W6 mRNA processing in Eukaryotes: Introns

Question 1

Consensus boundary sequences:

Answer 1

-Almost all introns begin with GU and end with AG

Question 2

Intron removal:

Answer 2

1) Cleave 5’ exon-intron boundary
2) Take free end and stick it to the branch point sequence within the intron
3) This forms lariat structure and the joint involves a 2’-5’ G-A bond
4) Cleave 3’ boundary to release lariat
5) Ligate exons together
6) Make lariat linear with debranching enzyme
7) Degrade linear introns

Question 3

Spliceosome:

Answer 3

-Splicing of pre-mRNA occurs exlusively in the nucleus
-Small nuclear ribonucleoprotein particles (snRNPs) plus some proteins form the spliceosome

Question 4

snRNPs:

Answer 4

-Typically contain 1 snRNA and about 10 proteins
-snRNA about 100-300 nt in length
-Proteins called ‘splicing factors’
-Major splicing snRNPs are U1, U2, U5 and U4/6 named after the RNA present

Question 5

snRNAs:

Answer 5

-snRNAs often have a sequence matching the splice consensus (important mechanism of binding)
-Mutations in this match region remove ability to splice introns

Question 6

Alternative splicing:

Answer 6

-Sometimes exons are spliced out of the pre-mRNA along with the introns
-This allows different combinations of exons to be assembled to form variety of mature mRNA sequences encoded from a single gene
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