CRISPR: An Old Biological Treasure
Scientists over the years have found new and exciting discoveries in areas such as biomimicry, neuroscience, and even gene expression and protein synthesis. However, none of these discoveries can even begin to compare to one of the most exciting new discoveries: CRISPR. This archaic but simple mechanism found in bacteria could be the key to solving the most complex problems puzzling geneticists today.
What is CRISPR?
First, let’s break down what CRISPR is. As a broad overview, CRISPR acts as the bacteria’s immune system. It neutralizes bacteriophage(virus that attack bacteria) by cutting a DNA segment and causing a malfunction in the virus. The bacteria then stores the cut DNA segment so it can recognize any future threats by the same virus. Now let’s go a bit deeper than this.
CRISPR stands for “Clustered Regularly Interspaced Repeating Palindromes”. It is a long strand of genetic code made up of viral DNA from bacteriophage that have previously attacked the bacteria. In order to separate one viral DNA from the other, the bacteria places an identical strand of genetic code between each viral piece of viral DNA. These strands are also palindromes, meaning that they read the same both forwards and backwards. The unique viral DNA is known as “spacer DNA” because it spaces out each repeating portion. This is why CRISPR stands for “Clustered Regularly Interspaced Repeating Palindromes”. The repeating palindromes are part of the bacteria’s DNA and in between each one is a unique segment of viral or bacteriophage DNA.
How Does CRISPR Work?
How does bacteria kill foreign pathogens and store their genetic code in a database? This is possible thanks to the help of enzymes and proteins known as Cas (CRISPR-associated) enzymes. There are numerous Cas proteins, all in charge of cutting unique segments of viral DNA. If the bacteria comes across a new pathogen, it will detect the bacteriophage as an invader and create new Cas protein to attack it. The Cas protein will cut a segment of the viral DNA, known as a protospacer, to insert into the bacterial genome as part of the CRISPR system.
However, what if the bacteriophage returns for another attack? In this case, the bacteria transcribes the DNA it has stored into crRNA(CRISPR-RNA). The bacteria also generates a corresponding Cas-protein in charge of cutting up that particular bacteria. The crRNA helps the Cas-protein identify which part of the bacteriophage to cut.
There’s A Problem However…
When studying CRISPR, scientists came across a puzzling question: When searching for the matching DNA sequence in the viral genome, how does the bacteria refrain from cutting the viral DNA that is stored in its own genome? After all, the spacer sequences in the bacterial genome are taken from different segments of the viral genome. This is where the PAM comes in.
PAM stands for Protospacer-Adjacent Motif. It sounds fancy, but it’s simply a small sequence of two to three base pairs that follows the targeted segment in the viral genome. This sequence gives the signal that it is ok for the CRISPR mechanism to cut the segment of DNA. The spacer stored in the bacteria’s genome is followed by a repeating palindrome, not a PAM sequence, preventing the CRISPR mechanism from cutting its own genome. Oftentimes, specific proteins known as Cas-1, Cas-2, and Cas-9 will look for the PAM sequence and only then check to see if the bacteriophage DNA matches the spacer sequence before cutting. This leads to a more efficient system: instead of unraveling the bacteriophage DNA in search for a sequence, the Cas-proteins can simply search for a PAM sequence and then try to discern whether the two genetic codes match.
CRISPR Development
When scientists started to understand the CRISPR mechanism, they realized that it could be used for a myriad of technological innovations. And so the search was on. The most famous discovery till date is the CRISPR Cas-9 system. This system was identified and developed by Jennifer Doudna and Emmanuelle Charpentier. Doudna and Charpentier studied the bacteria streptococcus pyogenes (S. pyogenes). S pyogenes has an incredibly simple system: it contains one Cas-protein, Cas-9, attached to a crRNA and a tracr-RNA(trans-activating RNA), which helps bind the protein and the crRNA. This tracr RNA base pairs with the crRNA to form something called a guide RNA (gRNA). The Cas-9 protein latches onto the tracr-RNA while the crRNA guides the Cas-9 protein to the bacteriophage. Doudna and Charpentier realized that if they could replace the crRNA with any other strand of code, the system could remove any segment of DNA required. The DNA would try to repair it, but it would likely create a mutation, inactivating that gene. This CRISPR Cas-9 system with a Cas-9 protein and gRNA grew vital to producing revolutionary technology in medicinal and biotech industries.
What is CRISPR’s Purpose?
Since its discovery and development, CRISPR has been used in various fields and continues to be a vital addition to new and emerging fields. CRISPR has proved useful in agriculture by helping vaccinate industrial cultures and engineer new probiotic cultures. It has also been used to make crops more tolerant to harsh conditions.
Of course, CRISPR looks to have a big future in medicine. For example, it can prevent disease by sterilizing disease vectors, which are the bacteria’s form of transmission. Also, scientists have proposed creating gene drives, genetic systems that can increase the chance of a parent passing a certain trait to their offspring. However the latter suggestion is a very polarized topic. While some say it could benefit the world and all of society, others raise ethical concerns, questioning not only the legality, but the morality of modifying someone’s genetic makeup. CRISPR definitely has a future in the world of medicine and genetics, but just how far can it influence these fields? That’s a question we can only answer in the future.
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