The key to rewriting genomes is a remarkable protein known as CRISPR-Cas9,
which naturally occurs in many bacteria as an immunity mechanism.
The Cas9 protein acts like a pair of
highly specific molecular scissors that can be directed to cut,
and therefore edit almost any gene in any organism.
Here's how it works.
To edit a cell's genome,
scientists introduced the Cas9 protein and the guide RNA with
a specific sequence that will direct where the Cas9 protein will make a cut in the DNA.
Cas9 binds to the guide RNA and scans the cell's DNA,
looking for a particular recognition sequence,
which is two G bases next to each other.
When it finds such a sequence,
Cas9 opens up the DNA double helix to check whether
the adjacent sequence is complimentary to those of the guide RNA.
If they're not, Cas9 continues scanning.
However, if all of the DNA bases match the targeting sequence of the guide RNA,
Cas9 will cut both strands of that DNA.
In order to repair the damage,
the cell will then do one of two things.
First, it can directly stick the ends together,
often creating mutations at the site of the break.
This allows scientists to use Cas9 to disrupt any gene.
On the other hand,
cells often tend to fix the damage without causing mutations.
To do so, the cells look to repair the DNA with matching sequences.
By introducing a matching DNA sequence that contains desire genetic alterations,
scientists can induce the cell to incorporate
just about any desired change into its genome.
This is how the CRISPR-Cas9 genetic editing system
allows scientists to precisely disrupt or modify genes.
