The separation of RNA on the basis of size by sucrose gradient fractionation is a technique frequently employed in a cloning strategy (1 –3 ). After production of poly(A)+ mRNA by affinity chromatography (seeChapter 16 ) the RNA can be fractionated once or twice on sucrose gradients to produce a subpopulation of mRNA enriched for a particular species. The RNA fractions from the gradient can be assayed by in vitro translation and subsequent immunoprecipitation or bioassay of the products (seeChapters 19 –Chapters 22 ).
