SET 2 — Transcription elongation, RNA Processing, Transcription Termination

Transcription elongation (eukaryotes)

RNA pol II adds ribonucleotides to the growing RNA chain; can start de novo without a primer.

CTD (C-terminal domain)

Subunit of poly II

Order of RNA processing (eukaryotes)

1. 5’ cap

2. Splicing

3. Cleavage

4. Polyadenylation

5’ cap addition and function

1. Occurs shortly after transcription starts; adds a modified guanine to the 5’ end of mRNA.

2. Protects RNA from degradation

Splicing

Removal of introns from pre-mRNA to produce mature mRNA.

Spliceosome

Large RNA-protein complex (made of snRNPs)

snRNPs

Small nuclear ribonucleoproteins

1. U1 snRNP

2. U2 snRNP

U1 snRNP

Binds to the 5’ splice site

U2 snRNP

Binds to the branchpoint A region in the intron

5’ splice site

Located at the start of each intron; GU (donor)

3’ splice site

Located at the end of each intron; AG (donor)

Splicing step 1

The 2’ OH of branchpoint A attacks the 5’ splice site, forming a lariat with a 5’–2’ bond.

Splicing step 2

The 3’ OH of the upstream exon attacks the 3’ splice site, joining the two exons.

Lariat structure

Formed during step 1 of splicing when the intron loops back to branchpoint A.

Cleavage

Creates the 3’ end of mRNA; occurs between the AAUAAA signal and the GU-rich sequence.

AAUAAA

Polyadenylation signal (PAS), located upstream of the cleavage site.

Cis-acting

GU-rich sequence

Located downstream of the cleavage site.

Cis-acting

CPSF (Cleavage and Polyadenylation Specificity Factor)

Binds to PAS

tTans-acting

CstF (Cleavage Stimulation Factor)

Binds to GU-rich

Trans-acting

Endonuclease in RNA processing

Cuts the RNA at the cleavage site between PAS and GU-rich sequence.

Trans-acting

Polyadenylation

Addition of a poly(A) tail to the 3’ end of mRNA

Poly(A) tail function

Protects mRNA from degradation

Transcription Termination uses the XRN2 exonuclease

Degrades the leftover RNA after cleavage