Britannica Sources used in this answer

How do CRISPR spacer sequences give bacteria a record of earlier viral infections?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

CRISPR-Cas9; gene editing — gene editing The CRISPR-Cas9 gene-editing complex from the bacterium *Streptococcus pyogenes*.

CRISPR spacer sequences function as a form of genetic memory that allows bacteria to record and recognize previous viral infections. When a virus attacks a bacterium for the first time, the cell captures a fragment of the viral DNA and integrates it into its genome as a new spacer element within the CRISPR region.12

Because these new spacers are typically inserted at one end of the CRISPR region, the sequence of spacers acts as a chronological record of the pathogens encountered by the bacterium and its ancestors. If the same virus attacks again, the bacterium transcribes these stored sequences into guide RNA. This guide RNA directs CRISPR-associated proteins, such as Cas9, to the matching viral genetic material, where the nuclease then cleaves and destroys the invading DNA to prevent the virus from reproducing.21