Intron recognition and alternative splicing

Question 1

What is intron definition model of splicing?

Answer 1

Splicing when there are short introns or very few introns (eg. in yeast)
-5’ and 3’ splice sites are recognised
-factors bridging U1 and U2 snurps recruit spliceosome

Question 2

What is the exon definition model of splicing?

Answer 2

Splicing when pre-mRNA contain long introns and short exons
-SR-proteins recognise exonic enhancer sequences
-cross-exon complexes assemble, connecting U1 and U2

Question 3

What is recursive splicing?

Answer 3

Splicing of very large introns (>50kb)
-introns removed in step-wise manner, regenerating functional 5’ splice site at spliced junction each time

Question 4

What is alternative splicing?

Answer 4

When distinct mRNAs are generated from the same pre-mRNA transcript
-Produce diff protein isoforms
-Regulate expression

Question 5

What is trans splicing?

Answer 5

Splicing between the 5’ splice site of one mRNA and the 3’ splice site and branchpoint adenosine of another mRNA

eg. in trypanosomes, a capped 5’ leader seq is spliced onto all mRNAs from nucleus