U1 and U2 snRNAs are important in recognising the start and end of introns and exons during pre-mRNA splicing.
U1 snRNA binds to the 5’ end of the intron, recognising where the intron begins. U2 snRNA binds to the 3’ end of the intron, identifying the end. This forms two sites that can then be identified by U4, U5, and U6 snRNAs for excision of the intron.
Having two snRNAs to identify the start and end of an intron allows for the intron to be of an indeterminate length. This is particularly beneficial with recursive splicing, allowing the excision of the intron in multiple sections by moving the 5’ splice site along by cutting out sections of the intron. In organisms that have large genomes with many split-genes (such as humans and other eukaryotic organisms), this is beneficial, allowing intron excision without having many different snRNAs for different lengths of intron.