The reason that scientists are excited about the ability to cut DNA in a living cell at very predicable/targeted positions is that these cuts are repaired by the cell in one of two ways.

The first way is for the cell to simply try to “glue” (the scientific term is “ligate”) the two ends together. This often results in the addition or removal of a few nucleotides in the process, which introduces new mutations that might inactivate the gene or make an inactive protein product. Inactivating (“knocking out”) a gene can be very useful in helping scientists figure out what a gene actually does in a cell.

In the second repair mechanism, the cell finds a “homologous” copy of the cut gene and repairs the damage by simply copying the undamaged DNA sequence over the damaged one. In this way, scientists can try to repair faulty genes by introducing cuts into them and providing the cell a “good” copy of the gene that the cell can use in the DNA repair.

Thus, scientists now have a powerful tool that allows them to “edit” DNA sequences. If the sequence of a faulty gene is known, this sequence can be used to produce a crRNA that can be used to target the faulty DNA sequence in a living cell. A good version of the gene can then be provided so that the cell’s own repair mechanism can use it to repair the cut introduced by CAS9, and in this way a faulty gene can be specifically replaced by a good copy.

Understandably, this new tool opens up some very exciting possibilities, but it also poses some challenges as Dr. Doudna indicated in her TED talk.

Much more research needs to be done before this tool can be used for therapeutic purposes. In the process, many ethical questions will also need to be addressed.