In this video, we're going to begin our lesson on the CRISPR Cas system. The CRISPR Cas system is, in its natural state, a bacterial cell defense mechanism against various phage infections. The CRISPR Cas system is used by bacteria to defend or protect themselves against viruses, bacteriophages, or phages that are capable of infecting them. CRISPR is actually an acronym that stands for clusters of regularly interspaced short palindromic repeats. This is a detailed description of a specific region in the bacterial cell's chromosome.
The way that the CRISPR Cas system works is that it protects the cell from infections that it once encountered by storing small pieces of phage DNA in its chromosome. These small pieces of phage DNA that are stored in the chromosome are referred to as spacers. Upon the first phage infection, a protein complex within the cell might degrade the phage DNA into small fragments, which are then sometimes incorporated into the chromosome as a spacer. These spacers within the bacterial cell's chromosome serve to act as a record of previous phage infections.
During any subsequent phage infection, such as a second infection by the same virus, the cell is able to transcribe those spacer fragments of various phage DNAs. The transcribed RNAs from the phage spacers form a complex with another enzyme within the cell called Cas, forming a Cas RNA complex. This Cas RNA complex is capable of targeting the infecting phage DNA and is able to cut it to inactivate the phage, thus protecting the bacteria from phages by inactivating them upon subsequent infections. Because it is incorporated into the chromosome, it can also be passed down to future generations of bacteria, allowing them to protect themselves against those same viruses.
If we take a look at our image below, we can better understand the CRISPR Cas system. On the left-hand side, representing the very first phage infection, the bacteriophage virus inserts its genetic material, which can be degraded and cut up into small fragments. These small fragments can be incorporated into very specific regions of the chromosome. The spacers serve to act as a record of previous phage infections as tiny fragments of phage DNA are incorporated into the chromosome as spacers.
Upon a second viral infection, the CRISPR system, including the Cas genes and the spacers, are transcribed, forming an RNA molecule. This RNA molecule forms a complex with the Cas enzyme, resulting in a Cas RNA complex capable of protecting the cell against subsequent infections. When the phage infects for a second time, it injects its genetic material, and the CRISPR system gets transcribed, activated, and forms this Cas RNA complex, which is capable of targeting and inactivating the phage DNA. The RNA in the Cas complex acts as a guide, complementary to a sequence in the phage DNA, guiding the Cas enzyme to cleave and inactivate that phage DNA. Thus, the phage DNA is degraded by the Cas RNA complex, providing the bacterial cell with a defense mechanism against this specific phage. The CRISPR system, being part of the chromosome, can be replicated and passed down to future generations of bacteria.
This concludes our brief lesson on the CRISPR Cas system and how it serves as a bacterial cell defense mechanism against phage infections. We'll be able to get a little bit of practice on these concepts as we move forward in our course. So, I'll see you all in our next video.
