RR8

Question 1

What is the CTD? What ind of sequence does it have?

Answer 1

On large subunit, it’s phosphorylated form is associated with highly transcribed genes
- composed of heptapeptide sequences

Question 2

What phosphorylation is actively taking place in event of transcription?

Answer 2

• CTD: the serine at position 5 then serine at position 2

Question 3

Describe the process of 5’ capping

Answer 3

• Capping enzyme recognizes serine 5, and sits near the 5’ end
• CDK7 phosphorylates on serine 5, and the phosphorylated P caps the 5’ end

Question 4

Why is 5’ capping important?

Answer 4

• protects pre-mRNA
• Facilitates nuclear export
• aids in recognition by translation factors

Question 5

What is an important step of RNA pol II after initiation?

Answer 5

RNA pol II pauses after inititiation
- allows a “changing of the guard” from Factors blocking elongation (NELF) to factors enhancing it (DSIF, SPT6 and PAF)
- This switch is mediated by CDK9

Question 6

What are the roles of CDK9 and CDK7?

Answer 6

-CDK7 phosphorylates on serine 5 after DNA is melted by TF2H- leading to capping
CDK9 adds additional phosphates to serine 2, causing conformational changes (changing of the guard)

Question 7

Explain the spliceosome cycle

Why is it necessary?

Answer 7

Eukaryotes have introns and exons- meaning that DNA -> premRNA (introns) -> mRNA -> proteins

RNA processing of premRNA takes place in order to remoe introns and join exons in correct order

Question 8

Explain how the spliceosome cycle takes place?

Answer 8

• First: 5’cap is added, poly(A) tail on 3’ end, introns are removed
• Exon-intron junctions almost always have GU(5’) and AG(3’) of intron, and an A upstream on the 3’ splice site (branch point)
• snRNPs (U1, U2, U5, U4 and U6) carry out spliceosome cycle
• U1 binds to 5’ splice site, U2 binds to A residue
• U5, U4 and U6 bind U1 and U2 together, creating the spliceosome complex, with a loop in the intron
• U1 and U4 are expelled, U6 pairs with U2 at 5’ splice site
• 5’ splice site is cleaved, then 3’ end is cleaved
• Ends of exons bind together
  Intron is released as ‘lariat’