What is the initial action of the CRISPR system upon viral DNA exposure?

The CRISPR system's initial action upon exposure to viral DNA involves the storing of viral DNA sequences as spacers for future recognition. This process is a fundamental aspect of the adaptive immune response in bacteria and archaea, allowing these organisms to "remember" specific viral sequences.

When a bacterium encounters a virus, it does not immediately cut or replicate the viral DNA. Instead, the system incorporates a short piece of the viral DNA into its own genome in a specialized region known as the CRISPR array. These segments, referred to as spacers, serve as a historical record of past infections. If the bacterium is later attacked by the same virus, it can quickly recognize the viral DNA using the stored spacer sequences, leading to a targeted response to eliminate the virus.

This ability to capture and store DNA is key to the effectiveness of the CRISPR system in providing immunity against viruses. The subsequent stages, which involve the cutting of viral DNA, occur only after the recognition of the DNA based on these stored spacers during future viral exposures.