CRISPR #3 - How Humans Use It

What could possibly be an application of a defense mechanism like CRISPR found in bacteria? To answer that, we first need to understand how the CRISPR system works. In the earlier blog, we understood that at a high level. But now we need to understand the details.

 

Scientists identified the pieces that played different roles:

  • tracrRNA: It acts as a handle to latch onto the attacking virus.
  • CRISPR RNA, aka crRNA: It uses the latched on tracrRNA and in turn acts as the guide to identify the attacking virus. Think of it like the torch that highlights the point to attack.
  • CRISPR associated enzyme, aka Cas enzyme: This is the scissor like enzyme that uses the guide crRNA to cut up the attacking virus.

See how it works? The tracrRNA attaches itself to the virus. The crRNA uses the tracrRNA to identify the virus to the Cas enzyme, who then cuts up the virus, scissor like.

 

Now we can get to the application that humans have devised based on this ancient bacterial defense system. Say, we know that a certain gene can cause a severe, medical problem. We would like to be able to edit the DNA of the person to replace this harmful gene. Here’s what CRISPR made possible:

  • Step 1: Create a guide RNA with the same genetic sequence as the gene that you want to edit in the human cell.
  • Step 2: The Cas enzyme would then use the guide RNA to “find” the gene in question within the cell; and cut it out, scissor like.
  • Step 3: This step isn’t from nature. Insert the gene of your choice at the location from which the offending gene was just cut out.

CRISPR had given us the idea on how to build a mechanism to do targeted and precise gene editing in human cells. That was the application.

 

It would set off a vicious competition among different scientists trying to commercialize this new found use. It would also set off ethical debates on the dangers and desirability of gene editing in humans. But the genie was now out of the bottle and it could not be put back in.

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