Enzyme measures RNA using natural ruler

For almost a decade, researchers have taken advantage of a powerful process called RNA interference (RNAi) to turn off certain genes in lab organisms and cell cultures (SN: 7/2/05, p. 7: Available to subscribers at Sound Off). Cells also use RNAi as a natural tool in immunity, development, and gene rearrangements within chromosomes. The process begins when a cell chews double-stranded RNA into bite-size fragments of about 25 base pairs, the chemical building blocks of RNA.