Summary: CRISPR-Cas9 is a revolutionary gene-editing tool derived from a bacterial immune defense. Using Cas9 as molecular scissors and a guide RNA as a targeting pattern, scientists can delete, replace, or insert DNA segments precisely. This technology enables breakthroughs in medicine, agriculture, and biotechnology, from curing genetic diseases to engineering resilient crops.
Picture this: you’re rummaging through your wardrobe to find that one slightly worn but very beloved shirt that never fails to make you feel good. A smile lights up your face when you see it; except, a quick examination reveals a snagged section right in the middle. Horrified, you yell for your mother the way one does when in a crisis.
She comes rushing in, fearing the worst. “But this is only a tiny tear,” she says nonchalantly, her voice heavy with relief when she sees you holding up the shirt. “I’ll fix it in no time,” she smiles.
Just like that, your old worn shirt now has a colourful patch of floral-printed fabric running across the centre — making it look newer than when you’d first purchased it!
Well, what if we told you that we could ‘alter’ our genes, albeit with a few extra steps, the same way we alter our jeans?
Humans figured out how to tinker with our genes roughly four decades ago when we realised that we could simply remove, add or alter our genome. But the one technique that truly revolutionised the gene editing scene in terms of ease and cost-effectiveness was CRISPR-Cas9. From deleting cancer-causing genes to adding spider DNA to silkworms to get them to produce fibre tougher than the Kevlar used in bulletproof vests, the sky’s the limit with this technology.
Now, if your curiosity is sufficiently aroused, let’s dive right into the fascinating tale of how we exploited what was primarily a bacterial immune response to suit our needs.
How does CRISPR-Cas9 work?
As its name suggests, this technology comes with two key players:
1. Cas9
Any tailoring project would be impossible without a trusty pair of scissors. Luckily, our cells come with in-built ‘molecular scissors’ called endonucleases. Once we decide which part of our genome we need to edit out, the Cas9 can just cut off the unnecessary bits.
2. Guide RNA
When you’re trying to fashion something new out of pieces of fabric, you don’t just want to run your scissors hither and thither. What really helps is a pattern that you can go back and refer to. In the world of CRISPR/Cas 9, that’s exactly what a guide RNA does.
Scientists created these readable forms of RNA that match the DNA segment we want to target. Not only does it fit with the DNA like pieces of a puzzle, but it also leads the Cas9 to this site so that it can cut it exactly where we want it to.
When we wish to merely ‘delete’ a mutation that is perhaps causing some disease in the body, we can design a guide RNA complementary to the defective site so it can bind to it. Then, the Cas9 follows it to the same spot and snips off the DNA strands.
What follows is the cell recognising the damaged DNA site and sewing it back together to repair it.
Conversely, if you wish to ‘alter’ your genome, you need to add something to replace the portion of your genome that needs to be gone. In this case, scientists introduce a customised DNA segment to take its place, before allowing the cell’s DNA repair machinery to do its thing.
How bacteria use CRISPR-Cas9 to defend themselves
Much like humans have white blood cells that swim through our bloodstream, looking for germs to launch attacks on and obliterate, scientists identified a comparable adaptive immune response in bacteria.
Bacteriophages are viruses that are natural enemies of bacteria. These microorganisms work by injecting their genetic material into the bacterial cell, where they proceed to multiply and ravage the host in the process. In order to protect themselves from the viral onslaught, bacteria began to store bits of the pathogen’s genetic material that it would pass down to progenies to “remember” past viral attacks — think a revenge notebook with photographs of every single person that hurt you.
A few generations later, the viral invaders’ DNA would become a part of the bacterial genome. Each segment of viral DNA would be flanked by repetitive palindromic sequences on either end. This results in what scientists call CRISPR arrays, where CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats (quite a mouthful, we know).
When a virus attacks the bacteria, the CRISPR DNA is translated into RNA segments that can recognise the viral enemy. While the sequence carries intel regarding the enemy, it still needs an accomplice to take down future invaders; enter: Cas9 or CRISPR-associated protein 9.
Cas9 can chop off the viral DNA into smithereens, easily dismantling it and allowing the bacteria’s digestive proteins to do away with the remains.
Scientists identified CRISPR arrays in bacteria and archaea (another group of microorganisms evolutionarily distinct from bacteria) in the 80s and 90s. But it wasn’t until 2012 when Jennifer Doudna and Emmanuelle Charpentier developed a powerful gene editing technology from it.
Since then CRISPR-Cas9 has been used to treat many diseases, including sickle cell, and has even been used to engineer crops that can adapt to the changing climate.
