Meet Cas9!

You may have heard of Cas9 before in discussions and news about gene editing with CRISPR! (The two women who established how to use CRISPR-Cas9 in the lab won the 2020 Nobel Prize in Chemistry.)

In nature, CRISPR is a bacterial acquired immune system. It recognizes invading DNA, like from viruses for example, and “saves” a piece in its own DNA. Then, the saved pieces help the immune system specifically recognize and destroy the invader in the future.

Left panel: First exposure to a virus. The viral DNA is recognized as foreign and cut up into pieces. Then, some pieces are saved into the bacteria's own DNA.
Right panel: Second exposure.
The bacteria's DNA creates guide RNAs that recognize viral DNA when it invades. These RNAs help tell Cas9 to destroy the viral DNA.
The CRISPR-Cas9 system catalogs pieces of viral DNA upon first exposure to recognize it again upon second exposure.

Cas9 plays the star role in this system. It is an endonuclease, which means it can cut DNA, and it specifically works on invading DNA.

A specific piece of RNA guides Cas9 to the right sequences. This is how Cas9 can tell the difference between the invading DNA (which needs to be destroyed) and the bacterium’s own DNA (which definitely should not be destroyed).

Cas9 can work with natural guide RNAs or a human-made sgRNA.

The RNA needs to recognize the invading DNA, so it is actually made from the part of the invading DNA that was “saved” during the first exposure. The RNA also needs to stick to Cas9. In the natural bacterial system, one RNA does each part of the job. In CRIPSR technology, scientists have invented a single guide RNA (called an sgRNA) that can do both parts of the job.

Three-dimensional structure model of Cas9 holding a piece of target DNA (white) matching its guide RNA (pink). The natural Cas9 cuts the DNA, but modified Cas9's can do other things.

With protein engineering, parts of Cas9 can be removed and new parts can be added. This is how scientists have been able to take the naturally occurring CRISPR system and used it for new purposes.

By deactivating Cas9’s ability to cut DNA, we can use dCas9 and an sgRNA to bring specific functions to very specific locations in the DNA. Some of these functions are shown below:

Gene Regulatory Proteins  
Activation Domains can turn a gene “on.”  
Repression Domains can turn a gene “off.”
Base Editors  
Adenine Deaminase can mutate A to G.  
Cytidine Deaminase can mutate C to T.
Histone Modifiers  
Histone Methyltransferases and Deacetylases can hide genes.  
Histone Acetyltransferases and Demethylases can reveal hidden genes.
DNA Imaging  
Green Fluorescent Protein Domains can label specific sequences under the microscope.

With the ability to do all of these powerful things, a natural question to ask is, “In what circumstances, if any, should we edit human genomes?” Surely the idea of designer babies is a big red flag for obvious reasons, but getting CRISPR right could mean fully curing genetic diseases and giving people resistance to deadly viruses like HIV. I’m not here to tell you the answer, but I am here to tell you that we all need to be looking for it because CRISPR-Cas9 gene editing is already here.

For a fun song:

For more about the 2020 Nobel Prize in Chemistry:

For more on CRISPR-Cas9 ethics:

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