Question

Draw the mechanism of lariat formation in a splicing pathway. Details are shown in Figure 38.7.

Answer 1

Simplified Mechanism of Splicin .2'-OH group of A within intron attackes the phosphodiester bond linking the first exon to the intron CopyrightThe McGraw-Hill Companies, Inc. Permission required tor reproduction or display OH Step 1 e A lariat is formed due to the GU at the 5' end of the intron forming a phosphodiester bond with the branchpoint A 5' 3' GOHAAGp .The free 3'OH on exon 1 Step 2 attacks the phosphodiester bond between the intron and exon 2 .The exons are then linked 14-10

RNA splicing . Process of cutting introns Copyright © The McGraw-Hil Companies, Inc Permission required for reproduction or display Start of transcription out of immature RNAs and stitching together the exons Gene:o to form final product is RNA splicing Intron 1 Intron 2 Exon 2 Exon 1 Exon 3 Transcription Introns are transcribed along with exons in the primary transcript Intron 2 Intron 1 Primary transcript: Exon 1 Exon 2 Exon 3 Splicing Introns are removed as the exons are spliced together Mature transcript: Exon 1 Exon 2 Exon 3

21.5 Pre-mRNA Splicing Proceeds Through a Lariat . Splicing requires the 5' and 3' splice sites and a branch The branch sequence is conserved in yeast but less well A lariat is formed when the intron is cleaved at the 5' site just upstream of the 3' splice site conserved in multicellular eukaryotes. splice site, and the 5' end is joined to a 2' position at an A at the branch site in the intron. Copyright O 2013 by Jones & Bartlett Learning, LLC an Ascend Learning Company www.jblearning.com