How "smart scissors" for DNA work

CRISPR/Cas Systems

Vera Mukhina, "Elements"

In the picture from the website artofthecell.com the mechanism of action of "smart scissors" for CRISPR/Cas DNA systems is depicted. This abbreviation hides a powerful tool for editing genomes, the mechanism of action of which, as it often happens, was spied on by nature and put at the service of man.

Many bacteria and archaea have an analogue of our immune system. It allows you to remember viruses and prevent repeated infection attempts. A special section of DNA (CRISPR cassette, English Clustered Regularly Interspaced Short Palindromic Repeats) is used as a storage device, which looks like many unique sequences separated by repeats. Unique sequences are pieces of the genetic code of viruses that once tried to infect a bacterium, in fact, their identifiers. Every time a bacterium copes with a viral infection, a piece of viral DNA is embedded at the beginning of the cassette. Thus, it turns out something like a medical record of a bacterium, where all its diseases are recorded in chronological order.

The so-called CRISPR-RNA (SRNA) synthesized from this site binds to the Cas complex of proteins also encoded in the bacterial genome. Like a sheriff with a portrait of a criminal, Cas (indicated in beige in the upper figure) with sgRNA (green) moves around the cell and when meeting with the criminal himself – a viral code with a homologous site (blue) – cuts it, suppressing infection.

In theory, this system should cut not only viral DNA, but also its own in the very place where the CRISPR cassette is located. This does not happen because a bacterium can distinguish its DNA from a foreign one by additional DNA marking sequences. In some types of systems, bacterial DNA is labeled, and in some – viral.

The complex with the Cas9 protein finds a DNA site (blue) homologous to the existing SRNA (green), hybridizes with it and makes a double-stranded incision, into which the desired site is then inserted using homologous recombination based on the sample. The sample consists of a homologous sequence (blue), provided with an insert (red) in the right place. Drawing from the website sites.tufts.edu

It turned out that Cas can work not only in bacterial cells. If you slip him some other sgRNA with a viral code instead, he will cut the DNA where there is a homologous fragment. This property was invented to use when editing the genome: first, these "smart scissors" make an incision of the desired DNA section, and then the damage is "healed" according to a given DNA sample, in which the required changes are incorporated.

Portal "Eternal youth" http://vechnayamolodost.ru  30.06.2016