[Campbell Biology P.453] The text describes both restriction enzymes and the CRISPR-Cas syst... | Practice Question

The text describes both restriction enzymes and the CRISPR-Cas system as bacterial defenses against phage DNA. Which of the following statements best highlights a key difference that makes the CRISPR-Cas system an example of adaptive immunity?

  • A: Restriction enzymes degrade all foreign DNA, whereas CRISPR-Cas specifically targets RNA molecules.
  • B: The CRISPR-Cas system incorporates segments of previously encountered phage DNA into its own genome, providing a targeted defense.
  • C: Restriction enzymes are active only after a phage infection, while CRISPR-Cas is constitutively expressed.
  • D: The CRISPR-Cas system protects bacterial DNA from methylation, a process restriction enzymes cannot perform.

Explanation

The defining characteristic of adaptive immunity is the ability to 'remember' past infections and mount a more specific and effective response upon re-exposure. The text explicitly states that when a phage infects a bacterial cell with the CRISPR-Cas system, 'the DNA of the invading phage is stored, integrated into the genome between two repeat sequences Figure 19.7 .' This 'spacer DNA' then serves as a 'homing device' for Cas proteins to identify and cut the same invading phage DNA in subsequent infections, which is the hallmark of adaptive immunity.