Flashcards in Transcription in Eukaryotes part 3 Deck (24)
When does the addition of the 5' cap happen?
As soon as the first 20-30 nt have exited the RNA pol.
What is the role of guanyl transferase
It converts and GTP into GMP, and then adds it to the rNDP at the 5' end to make an unusual 5'-5' linkage
What enzyme adds a methyl group to position seven of the final guanine on a strand at the 5' end?
ribonucleotide methyl transferase
What are three functions of the 5' cap?
1. distinguish transcripts made by RNA pol. II
2. protect mRNA from degradation by exoribonucleases
3. facilitate interaction with the Cap Binding complex
Define the first transesterfication reaction in the context of splicing
A special "A" in a sequence motif in an intron attacks the phospodiester bond present between G/G sequence present at the beginning of the intron sequence, cutting it.
Define the second transesterfication reaction in the context of splicing
The free -OH group on the first exon attacks the phosphodiester bond present between the G/G at the end of the intron sequence, fully separating the intron from the second exon.
In the context of the spliceosome, what is responsible for cleaving introns?
small nuclear RNA (snRNA) with catalytic activity, linked to a complex of proteins called small nuclear ribonucleoprotein (snRNP). Often have names such as U2, U4, U5, etc.
What two proteins attract U2 snRNP to the 3' splice site?
BBP and U2AF
What three snRNPs form the "triple snRNP?
U4, U5, U6
What proteins put the two exons together at the end of splicing?
the exon junction complex
What is the role of RNA helicase in splicing
It modifies the RNA-RNA hybrids in such a way that the catalytic centers are optimally oriented
What snRNP replaces for U1?
What snRNP helps facilitate joining of the exons together?
What is the exon definition hypothesis?
predicts that the relative constancy of the exon size in eukaryotes (100-200nt) is used to facilitate formation of splice-site machinery
1. heterogenous nuclear ribonucleoprotein
2. preferentially bind to cryptic bind sites in introns to mask them
3. condense large introns to help facilitate their excision
Define SR proteins
Proteins high in serine and arginine that bind to exon splice sites to help facilitate attraction of snRNPs.
Define minor form of spliceosome
just recognizes different consensus sequecne
Define trans splicing
a single 5’ exon is spliced onto the 5’ end of many different mRNAs
Phosphorylation of the CTD allows the recruitment of what two factors?
CPSF (cleavage and polyadenylation specificity factor) andCstF (cleavage stimulation factor F)
What is PAP and what does it do?
Poly A polymerase. It adds close to 200 A's to the 3' end of the mRNA.
What is the role of the poly-A binding protein?
Bind to the poly A region at the end of the mRNA. They are responsible for preventing degradation from the 3' direction
What six factors must be present in order for mRNA to be "acceptable" to leave the nucleus?
1. the cap binding complex
2. SR proteins
5. Export factors
6. Exon Junction complex
The CTD is made of a heptamter that is repeated 52 times in humans. What is the sequence of the heptameter?