Dicing Up Dsrna
Under most circumstances, RNA in a cell is present as a single-stranded molecule only. For instance, mRNA is created in the cell nucleus and transported to the ribosomes in the cytoplasm as a single strand. Double-stranded RNA (dsRNA), in which two complementary strands pair up, is normally present only in circumstances that pose a threat to the cell. This can occur when a dsRNA virus infects the cell, or from infection by some other viruses whose genomes are temporarily copied into dsRNA. It also occurs when certain types of transposable genetic elements (transposons) copy themselves in preparation for reinserting elsewhere in the cell's genome. Though the RNA copies are single-stranded, most transposons have sequences at their ends that, when transcribed into RNA, can fold back on themselves to form dsRNA.
When a cell detects dsRNA, it uses a nuclease enzyme to cut it into small fragments, twenty-one to twenty-three nucleotides long (the Drosophila enzyme is whimsically but accurately named "dicer"). This inactivates the RNA, so that it cannot be used to carry out the viral replication cycle or be reinserted into the genome (in the case of a transposon), thus protecting the cell from its harmful effects.