Lecture 27 - RNA Splicing

Question 1

What are the three types of introns?

Answer 1

Spliceosome, Group 1, Group 2 (bacteria only)

Group 1 and 2 are self-splicing –> ribozymes that catalyze their own excision

Question 2

What are the conserved sequences in Spliceosome and then self-splicing introns

Answer 2

Spliceosome: upstream exon ends in AG, intron begins with GU and has a branch site A and then has a string of 10 pyrimidines followed by any base and then a U and then AG.
Self-splicing: first nucleotide of intron is an A, last nucleotide of upstream exon is a U, first nucleotide of the downstream exon is a U

Question 3

Describe the process of spliceosome introns being excised

Answer 3

U1 binds to the 5’ splice site. U2 binds to the branch site, allows U4, U5, U6 to come and make the spliceosome. U2 causes the A to bulge, the 2’OH goes and attacks the 5’ phosphate of the first nucleotide of the intron (2’-5’ linkage). This frees up the 3’OH of the last nucleotide of the upstream exon to attack the phosphodiester bond of the first nucleotide of the downstream exon. The two splice sites are brought together and joined. The 5’ splice site is associated to the C-terminal domain of RNA pol II after it is transcribed, the 3’ splice site is captured by the spliceosome complex to bring the splice sites together.

Question 4

What is the spliceosome complex made of?

Answer 4

snRNPs small nuclear ribonucleoproteins made up of snRNAs and proteins

Question 5

How do Group 1 introns get spliced?

Answer 5

free GMP attacks the 5’ splice site, frees the 3’OH of the exon to attack the 3’ splice site

Question 6

How do Group 2 introns get spliced?

Answer 6

Very similar to spliceosome except there is no spliceosome formed. 2’OH of branch site A attacks 5’ splice site, now-free 3’OH of exon attacks 3’splice site