when they are exported?
Circularization
* Indicates a mRNA is processed properly
How does mRNA undergo circularization?
- elF4F “cap recognition” protein
- poly(A) binding protein (PABP)
Interacts with the 5’ cap and poly(A) tail. Result reduces mRNA degradation
List in order the steps of mRNA decay
- Deadenylase: Takes off 3’ PolyA tail
- Decapping enzyme: Removes 5’ cap
- 5’ and 3’ exonuclease: Degrade mRNA from both ends
What is special about tRNA transcripts?
Covalently modified
* Highly modified, and often do not function UNLESS modified
Are rRNA modified?
Yes they are also covalently modified
What are some forms of covalent modifications?
- Methylation
- Hydroxylation
- Deamination
What is sequence specific RNA degradation known as?
RNA interference
What does RNA interference require?
- Binding of second complementary RNA strand
- With help of several proteins, the mRNA is cleaved and degraded
What is the second complementary RNA strand in RNA interference known as?
Small interfering RNA or microRNA
