Molecular Biology 5th Edition by Robert Weaver

Exercise 3

You are working in a research laboratory that studies the biochemisty of mRNA processing. You have developed an in vitro assay for both splicing and polyadenylation. You produce in vitro the following radioactive mRNA substrates (see table, next page) that either include a 5-cap or lack the 5-cap. You incubate these radioactive mRNA substrates with HeLa nuclear extract for 20 min at 308C and electrophorese the products on a high resolution gel. You then distinguish the splicing products based on their relative sizes in the gel. You count the amount of radioactivity found in the unprocessed mRNA (premRNA), the amount with intron 1 removed (splice 1), the amount with intron 2 removed (splice 2), both introns removed, and the amount of polyadenylated (poly A). You get the following results, where the number of pluses is related to the relative amount of radioactivity found in that band on the gel:
Propose a hypothesis that explains all these results.