Final answer:
The CRISPR-Cas9 system, derived from an adaptive immune response in bacteria, allows for the recognition and cleavage of viral DNA, preventing infection. CRISPR sequences record past viral infections, and when these are transcribed and combined with Cas proteins, they can target and degrade matching viral DNA. This mechanism has been adapted for precise DNA editing in genetic engineering.
Step-by-step explanation:
The CRISPR-Cas9 system is a part of the adaptive immune system in bacteria and archaea, which allows these organisms to detect and destroy foreign DNA, such as that from bacteriophages. In this system, CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) sequences in bacterial DNA, which contain segments of DNA from previous viral invaders, are transcribed along with tracrRNA (trans-activating CRISPR RNA). The CRISPR RNAs, containing the "spacer" sequences, form complexes with the tracrRNA and the Cas9 nuclease. When a previously encountered virus infects the bacterium again, these RNA-guided Cas9 complexes will recognize and cleave the viral DNA that matches the spacer sequence, thus preventing infection. Essentially, this system serves as a molecular memory and defense mechanism against phage infections.
In the context of genetic engineering and biotechnology, this system has been adapted to allow for precise editing of DNA within various organisms, including humans. The capability of Cas9 to create double-stranded breaks at specified locations in the DNA under the guidance of a guide RNA (gRNA) has revolutionized genome editing. This technology has significant implications in medicine, as it can be used to correct genetic mutations at their source.