CRISPR

Illustration of process, which starts in the upper left and proceeds clockwise with the single phase representing two invasions by different phages.

CRISPR (Clustered regularly interspaced short palindromic repeats) are segments of naturally-occurring prokaryotic DNA containing short, repetitive base sequences. This makes precise editing of bacterial DNA sequences possible.

Biology

These segments are sequences in the DNA of bacteria are copied from invading plasmids and phages, saved in the bacteria's own DNA in a library-like fashion and provide the bacteria with an immune system as a form of acquired immunity. Different versions of the Cas molecule (CRISPR associated protein) play a role in three steps of the immune system sequences.

In the final step of the immune system sequence, Cas uses the copied DNA sequences to attack later invading DNA from the same invading species to cut the invading DNA. The DNA copy is translated into RNA. The resulting RNA molecule is complexed with the Cas molecule where its is referred to as "guide RNA" (gRNA). The gRNA guides the complex to the target invading DNA, where the Cas molecule cuts invading DNA at a precise point. As of 2017, new variations on this phenomenon are continuing to be discovered.

Technology

In 2013, Cas9 was complexed with synthetic RNA to create a dramatic improvement in the specificity, speed and cost of DNA editing. This technique is often also called "CRISPR" in the press. The use of CRISPR/Cas9-gRNA complex for genome editing was the American Association for the Advancement of Science's choice for breakthrough of the year in 2015.

CRISPR has been demonstrated to be able to edit the genome of human zygote cells (fertilized eggs) but all such experiments have a protocol to destroy the developing embryo after several days. Anti-CRISPR proteins (that interfere with attempts at CRISPR gene editing) are also being discovered.