Bacterial Adaptive Immunity with CRISPR/Cas9

Summary

CRISPR systems, short for Clustered Regularly Interspaced Short Palindromic Repeats, are crucial for bacterial immune responses. These systems consist of arrays of Cas genes and incorporate short DNA sequences from foreign viruses or plasmids to build immunity. The process involves transcription of the CRISPR repeat array into a primary transcript, which interacts with tracer RNA to form crRNAs. These crRNAs guide the Cas9 protein to foreign DNA, where the complex recognizes specific sequences (PAM) and cleaves the DNA, effectively providing immunity through a precise targeting mechanism.

Takeaways

• 😀 CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats and is part of bacterial immune systems.
• 😀 Type II CRISPR systems incorporate DNA sequences from foreign viruses or plasmids to provide immunity.
• 😀 CRISPR repeat arrays are transcribed into a primary RNA transcript, which is later processed into individual CRISPR RNAs (crRNAs).
• 😀 TracrRNA (trans-activating CRISPR RNA) pairs with the primary transcript to guide the formation of crRNAs.
• 😀 Each crRNA contains a 20 base pair sequence complementary to foreign DNA, allowing it to target specific invaders.
• 😀 The crRNA-tracrRNA complex interacts with the Cas9 protein, forming an RNA-guided nuclease.
• 😀 The Cas9 protein searches for a specific sequence called the PAM (Protospacer Adjacent Motif) in foreign DNA.
• 😀 If the PAM sequence is present and the crRNA matches the target DNA, the Cas9 protein cleaves the foreign DNA.
• 😀 CRISPR-Cas9 functions as a defense mechanism by cutting viral or plasmid DNA to neutralize potential threats.
• 😀 This immune system mechanism allows bacteria to remember and defend against previous invaders through stored genetic sequences.

Q & A

What does CRISPR stand for?

CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats, a system involved in bacterial adaptive immune responses.

How are CRISPR systems structured?

CRISPR systems are made up of arrays and clusters of CRISPR-associated (Cas) genes.

What is the role of Type II CRISPR systems?

Type II CRISPR systems incorporate approximately 20 base pair repeats of DNA from foreign viruses or plasmids to confer immunity against these invaders.

What is transcribed from the CRISPR repeat array?

The CRISPR repeat array is transcribed into a primary transcript, which contains repeats of conserved sequences complementary to a trans-activating CRISPR RNA (tracer RNA).

What is the function of the tracer RNA?

The tracer RNA base pairs with the primary transcript and is processed into individual CRISPR RNAs (crRNAs), which guide the system in recognizing foreign DNA.

What do crRNAs do in the CRISPR system?

Each crRNA contains a 20 base pair sequence complementary to foreign DNA, and it interacts with the Cas9 protein to form an active RNA-guided nuclease complex.

What is the Cas9 protein’s role in CRISPR?

The Cas9 protein forms an active ribonucleoprotein complex with the crRNA and tracer RNA, acting as a nuclease that cleaves the target DNA when it finds complementary sequences.

What is the PAM sequence, and how does it relate to Cas9's activity?

The PAM (Protospacer-Associated Motif) sequence is found in the foreign DNA and is necessary for Cas9 to bind to and cleave the target DNA.

How does the Cas9 complex identify its target DNA?

The Cas9 complex binds to the PAM sequence in the foreign DNA and checks for complementarity between the crRNA and the target DNA sequence. If both align, Cas9 cleaves the DNA.

What happens when Cas9 successfully binds to its target?

Once Cas9 successfully binds to the target DNA via the PAM sequence and crRNA complementarity, the Cas9 nuclease cleaves the target DNA, thereby disrupting the foreign DNA.