The CRISPR-Cas9 system functions naturally in bacteria to provide immunity against viral infections

• When a virus infects a bacterial cell, snippets of viral DNA are pasted into the bacterial genome to form a CRISPR locus

• These snippets act as genetic memory bank (CRISPR = ‘clustered regularly interspaced short palindromic repeats’)

• A CRISPR sequence is transcribed into a guide RNA strand (gRNA) that binds to a CRISPR-associated nuclease (Cas9)

• The gRNA-Cas9 complex drifts throughout the cell until the gRNA locates and binds with any complementary viral DNA

• This enables the Cas nuclease to then destroy the viral DNA sequence and hence prevents any subsequent infection

The CRISPR-Cas9 system has been modified by scientists to selectively remove any targeted sequence, allowing for precise gene editing

• The Cas9 protein is complexed with a synthetically derived gRNA molecule that is complementary to a target sequence

• The gRNA will bind to the target sequence, prompting its excision by the Cas nuclease (i.e. gene knockout)

• Following the removal of the target sequence, another sequence of DNA can be integrated in its place (gene editing)

Gene editing via the CRISPR-Cas9 system has been used to address a variety of agricultural issues associated with food production

• Certain metabolic pathways have been enhanced to improve nutritional content (e.g. higher starch production)
  

• Plant absorption spectra have been modified to increase photosynthetic efficiencies (e.g. new pigments introduced)
  

• Higher tolerances to biotic pathogens (viral, bacterial, fungal) or abiotic stresses (cold, drought, salt) have been achieved
  

• Resistance to particular herbicides have been incorporated into crops to allow for elimination of competing weed species