Practice question

Major class introns removed by spliceosome share highly conserved terminal dinucleotides, GU at 5' end and AG at 3' end, recognized by U1 and U2 snRNPs respectively. These U2-type GU-AG introns constitute vast majority, over 99% of introns in eukaryotic nuclear pre-mRNA, distinguished by consensus sequences including branch point adenine. They reside in protein-coding genes transcribed by RNA polymerase II in nucleus, removed co-transcriptionally. Minor U12-dependent AU-AC introns represent rare variant. GU-AG is absent from tRNA introns or group I/II self-splicing introns found in organelles.