Splice sites are directional in nature. 5’ splice site -the left, or donor, site and the 3’ splice site - right, or acceptor, site. 

1. U2 addition to base pair with the branch site in the presence of ATP

2. U1 and U4 release Formation of the catalytic center in which U6 base pairs with the 5' splice site; U6 also base pairs with U2; U2 remains base paired with the branch site; U5 interacts with both exons through its loop.

So, 1 and 2 are the event of complex formation which are 

TLS Online TPP Program

#Question id: 6594

#General Aptitude

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464	?	903

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#Question id: 6595

#General Aptitude

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F	I	M
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#Question id: 6596

#General Aptitude

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D	I	N
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#Question id: 6597

#General Aptitude

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R	O	?
?	G	C

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#Question id: 6598

#General Aptitude

F	W	O
A	J	K
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#Question id: 6599

#General Aptitude

H	K	Q
C	G	O
E	J	?