The 5′ cap requirement for ribosome binding and translation. Radiolabeled mRNAs with or without a 5′-methyl cap were incubated with ribosomes, and the complexes were separated from free mRNA by sucrose density gradient centrifugation. Denser particles migrate farther in the gradient, as shown on the left side of the graphs. Radioactivity levels (in counts per minute) of capped (top panel) and uncapped (bottom panel) mRNAs show that only the mRNA with a 5′ cap migrates with ribosomes.