Captain CRISPR is our hero of the gene editing world, but what makes him so special and what exactly even is CRISPR? Well in short, it is an immune system found in most prokaryotes for protection against invading pathogens (disease-causing bad guys). Whoa! That may seem complicated, but let’s break it down:

First, the immune system is the body’s defense system. So, for example, when you get sick with a cold, your body turns on specialized equipment to fight off the infection. The other thing it does is remember who was responsible so that you won’t get sick as badly from the same invader next time—this is called adaptive immunity. Essentially, the immune system creates a wanted poster so that if the same crook comes back for round 2, you’ll be ready!

Cool, so now we know what an immune system is, then what the heck is a prokaryote? Well, prokaryotes are simple life-forms, like bacteria. You’re made of multiple cells that are highly organized, almost like a factory. Basically, there are designated areas that have their own jobs and they’re walled off from the others by a protective layer. Prokaryotes don’t have such compartments, or specifically, membrane bound organelles, and are more like a pool with a bunch of toys floating around. Remember, they’re simple life-forms compared to you!

Finally, what are pathogens? These are viruses or bacteria that cause disease, but let’s stick with what viruses do in this case. Viruses are even simpler than prokaryotes, as all they principally possess is a copy of their genes. So, to “reproduce” they need to take over an organism that has that necessary machinery. They do this by inserting their DNA or RNA (a.k.a: their genetic instructions) into a host cell like a syringe, and from there, that genetic material will incorporate itself and force the cell to do its bidding (how despicable!). As for what DNA actually is, well think of it as a really long ladder where the two sides (or strands) are made of sugars and the rungs are made by chemicals called bases, of which there are 4: A, T, C, or G. Also, it is important to know that A will always bind to T while C will always bind to G. RNA is the more active form of DNA that has a wide variety of functions, including helping to build proteins. Unlike its DNA counterpart, RNA only comprises of one strand, and it also uses a U base instead of a T. When DNA is sourced to make RNA, we say that it is transcribed, and this occurs when a protein called an RNA polymerase binds to a promoter region on the DNA and begins to add complementary base pairs. For example, if one of the DNA strands has a sequence of AGCTA, then the RNA that’s transcribed will have a sequence of UCGAU.

Phew! Alright now that we’ve gotten the basics out of the way, we can finally talk about CRISPR. So, when a virus injects its DNA, many prokaryotes will spot that viral DNA, and stop it before it can get on with its dastardly deed. CRISPR-associated enzymes (or “Cas” for short) will identify and cut out a part of the viral DNA, called a protospacer, and it will stick it into a region of the prokaryotes’ genome (genome = the entire DNA within the cell) called the CRISPR array. On both sides of the protospacer are palindromic DNA sequences (palindromic means a word that is spelled the same when written backwards, like racecar!). For example, the top strand might read GGATCC, and the bottom would read CCTAGG. Now here’s what we’ve been waiting for: these are the clustered regularly interspaced short palindromic repeats, also known as the CRISPR! As a member of the CRISPR array, the protospacer is renamed to spacer, and a new palindromic repeat is built in front of it. In other words, the spacers are the wanted posters while the palindromic sequences are like shelves to keep things organized.

So, when a virus returns and inserts its DNA into the cell, pre-cRNA is constructed from the CRISPR array, and a different decently sized RNA called a tracrRNA, binds to a portion of a repeat that is next to the spacer. The pre-cRNA is then broken up by an enzyme called RNAase into tag teams of spacer and tracrRNA. Together these two join to make the guide RNA, and along with Cas enzyme(s), this complex hunts the complementary piece of viral DNA. (One such enzyme is Cas9, which is found in a bacteria called streptococcus pyogenes, but other species use different and sometimes even more than one Cas enzyme!). Once the CRISPR-Cas complex binds to the enemy DNA, the Cas enzyme cuts the viral DNA - It’s really like having a hole in the middle of a sheet of instructions, which makes the viral DNA dysfunctional. The target also has to have a Protospacer Adjacent Motif site (PAM) next to it, which is a short DNA sequence that the Cas enzyme has to spot before making a cut. When protospacers are brought into the CRISPR array, the PAM site doesn’t come with them. Because of that, the spacers don’t have a PAM site, which prevents the prokaryote from attacking its own DNA. How tactical!

This is how CRISPR acts as a natural “immune system” in bacteria against viral attacks! Scientists like Jennifer Doudna and Emmanuelle Charpentier saw this and realized it can be used as a tool to edit human and plant DNA. Being able to edit human DNA can mean curing diseases, and editing plant DNA can allow for making plants that are resistant to drought/disease!