The discovery of the Cas9 protein has simplified gene editing, and may even make it possible to eliminate many hereditary diseases in the near future. Using Cas9, researchers have the ability to cut DNA in a cell to correct mutated genes, or paste new pieces of genetic material into the newly opened spot. Initially, the Crispr-Cas9 system seemed to be extremely accurate. However, it is now apparent that Cas9 sometimes also cuts other DNA sequences similar to the sequences it was programmed to target. Scientists at Delft University of Technology have developed a mathematical model that explains why Cas9 cuts some DNA sequences while leaving others alone.
The Crispr-Cas9 system is a defence mechanism that protects bacteria from viruses. If a virus enters a bacterium but does not take over the cell, the defence system cuts out some genetic material from the virus and stores it in the bacterium’s own genome. The built-in viral DNA acts as a genetic memory. If the same virus attacks the bacterium (or its descendants), it quickly recognises the attacker and can send out Cas9 proteins to track it down. Using viral RNA as a sort of “cheat sheet,” the protein hunts for hostile DNA in the cell. If it finds a match, the Crispr-Cas9 system then cuts the viral DNA, incapacitating the threat.