CRISPR was first observed in a bacterium (Escherichia coli). The bacterial CRISPR-Cas immune system against viruses found in many prokaryotes has been copied and modified to cut specific genes in plants and animals and stop them from working. Stopping just one particular gene from functioning in one germ cell may be partially achievable. However, other side effects of CRISPR-Cas in plants and animals can be unpredictable and uncontrollable. CRISPR-Cas is a segment of genetically modified DNA that can produce RNA segments for binding to a specific segment of DNA in the cell. CRISPR is made to recognize specific target sequences in DNA. Cas is a DNA cutting enzyme from a bacterium. CRISPR sequences of DNA and the associated Cas enzyme for cutting DNA are now used as a tool in genetic engineering to cut out or add base pairs somewhere in the genes of plants and animals.
CRISPR is a stretch of modified DNA that contains short repeats of base sequences, which will bind to a specified site of a host's DNA. The CRISPR-related Cas-6 enzyme will cut or cut DNA from the host cell at the site(s) where CRISPR can place it. CRISPR-Cas in nature is part of the defense mechanism for bacteria and single-celled organisms (prokaryotes). In bacteria, CRISPR DNA will recognize the site on the gene in a targeted manner and bind there. The associated Cas enzyme will cut the DNA at the same site. In some cases, this can stop viruses from producing their foreign proteins in the bacteria.
Removing a base pair of DNA from a specific gene can be used to disable the gene that codes for an unwanted protein. But there are many other effects apart from a perfect cut and paste on a specific DNA region, and these side effects can lead to unwanted pathogenesis. Cas (CRISPR-associated protein) is an RNA-guided DNA endonuclease enzyme found in prokaryotes. They are probably there to damage and neutralize foreign genes. There are many different types and their properties are tested on eukaryotes (plants and animals). They will be used to create virus resistance, or to silence the production of unwanted proteins from "defective" genes.
In the United States, under its regulations on biotechnology, the USDA does not currently regulate, or has any plans to regulate, plants that could otherwise have been developed through traditional breeding techniques. This is as long as these plants have been developed without the use of plant pests such as donors (e.g. Fusarium) or vectors (e.g. Agrobacterium) and they are not themselves harmful (e.g. weeds). In the EU, CRISPR is regulated using existing regulations for GMOs, genetically modified organisms. CRISPR-Cas are originally from bacteria and they are genetically modified for insertion into specific plants and animals.
CRISPR-Cas can cause many changes with modification. These are characterized as insertions and deletions, small and large, in regions close to and far from the target. CRISPR is accurate up to the point that it can cut one gene. The natural repairs then can lead to many other different outcomes. This makes CRISPR-Cas the cause of many different speculative end results in the genome. It has been shown that the repair of DNA strand breaks induced by CRISPR-Cas9 can lead to large deletions of DNA and complex rearrangements of the DNA strands. In nature, CRISPR-Cas is the natural defense of prokaryotes. Beating it into the defenses of eukaryotes could prove to be a very risky challenge. Damage has been observed in cells caused by CRISPR-Cas editing which can have pathogenic consequences.
Kosicki M., Tomberg K., and A. Bradley. 2018. Repair of double-strand breaks induced by CRISPR–Cas9 leads to large deletions and complex rearrangements. Nature Biotechnology, Letters.
Rath D., Amlinger L., Rath A., and M. Lundgren. 2015. The CRISPR-Cas immune system: Biology, mechanisms and applications. Biochemistry 117, 119-128. http://dx.doi.org/10.1016/j.biochi.2015.03.025