Explain the 4 steps of 5’ capping of mRNA.
- Pre-mRNA capped during transcription when it gets to 25 nucleotides long
- 5’- triphosphate removed by phosphohydrolase
- GTP added in 5’-5’ leakage by guanylyl transferase
- Several methyl groups added to complete it
3 big purposes of 5’ cap:
- prevents exonuclease activity
- landmark for positioning on ribosome
- serves as splicing marker
Explain the 4 steps of mRNA 3’ cleavage and polyadenylation
- PAP binds to AAUAAA and GU sequence motifs (both are poly A signals)
- This complex cleaves 10-35 nucleotides from 3’ end
- After cleavage, PAP slowly adds 12 A residues
- PAPBII joins complex, and then PAP quickly adds 200 A residues
The spliceosome contains what?
The snRNAs U1, U2, U4, U5, U6, and about 100 splicing proteins.
Explain the 3 steps of of splice mechanism by splicesome
- The adenine residue attacks phosphodiester bond at 5’ splice site between intron and exon.
- 3’ side of the exon attacks the 3’ splice site
- Introns are cut out of mRNA in shape of lariat, and exons are fused together
Where are group I self splicing introns located?
Present in mitochondria, chloroplasts, and some bacteriophages
where are group II self splicing introns located?
mitochondria of fungi, and in chloroplasts and cyanobacteria
Which group of self splicing introns shows the evolutionary link to the eukaryotic spliceosomes?
Group II introns, since their process is the most similar.
What is beta thalassemia?
characterized by abnormal, easily destroyed rbcs. caused by mutation in beta globin gene that alters splice sites
explain A to I editing and what does it
A is deaminated to I by the enzyme ADAR. Very prevalent in humans.
explain C to U editing and what does it. Example?
C is deaminated to U by cytidine deaminase. An example is ApoB. This is created by editing an mRNA transcript to making a stop codon instead of glutamate. Becomes a cholesterol transporter.
What do trypanosomes cause? What happens with them, and where does it happen?
sleeping sickness and chagas disease. Extensive mRNA editing. Us are added by uridydyl transferase and guide RNAs. All RNA editing is co-transcriptional, i.e. it happens right when things are transcribed.
Explain rRNA processing
Precursor rRNA segment comes from the nucleolus. Modified by snoRNAs, which cleave the 45S segment, and carry out certain chemical modifications such as direct methylation and direct pseudouridylation.
What are 5’ UTR sequences?
They are located before the start codon and are NOT translated into protein.
Explain tRNA processing and where it happens. 4 steps.
Happens in the nucleus.
- Cleavage- RNase D cleaves 3’ end and RNase P cleaves the 5’ end.
- splicing- by PROTEINS, not snRNAs
- Us are switched out for CCAs on 3’ end, which is important for amino acyl tRNAs
- chemical modif- a small percentage of bases are modified via snoRNAs
snRNAs, group I and II introns, and the RNA portion of RNase P are examples of?
ribozymes.
