Viruses pose a threat to all organisms, even to bacteria.
They latch on to cells and inject their DNA into them.
The DNA is integrated into the genome of the bacterium, which then produces new viruses.
But bacteria are not defenceless.
They remember the pathogens and are therefore able to develop immunity against that specific virus.
Enzymes cut a piece out of the viral DNA and insert it at a specifically defined location in the bacterial genome.
The cut-out section is known as a CRISPR segment.
The cell then translates this segment into an RNA molecule, known as CRISPR RNA.
It contains information from the genome of both the virus and the bacterium.
A second molecule now comes into play: tracrRNA. Together, they bind to the Cas9 enzyme.
This enzyme now protects against this specific virus, because its crisprRNA is able to bind to the corresponding viral DNA.
The Cas9 protein then cuts the DNA strand into pieces, rendering it harmless and fending off the attack by the virus.
The crisprRNA therefore determines where Cas9 cuts the viral DNA. But that works only if the tracrRNA is also bound to the molecule.
Two discoveries have made the CRISPR/Cas9 system into a revolutionary tool for gene technology.
Firstly, the tracrRNA and CRISPR RNA can be fused into a single molecule, making it easier for scientists to synthesise it.
Secondly, the system also works with a modified Cas9 protein in higher organisms having nucleated cells.
Researchers can vary the sequence of the fused RNA molecule at will.
In this way, THEY determine where the Cas9 protein cuts a DNA strand. This enables them to target specific genes and switch them off.
The cell attempts to re-join the loose ends. However, it is unable to do so without errors, and the gene can no longer be read.
It is also possible to swap entire genes.
This method is much less costly and more efficient than any previously known technique, as only one RNA molecule has to be modified.
CRISPR/Cas9 can be used to modify the genome of any desired cell.
One day this precise tool may even help to cure hereditary diseases in humans.
