Bacterial Immune System

CRISPR was not invented — it was discovered in nature. Bacteria evolved this defense system long ago to survive viral attacks. When a bacterium beats a virus, it keeps a snippet of that virus's DNA inside its own genome, like a molecular wanted poster for spotting the same enemy next time.

If that virus returns, the bacterium builds a short guide RNA that matches the stored snippet and clips it onto a cutting enzyme called Cas9. The guide steers Cas9 straight to the matching viral DNA, and Cas9 slices it apart. The breakthrough idea was simple: change the guide, and you can aim those scissors at almost any DNA you choose.