Question

For splicing or pre-mRNA, which snRNA molecules is needed for recognition of the 5' splice of the intron?

A	U1
B	U2
C	U5
D	U4

For splicing or pre-mRNA, which snRNA molecule is needed for recognition of branching point A in the intron?

A	U4
B	U2
C	U1
D	U5

Answer 1

A.: U1 snRNA

All snRNAs involved in splicing are bound to specific RNA binding proteins and exists as ribonucleotide particles. Therefore U1 snRNA exists as U1 snRNP. Binding of U1 snRNP to the 5′ splice site is the first step in splicing. The human U1 snRNP contains the core Sm proteins, three U1- specific proteins and U1 snRNA. The secondary structure of U1 contains four domains. U1 snRNA interacts with the 5′ splice site by base pairing between its single-stranded 5′ terminus and a stretch of four to six bases of the 5′ splice site.The U1 snRNA pairing reaction with the 5′ splicing is stabilized by protein factors.

B: U2 snRNA

The branch point binding protein (BBP, also known as SF1) interacts with the branch point sequence and splicing factor U2AF  binds to the polypyrimidine tract just upstream of the dinucleotide AG at the 3’ end of the intron. This results in the formation of a relatively stable complex called the commitment complex. The commitment complex is also known as early pre-splicing complex (E-complex) in mammalian cells. After the formation of E complex other snRNPs and splicing factors involved in splicing associate with the E-complex in a defined order. Now U2 snRNA binds to the branch site converting E-complex into A - complex. The pairing between U2 and branch site occurs in such a way that the branch site Adenine residue is excluded from pairing and it exists as a single nucleotide bulge. The 2'-OH of this Adenine residue initiates the first trans-esterification reaction.