CRISPR Immunity Explained: How Cas9 Protects Bacteria from Viruses

Summary

Bacteria have an adaptive immune system known as CRISPR, which helps them protect against viruses. When a virus injects its DNA into a bacterial cell, a segment of the viral DNA is integrated into the bacterial genome at a site called CRISPR. This process records past infections. The bacterial cell then creates RNA from these sequences and forms a complex with the Cas9 protein to recognize and cut viral DNA. The Cas9–RNA machine halts the viral infection by chopping up the DNA, rendering it harmless.

Takeaways

• 😀 Bacteria use an adaptive immune system called CRISPR to defend against viruses.
• 😀 CRISPR works by integrating a piece of viral DNA into the bacterial genome during infection.
• 😀 The integrated viral DNA is stored at a site in the bacterial genome called 'CRISPR'.
• 😀 The CRISPR site contains repeated sequences that flank each inserted viral DNA piece.
• 😀 This integration creates a permanent genetic record of previous viral infections.
• 😀 The viral DNA sequences in the CRISPR region are copied into RNA.
• 😀 The RNA is processed into smaller units, each containing one viral sequence plus the surrounding repeat.
• 😀 A separate RNA called 'tracrRNA' helps to form a surveillance complex with the Cas9 protein.
• 😀 The Cas9-RNA complex locates matching DNA sequences in future infections.
• 😀 The Cas9-RNA complex unwinds the DNA and cuts each strand of the DNA double helix.
• 😀 In bacteria, the broken viral DNA is destroyed by other proteins, stopping the infection.

Q & A

What is CRISPR, and how does it help bacteria?

CRISPR is an adaptive immune system used by bacteria to protect themselves from viruses. It works by storing viral DNA within the bacterial genome, which is then used to recognize and destroy future viral infections.

How does a virus infect a bacterial cell?

A virus injects its DNA into a bacterial cell, where it is integrated into the bacterial genome at a site called 'CRISPR.'

What is the role of the CRISPR site in bacteria?

The CRISPR site in bacteria contains repeated sequences that flank inserted viral DNA. This forms a permanent genetic record of past infections, allowing the bacteria to recognize and defend against future attacks by similar viruses.

What happens to the viral DNA after it is integrated into the bacterial genome?

The viral DNA is copied into RNA, which is then chopped into functional units. Each unit contains one viral sequence plus the flanking repeat sequences from the CRISPR region.

What is tracrRNA, and how does it contribute to CRISPR immunity?

tracrRNA is a separate RNA molecule that binds with the RNA derived from the CRISPR sequence. Together, they form a complex with the Cas9 protein that helps identify and destroy matching viral DNA.

How does the Cas9 protein function in CRISPR immunity?

The Cas9 protein binds with the CRISPR-derived RNA and helps find DNA with a matching sequence. Once a match is found, the Cas9 protein unwinds the DNA and cuts both strands of the DNA double helix.

What happens to viral DNA once it is cut by the Cas9 protein?

Once the viral DNA is cut by the Cas9 protein, other proteins in the bacterial cell chop it up and destroy it, preventing the virus from infecting the bacteria.

What is the significance of the RNA-chopping process in CRISPR?

The RNA-chopping process ensures that each viral sequence, along with the repeat sequences from CRISPR, is stored in a form that can be used by the bacterial cell to recognize and defend against future viral infections.

Why is CRISPR described as an adaptive immune system?

CRISPR is called an adaptive immune system because it allows bacteria to 'learn' from past viral infections by storing and utilizing sequences of viral DNA to protect themselves from future attacks.

What is the function of the surveillance complex in CRISPR immunity?

The surveillance complex, formed by the Cas9 protein and RNA, scans the bacterial environment for viral DNA sequences that match the stored viral records in the CRISPR region. Once a match is found, it leads to the destruction of the viral DNA.