DNA is the instruction manual for life. It encodes the fundamental properties of an organism- how it develops, functions, and reproduces. Changing a DNA sequence in a living cell is known as genome editing. For a long time, this was either impossible or extremely challenging.
A new technology called CRISPR-Cas9 has made this process much easier. CRISPR is a bacterial immune system that has been repurposed for making precise breaks in DNA. Scientists use the Cas9 protein like a molecular scalpel to slice a DNA site in two. The cell will die if it does not repair the damage to its genome, so it has two main ways to fix the DNA break.
In the non-homologous end joining (NHEJ) pathway shown on the left half of the diagram, random bits of DNA are inserted or deleted at the cut site before the free DNA ends are reattached. When a gene's sequence is disrupted in this way, it will no longer function, so NHEJ is a great way to stop the effects of a harmful gene.
Another pathway called homology-directed repair (HDR) is shown on the right side of the diagram. Researchers add a piece of DNA containing sequences that match the site of the break, and the cell uses it as a “patch” to repair the cut. In this manner, scientists can deliver new genes with useful functions or replace a mutation with a healthy sequence.
In this 2015 WNYC Radiolab episode, "Antibodies Part 1: CRISPR," Jad and Robert explore the world of CRISPR with Jennifer Doudna, Eugene V. Koonin, Beth Shapiro, and Carl Zimmer.
"Hidden inside some of the world’s smallest organisms is one of the most powerful tools scientists have ever stumbled across. It's a defense system that has existed in bacteria for millions of years and it may some day let us change the course of human evolution."