CRISPR Gene Editing

Amy Decaire

CRISPR is a family of DNA sequences found in the genomes of prokaryotic organisms such as bacteria and archaea. These sequences are derived from DNA fragments of bacteriophages, viruses that infect and replicate within bacteria, that had previously infected the prokaryote, a microscopic single-celled organism that does not have a nucleus. They are used to detect and destroy DNA from similar bacteriophages during infections. CRISPR is a technology that can be used to edit genes.

Essentially, it’s a way of finding a specific bit of DNA inside a cell. After that, the next step in CRISPR gene editing is usually to alter that piece of DNA. However, CRISPR has also been adapted to do other things too, such as turning genes on or off without altering their sequence. CRISPR has been used to experiment with gene-edited mosquitos to reduce the spread of malaria, for engineering agriculture to withstand climate change, and in human clinical trials to treat a range of diseases, including cancer and protein disorders. 


The CRISPR-Cas9 system consists of two key molecules that introduce a change into the DNA. One of these molecules is an enzyme called Cas9. This acts as a pair of ‘molecular scissors’ that can cut the two strands of DNA at a specific location in the genome so that bits of DNA can then be added or removed. The second molecule is a piece of RNA called guide RNA (gRNA). This consists of a small piece of pre-designed RNA sequence, about 20 bases long, located within a longer RNA scaffold. The scaffold part binds to DNA and the pre-designed sequence ‘guides’ Cas9 to the right part of the genome. This makes sure that the Cas9 enzyme cuts at the right point in the genome. The guide RNA is designed to find and bind to a specific sequence in the DNA. The guide RNA has RNA bases that are complementary to those of the target DNA sequence in the genome. This means that, at least in theory, the guide RNA will only bind to the target sequence and no other regions of the genome. The Cas9 follows the guide RNA to the same location in the DNA sequence and makes a cut across both strands of the DNA. At this stage the cell recognises that the DNA is damaged and tries to repair it. Scientists can use the DNA repair machinery to introduce changes to one or more genes in the genome of a cell of interest.

There are concerns about the ethical side of CRISPR. Scientists generally agree that CRISPR should be allowed for use in the creation of human disease models, and in understanding the development and molecular mechanisms of diseases. However, it should be prohibited for the purposes of eugenics or enhancement.

While CRISPR has the ability to cure some diseases, studies have shown that it could lead to mutations that lead to others down the line. If genetic edits are made to embryos, or to egg or sperm cells, these changes will be inherited by all future generations.