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#CRISPR-Cas9

3 public questions tagged with this topic.

Gene Tar getting

CRISPR interference phase involves:

Bacterial CRISPR adaptive immunity comprises three functional phases. Adaptation involves spacer acquisition by Cas1-Cas2. Expression involves transcription of CRISPR array into pre-crRNA processed into mature crRNAs with tracrRNA assistance. Interference phase represents actual defense where effector complex loaded with crRNA surveys invading DNA for complementary protospacer adjacent to PAM and mediates degradation. In type II systems Cas9 itself cleaves target DNA three base pairs upstream of PAM, generating double strand breaks, thereby neutralizing bacteriophage replication during interference phase efficiently and rapidly.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Gene Tar getting

Which genome editing method does not require dimerization?

Zinc finger nucleases and TALENs use FokI endonuclease domain which lacks catalytic activity as monomer and must dimerize to cleave DNA. Therefore they require paired binding sites in tail-to-tail orientation separated by spacer. Meganucleases act as single protein but still involve large protein-DNA recognition interface. CRISPR Cas9 avoids protein dimerization; a single Cas9 protein guided by single guide RNA recognizes target via RNA-DNA pairing and PAM, then introduces double strand break. This monomeric architecture simplifies multiplexing and delivery for genome editing.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Gene Tar getting

In CRISPR-Cas9, what sequence is essential for Cas9 to recognize the target?

Streptococcus pyogenes Cas9 requires a protospacer adjacent motif PAM downstream of target for binding and cleavage. The canonical PAM is 5'-NGG-3' on non-target strand. Guide RNA pairs with 20 nucleotides upstream of PAM, but Cas9 first scans DNA for PAM, triggering DNA unwinding and R-loop formation. Without PAM, even perfect guide complementarity fails to induce double strand break. PAM ensures self versus non-self discrimination since bacterial CRISPR array lacks PAM. Engineering higher fidelity Cas9 variants often alters PAM recognition profiles.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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