Diagram the mechanism by which CRISPR–Cas functions in the immune system of bacteria and archaea.

Verified step by step guidance

1. Understand the context: CRISPR–Cas is an adaptive immune system in bacteria and archaea that provides defense against invading genetic elements such as viruses (phages) and plasmids. It involves three main stages: adaptation, expression, and interference.
2. Step 1: Adaptation - When a bacterium is infected by a virus, the CRISPR–Cas system captures a short segment of the viral DNA and incorporates it into the CRISPR array in the bacterial genome. This segment, called a 'spacer,' is stored between repeat sequences in the CRISPR locus.
3. Step 2: Expression - The CRISPR array is transcribed into a long RNA molecule, which is then processed into smaller CRISPR RNAs (crRNAs). Each crRNA contains a spacer sequence that matches the DNA of a previously encountered virus.
4. Step 3: Interference - When the same virus infects the bacterium again, the crRNA guides the Cas protein to the matching viral DNA sequence. The Cas protein, which has nuclease activity, binds to the viral DNA and cleaves it, thereby neutralizing the threat.
5. Diagram the process: To visualize this mechanism, create a diagram with three sections: (1) Adaptation - show viral DNA being integrated into the CRISPR array, (2) Expression - depict the transcription and processing of crRNA, and (3) Interference - illustrate the crRNA-Cas complex targeting and cleaving the viral DNA.

Key Concepts

CRISPR-Cas System

The CRISPR-Cas system is a bacterial and archaeal adaptive immune mechanism that protects against viral infections. It consists of CRISPR sequences, which are short segments of DNA derived from previous viral infections, and Cas proteins that perform various functions, including cutting foreign DNA. This system allows organisms to 'remember' past infections and mount a targeted response upon subsequent encounters with the same virus.

Guide RNA

Guide RNA (gRNA) is a crucial component of the CRISPR-Cas system, serving as a template that directs the Cas proteins to the specific DNA sequence of the invading virus. The gRNA is complementary to the target DNA, ensuring precise binding and cleavage. This specificity is vital for the effectiveness of the immune response, allowing the organism to eliminate only the viral DNA without affecting its own genetic material.

DNA Cleavage

DNA cleavage is the process by which the Cas proteins, guided by the gRNA, cut the DNA of invading viruses. This action effectively neutralizes the threat by preventing the virus from replicating and expressing its genes. The ability to cleave DNA at specific sites is what makes the CRISPR-Cas system a powerful tool not only for bacterial immunity but also for genetic engineering applications in various organisms.