How does genome editing work?

What is genome editing and CRISPR/Cas9?

The earliest method scientists used to edit genomes in living cells was homologous recombination. Homologous recombination is the exchange (recombination) of genetic information between two similar (homologous) strands of DNA.1 Scientists began developing this technique in the late 1970s following observations that yeast, like other organisms, can carry out homologous recombination naturally.

To perform homologous recombination in the laboratory, one must generate and isolate DNA fragments bearing genome sequences similar to the portion of the genome that is to be edited. These isolated fragments can be injected into individual cells or taken up by cells using special chemicals. Once inside a cell, these DNA fragments can then recombine with the cell's DNA to replace the targeted portion of the genome.

CRISPR is a game-changing technology; unlike its predecessors, CRISPR is a simple technology with little assembly required. CRISPR associated DNA sequences were first observed in bacteria in the early 1990s, but it was not until the 2000s that the scientific community understood its ability to recognize specific genome sequences and cut them via the Cas9 protein, a protein that works with CRISPR and that has DNA-cutting abilities. In nature, CRISPR is used by bacteria as an immune system to kill invading viruses, but it has now been adapted for use in the lab.

With CRISPR, researchers create a short RNA template that matches a target DNA sequence in the genome. Creating synthetic RNA sequences is much easier than engineering proteins as is those required for ZFNs and TALENs. Strands of RNA and DNA can bind to each other when they have matching sequences. The RNA portion of the CRISPR, called a guide RNA, directs Cas9 enzyme to the targeted DNA sequence. Cas9 cuts the genome at this location to make the edit. CRISPR can make deletions in the genome and/or be engineered to insert new DNA sequenc