CRISPR uses "guide RNAs" to selectively target specific DNA sequences. Guide RNAs can be designed to add, replace, or remove specific DNA sequences.
The guide RNAs, corresponding to the targeted viral sequences embedded in a viral genome, attract CRISPR-associated protein 9 (Cas9), which then cuts the viral genome and destroys it so it can't produce new virus. Cas9 searches the DNA for matching spacer sequences and cuts them by breaking the double helix across both strands. In this way, a bacterial cell can recognize bits of viral DNA in future encounters and promptly remove them, akin to an animal's immune system.
Cas9 was the first DNA-cutting enzyme adapted from CRISPR. Others, with differing targets, include Cpf1 (Cas12a) and C2c1 (Cas12b).
Learn more about how Cas9 acts as a genetic editing tool.
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Cite this: Kiran Musunuru. Fast Five Quiz: Genomic Medicine — CRISPR Gene Editing - Medscape - Dec 23, 2021.