RNA processing (exam 2) Flashcards
5’ cap
7-methyl-guanosine cap
Provides handle for ribosomes to attach for translation
Protects mRNA from endonucleases
How does 7-methyl-guanosine link to 5’ end?
5/,5’ triphosphate link
Polyadenylation
poly A tail on 3’ end
Molecular condensation with GTP with the 5’NTP
Capping enzyme
Guanylyl transferase
Poly A tail cleavage signal
AAUAAA
Only RNA made by ____ is capped because guanylyl transferase is associated with the CTD of this polymerase
RNA Pol II
What determines the 3’ end of an mRNA
The processing event of the addition of the poly A tail
Poly A Polymerase
template-independent
requires 3’ OH after cleavage
Why is the poly A tail made of adenosine as opposed to a different nucleotide?
Because most endonucleases don’t cleave between adenosine units
Histone mRNA
3’ end has a self-complementary sequence
Stem Loop Binding Protein
Prevalence of introns
Bacteria: not present
Yeast: present and small, not prevalent
Yeast mitochondria: many self splicing introns
Mammals: Common and large compared to exons
What components of mammalian cells do not contain introns
mitochondria and histone mRNA
Group I and II introns
self-splicing
require no additional proteins or ATP
Spliceosomal introns
spliced by spliceosomes
most common
frequent in protein-coding regions
tRNA modification
spliced by protein-based enzymes
primary transcript spliced by endonuclease
exons joined by ATP-dependent ligase
Nuclear pre-mrna intron splicing mechanism
Spliceosome, internal 2’ OH provides free OH group
Group I self-splicing intron splicing mechanism
guanine nucleotide provides 3’ OH
more common than group II
found in organelles and nucleus in lower eukaryotes
Group II self-splicing intron splicing mechanism
internal 2’ OH provides free OH
Found in mitochondria, chloroplast, tRNA, and pre-mRNA
What intron sequences are conserved during splicing?
Small regions immediately adjacent to splice sites
Spliceosomes are made of
snRNPs
snRNPs are made of
snRNAs
Step 1 of splicing mechanism
2’ hydroxyl of branch-point A attacks bond at upstream end of intron, forming a loop, and leaving a 3’ hydroxyl upstream
Conformational change that occurs during step 1 of splicing mechanism
transesterification
Is ATP required for step 1 of splicing mechanism
No
Step 2 of splicing mechanism
The free 3’ OH of the upstream exon can now attack bond at downstream end of intron
What is the structure that is formed during the splicing mechanism
lariat
Step 3 of splicing mechanism
The intron is excised as a lariat and the upstream and downstream exons are covalently spliced together by a new phosphodiester bond
rRNA processing by Pol I
a series of cuts by endonucleases occur in the nucleolus to convert precursor into 3 products that become part of each ribosome
T/F There is no splicing involved in rRNA processing
true
rRNA processing requires
snoRNAs: small nucleolar RNAs
3’ end of tRNA are modified to be
CCA-3’
What is the purpose of the modified end of tRNA
to provide a binding site for amino acids
tRNA modifications
- CCA-3’ end
- Addition of methyl and isopentenyl groups. Methylation of 2’ OH in ribose
- Conversion of specific uracil bases to uracil-adjacent structures
Transport of mature RNA out of the nucleus is facilitated by
nuclear pores
rRNA is transported out of the nucleus as
pre-ribosomal particles
