BIO 205 RNA PROCESSING Flashcards
simultaneous transcriptions and translation… where?
only in prokaryotes
no significant mRNA processing in __
prokaryotes
Processing of eukaryotic messenger RNAs
- 5’ processing (5’ cap)
- 3’ processing (poly-A tail)
- intron removal (splicing of exons)
UTRs
untranslated regions at the 5’ and 3’ ends
5’ UTR function
ribosomal binding site
3’ UTR function
signal for polyadenylation (Poly-A)
5’ modification (5’ cap)
added WHEN
shortly after transcription when mRNA is ~20-30 nts
functions of 5’ Cap in Eukaryotic mRNA
- protects mRNA from 5’ exonucleases (Stability)
- helps transport mRNA to cytoplasm
- necessary for ribosome binding in translation
3’ cap (poly-A tail addition) process
- Polymerase II continues to synthesize mRNA into 3’UTR region
- polyA polymerase recognizes the sequence and add string of As
poly-A tail addition stands for
polyadenylation
Functions of 3’ poly-A tail addition in eukaryotic mRNA
- protects mRNA from 3’ exonucleases (Stability)
- helps transport mRNA to cytoplasm
- will get chewed up, but UNIMPORTANT
exonucleases
enzyme which removes successive nucleotides from the end of a polynucleotide molecule
exons and introns aka
exons: coding regions
introns: noncoding
5’ cap formation
7-methylguanosine ------- 5'-to-5' triphosphate bridge ------- 5' end of primary transcript
R-Looping Experiments
in vitro
- Denature double strand DNA
- big loop - anneal (recombine) single-strand DNA + RNA from same gene
- double-stranded DNA is now INTRON in between 2 r-loops - Observe DNA-RNA hybrids under microscope
why must splicing occur precisely
to prevent deleterious translational defects
Splicing disorder (non-precise)
thalassemia
splicing needed
to make transcript
sequences required for intron removal
EXON 1: —AG
BRANCH SITE: “A”
EXON 2: G—
EXON 1 must:
end in —-AG
EXON 2 must:
start w G—
BRANCH SITE must:
have an “A”
WHY DOES SPLICING OCCUR
BECAUSE INTRONS ARE NONCODING (IRRELEVANT) AND NOT-NEEDED
branch point sequence
connects introns together - forming lariat
lariat
only intron sequencing
linkage of splicing
5’ –> 2’
spliceosome complex
- consists of small nuclear RNAs (snRNAs) and proteins (U’s)
* form small nuclear ribonucleoprotein particles —–(snRNPs or “snurps”)
snRNPs
small nuclear ribonucleoprotein particles
what does spliceosome complex DO
• mediates RNA splicing
• specific base-pairing
between snRNA (of SNURP) and mRNA directs splicing process
Gene Splicing Mechanism
• intron sequence has 2 exons on end
• 2’ OH from BRANCH SITE loops to bind to exon 1
• exon 1 removes itself, moving to bind to exon 2
• excised intron sequence forms LARIAT
• 5’ —> 2’ linkage
• 3’ end: -OH coming off lariat
• remaining:
- lariat & portion of spliced pre-mRNA
when does gene splicing occur
after mRNA is formed from transcription
alternative splicing used when
to produce various mRNAs
alternative splicing:
-exon1—exon2—exon3-
REMAINS
1-2-3
2-3
1-2
Alpha-Tropomyosin gene
- alternative splicing
* multiple proteins possible from one gene
end of RNA processing…
mRNA now mature