Question

1. You are studying a gene that has four exons. You find that in some cell types the mature mRNA contains all four exons while in others exon 3 is missing from the transcript. You suspect that cells that lack exon 3 express a splicing repressor. Which splice site do you think this repressor acts on to inhibit inclusion of exon 3 (include a drawing)? Explain your reasoning.

Answer 1

Splicing is the process by which the protein coding exons are joined together after the removal of large non-coding intronst form the coding sequence. In some genes, this process leads to one outcome and thus named constitutive but in most cases it leads to more than one outcome and thus called alternative Both processes are mediated by the spliceosome, specialized machinery that recognizes consensus RNA sequences .T spliceosome component U1snRNP binds the 5’ splice site (5’ss), the splicing Factor 1 (SF1) binds the branch point site (BPS) adenine, U2 auxiliary factor 65 kDa subunit (U2AF65) binds the poly-pyrimidine tract (PPT) and U2 auxiliary factor 35 kDa subunits (U2AF35) binds the 3’ splice site (3’ss). The last two component are further replaced by U2snRNP and following complex base-paring rearrangements and RNA-protein interactions involving hundreds of protein, the spliced mRNA, the intron by-product and the spliceosome component are release.Chemically speaking, the splicing reaction proceeds in two trans-esterification steps . The first step involved the attack by the 2’ hydroxyl of the branch point adenine on the phosphate at the 5’ss, releasing at the same time the 3’end of the mRNA. The second step involved the attack by one of the hydroxyl of the terminal phosphate on the phosphate at the 3’ss, liberating the intron in the form of a lariat.