single guide RNA

A single guide RNA is the convenient, all-in-one version of the RNA that steers Cas9. In nature the guide comes in two separate RNA pieces that must find each other; researchers cleverly fused them into one molecule, like combining a key and its keyring into a single handy unit.

The single guide RNA stitches together the two natural components: the crRNA, which carries the roughly 20-letter spacer that base-pairs with the DNA target, and the tracrRNA, which forms the scaffold that Cas9 grips. Linked into one strand, it does both jobs at once — specifying the target and holding onto the enzyme — so a working Cas9 editing complex needs just this one RNA plus the protein.

This fusion, introduced in the landmark 2012 work on programmable Cas9, made CRISPR dramatically easier to use: change the 20-letter targeting portion and you retarget the system anywhere with a suitable PAM. People often say “guide RNA” loosely to mean the single guide RNA used in editing, though strictly the term also covers the natural two-part guide.