NYT on Crispr-Cas9:
A Powerful New Way to Edit DNA
Scientists Seek Ban on Method of Editing the Human Genome

The repeated DNA sequences ["clustered regularly interspaced short palindromic repeats" or CRISPR] in the bacterial genome are separated from one another by other sequences. These “spacers” are excerpts from the sequences of viruses that have attacked the bacterium or its ancestors. They are like genetic mug shots, telling the bacterium which bad guys to watch for. The Crispr defense system will slice up any DNA with that same sequence, so if the same virus invades again, it will be destroyed.

The real frenzy, however, started in 2012, when [Charpentier & Doudna] demonstrated a way for researchers to use Crispr to slice up any DNA sequence they choose.

Scientists must synthesize a strand of DNA’s chemical cousin RNA, part of which matches the DNA sequence to be sliced. This “guide RNA” is attached to a bacterial enzyme called Cas9. When the guide RNA binds to the corresponding DNA sequence, Cas9 cuts the DNA at that site.

The cell tries to repair the cut but often does so imperfectly, which is enough to disable, or knock out a gene. To change a gene, scientists usually insert a patch — a bit of DNA similar to where the break occurred but containing the desired change. That patch is sometimes incorporated into the DNA when the cell repairs the break.

-Andrew Pollack

(Image credit: nytimes.com)