Step (Name)

Event

Energy?

CC (commitment complex)
U1 joins with the pre-mRNA (the splicing substrate), attaching at the 5' intron/exon boundary.
None required

A (A complex)
U2 binds to the complex at the branch point A
ATP

B1 (B1 complex)
The U4/U6 complex, along with U5 binds.
None required

B2 (B2 Complex)
U4 dissociates, allowing U6 to base pair with the snRNA in U2
None required

C1 (C1 complex)
Step one of the splicing mechanism (separation at the 5' exon/intron boundary and formation of the lariat).
ATP

C2 (C2 complex)
Step two of the splicing mechanism (joining of the exons and excision of the lariat structure intron).
ATP

I (I complex)
Joined exons dissociate from complex, leaving the lariat structure behind.
None required

(completion)
Spliceosome dissociates – snRNPs recycle – intron (lariat structure broken into monomers.
None required

From: The Spliceosome

Step (Name)	Event	Energy?
CC (commitment complex)	U1 joins with the pre-mRNA (the splicing substrate), attaching at the 5' intron/exon boundary.	None required
A (A complex)	U2 binds to the complex at the branch point A	ATP
B1 (B1 complex)	The U4/U6 complex, along with U5 binds.	None required
B2 (B2 Complex)	U4 dissociates, allowing U6 to base pair with the snRNA in U2	None required
C1 (C1 complex)	Step one of the splicing mechanism (separation at the 5' exon/intron boundary and formation of the lariat).	ATP
C2 (C2 complex)	Step two of the splicing mechanism (joining of the exons and excision of the lariat structure intron).	ATP
I (I complex)	Joined exons dissociate from complex, leaving the lariat structure behind.	None required
(completion)	Spliceosome dissociates – snRNPs recycle – intron (lariat structure broken into monomers.	None required