Q.21 CRISPR-Cas system is associated with
(A) adaptive immunity in eukaryotes
(B) adaptive immunity in prokaryotes
(C) innate immunity in eukaryotes
(D) innate immunity in prokaryotes
CRISPR-Cas System: Adaptive Immunity in Prokaryotes Explained
The CRISPR-Cas system provides adaptive immunity in prokaryotes, making option (B) the correct answer for this multiple-choice question. This mechanism allows bacteria and archaea to defend against viruses by acquiring and using genetic memories of invaders.
Correct Answer
(B) adaptive immunity in prokaryotes
CRISPR-Cas, or Clustered Regularly Interspaced Short Palindromic Repeats paired with Cas proteins, evolved in bacteria and archaea as a defense against bacteriophages and plasmids. It works through three stages: adaptation (capturing invader DNA as spacers), crRNA biogenesis (processing RNA guides), and interference (targeting and cleaving matching foreign DNA).
Option Analysis
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(A) adaptive immunity in eukaryotes: Incorrect, as eukaryotes lack a native CRISPR-Cas system; prokaryotes developed it first, though humans now adapt it for genome editing.
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(B) adaptive immunity in prokaryotes: Correct, confirmed across studies as the only known adaptive immune system in prokaryotes, unlike innate defenses like restriction enzymes.
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(C) innate immunity in eukaryotes: Incorrect; eukaryotic innate immunity involves pattern recognition receptors, not CRISPR-Cas, which is prokaryote-specific in origin.
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(D) innate immunity in prokaryotes: Incorrect; prokaryotic innate immunity uses static barriers like restriction-modification systems, while CRISPR-Cas is adaptive due to its memory acquisition.
Mechanism Overview
The system stores viral sequences as spacers in CRISPR arrays, transcribes them into crRNA, and uses Cas proteins (e.g., Cas9) to recognize and destroy matching invaders via base-pairing and cleavage. This specificity distinguishes it from broad innate responses. Found in about half of bacteria and most archaea, it integrates into “defense islands” on genomes.
