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Chapter 15: Problem 17

How does the molecular mechanism of the CRISPR-Cas system use a viral DNA sequence against that same virus?

Short Answer

Expert verified
Question: Explain how the CRISPR-Cas system uses a viral DNA sequence against the same virus in a step-by-step manner. Answer: The CRISPR-Cas system uses a viral DNA sequence against the same virus by carrying out the following steps: 1. Integration of viral DNA fragments (spacers) into the bacterium's CRISPR array during an initial infection, generating a genetic memory. 2. Transcription and processing of CRISPR array into a mature crRNA molecule containing the spacer sequence complementary to the viral DNA during a subsequent infection. 3. Formation of the CRISPR-Cas complex, where the crRNA molecule guides the Cas enzyme. 4. Targeting and cleavage of viral DNA based on the complementary sequence in the crRNA, inactivating the viral DNA and protecting the bacterium from the virus.

Step by step solution

01

1. Understand the CRISPR-Cas System

The CRISPR-Cas system is a prokaryotic adaptive immune system that provides resistance to foreign genetic elements, such as viruses, by targeting and cleaving specific DNA sequences. It consists of two main components: CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) RNA and the Cas (CRISPR-associated) proteins.
02

2. Initial Infection and Integration

When a bacterium encounters a viral infection, small fragments of the viral DNA, called spacers, are integrated into the bacterium's CRISPR array. These spacers are interspersed with repeat sequences, creating a unique genetic memory of the infection.
03

3. Transcription and Processing

In response to another infection by the same virus, the CRISPR array is transcribed into a long precursor RNA (pre-crRNA), which is then processed by a Cas enzyme called Cas9 or Cas12, depending on the type of CRISPR-Cas system. This processing generates a mature CRISPR RNA (crRNA) molecule that contains the spacer sequence complementary to the viral DNA.
04

4. Formation of the CRISPR-Cas Complex

The mature crRNA molecule guides the Cas enzyme to form a CRISPR-Cas complex. This complex seeks out and identifies the viral DNA based on the complementary sequence in the crRNA.
05

5. Targeting and Cleavage of Viral DNA

Once the CRISPR-Cas complex identifies the target viral DNA, the Cas enzyme cleaves the viral DNA at the specific site complementary to the crRNA spacer. This cleavage inactivates the viral DNA, thereby protecting the bacterium from the virus. In summary, the CRISPR-Cas system uses a viral DNA sequence against that same virus by incorporating a fragment of it into the bacterium's CRISPR array, producing a specific crRNA molecule that guides the Cas enzyme to recognize and cleave the viral DNA, effectively neutralizing the infection.

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Most popular questions from this chapter

Predict the effect on the inducibility of the lac operon of a mutation that disrupts the function of (a) the crp gene, which encodes the CAP protein, and (b) the CAP-binding site within the promoter.

What is the major difference between the mechanism involved in attenuation and riboswitches and the mechanism involved in the regulation of the lactose operon?

Bacterial sRNAs can bind to mRNAs through complementary binding to regulate gene expression. What determines whether the sRNA/mRNA binding will promote or repress mRNA translation?

Describe the experimental rationale that allowed the lac repressor to be isolated.

In the publication that provided the first evidence of CRISPRCas as an adaptive immune system [Barrangou, R., et al. (2007). Science. $315: 1709-1712$ ], the authors state that CRISPR-Cas "provides a historical perspective of phage exposure, as well as a predictive tool for phage sensitivity." Explain how this is true using what you know about the CRISPR locus.
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