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Chapter 9: Problem 8

What are the three stages by which CRISPR-Cas immunity occurs? What takes place in each stage?

Short Answer

Expert verified
CRISPR-Cas immunity occurs in three stages: adaptation, expression and processing, and interference, where viral DNA is captured, crRNA is produced, and invading DNA is degraded, respectively.

Step by step solution

01

Adaptation

In the first stage, adaptation, the CRISPR-Cas system acquires new spacers. When a bacterium is infected by a virus, specific sequences of the viral DNA are recognized and captured. These sequences are then integrated into the CRISPR array of the bacterial genome as new spacers, providing a genetic record of past infections.
02

Expression and Processing

During the second stage, expression and processing, the CRISPR array is transcribed to create a long precursor CRISPR RNA (pre-crRNA). This pre-crRNA undergoes processing to produce mature crRNA. The crRNA is then incorporated into a CRISPR-associated (Cas) protein complex, forming a surveillance complex.
03

Interference

In the final stage, interference, the CRISPR-Cas complex (containing the crRNA) interrogates and binds to invading viral DNA that matches the crRNA sequence. Upon binding, the Cas proteins cleave and degrade the viral DNA, thereby preventing the virus from replicating and establishing an infection.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Adaptation
During the adaptation phase, the CRISPR-Cas system kicks into gear when a bacterium encounters a virus or plasmid. This stage is like building a library of threats. When a virus infects the bacterium, certain sequences from the viral DNA, termed "protospacers," are identified. These sequences are then copied and integrated into the bacterium's own genome at the CRISPR locus, forming what are called "spacers." This creates a genetic memory within the bacterial DNA that can recognize previous invaders.
  • The spacers are akin to a photographic memory of past infections.
  • Each spacer is followed by repeat sequences that serve as markers between individual spacers.
This step is crucial for building the immune defense database that helps the bacterium recognize and respond faster in future encounters with the same virus.
Expression and Processing
The second stage, expression and processing, involves preparing the immune system for action. Here, the CRISPR locus that holds the repeated sequences and spacers is transcribed into a long RNA molecule known as precursor CRISPR RNA (pre-crRNA). This transcription is an important step in mobilizing the immune response.
  • The pre-crRNA is like a rough draft that needs refining.
  • This RNA is processed into shorter segments known as mature CRISPR RNAs (crRNAs).
Each crRNA segment corresponds to a specific spacer and its corresponding viral sequence. These crRNAs are then loaded into a complex with Cas proteins to form a seeking-and-destroying unit ready for action. This sophisticated process ensures the bacterial defense machinery is ready and waiting to intercept any incoming threats.
Interference
Interference is the action-packed third stage where the CRISPR-Cas system neutralizes threats. With the crRNA serving as a guide, the CRISPR-Cas complex patrols the cell, searching for viral DNA that matches the sequence encoded by the crRNA. When it finds a match, the crRNA guides the Cas proteins to the target.
  • Upon binding to the viral DNA, the CRISPR-Cas complex cleaves the DNA.
  • This effectively deactivates the virus, preventing it from multiplying and spreading infection.
This stage functions like a molecular ambush, protecting the bacterium by destroying the invader's genetic material. The efficiency and precision of this immune response are remarkable, highlighting the CRISPR-Cas system's role as an adaptive powerhouse in bacterial defense.

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Most popular questions from this chapter

Place the following components of CRISPR-Cas systems in the order in which they function during generation of CRISPR-Cas immunity. a. Effector complex binds to the foreign DNA b. Transcription of CRISPR array into a long CRISPR precursor RNA c. crRNA combines with a Cas protein to form an effector complex d. Incorporation of spacers into CRISPR array e. Cas protein in the effector complex cleaves the foreign DNA f. CRISPR precursor RNA (pre-crRNA) is cleaved and processed into crRNAs

In Chapter 1 , we discussed the theory of the inheritance of acquired characteristics, an idea that was later discredited. Recently some researchers have claimed that the CRISPR-Cas system represents a form of inheritance of acquired characteristics. How might the CRISPR-Cas system be considered a type of inheritance of acquired characteristics? Do you think CRISPR-Cas represents inheritance of acquired characteristic? Why or why not?

Briefly explain the differences between \(\mathrm{F}^{+}, \mathrm{F}^{-}\), Hfr, and \(\mathrm{F'}\) cells.

What problem is solved by the requirement that a PAM sequence be adjacent to a protospacer in order to be recognized by CRISPR-Cas systems?

Explain how a retrovirus, which has an RNA genome, is able to integrate its genetic material into that of a host having a DNA genome.
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