CRISPR: A New Way for Scientists to Edit DNA

Just like humans, bacteria can get sick. Some bacteria have a defense system called CRISPR/Cas9 that protects them from infection with viruses. Over the last few years, scientists have adapted this bacterial defense system to be used in the laboratory to alter the DNA of various organisms. This article will explain how CRISPR/Cas9 is used to edit genes and will provide examples of how this technology is useful. Experiments using CRISPR/Cas9 must be carried out ethically, that is, scientists must ensure that all research respects human rights and animal welfare and complies with the law.


CRISPR/CAS : THE DEFENSE SYSTEM OF BACTERIA
No one likes to get sick. We cough, sneeze, have fever, and pain. All these things are signs that the body's defenses, called the immune system, are trying to protect us from tiny, disease-causing invaders, including viruses causing diseases like the common cold or Covid-. Did you know that viruses can infect many other organisms, too-including tiny bacteria? The viruses that infect bacteria are called BACTERIOPHAGE A type of virus that only infects bacteria.
bacteriophages. Although it is quite di erent from the human immune system, bacteria also have a defense system that protects them against infections. This system is called CRISPR/Cas [ ]. When a bacteriophage infects a bacterium, the virus injects its genetic material into the bacterium. The bacterium recognizes this invader, and with the help of CRISPR/Cas , cuts the genetic material into pieces to stop the infection. Afterwards, the bacterium keeps a piece of the bacteriophage's genetic material, so if the same bacteriophage infects again, the bacterium can react more quickly to the invader.

WHAT IS CRISPR/CAS ?
The name CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats. This sounds like a mouthful, but keep reading and you will understand this complicated name! Like all organisms, bacteria contain a genetic material called DNA. DNA is made of four di erent building blocks, called nucleotides: NUCLEOTIDE One of the four building blocks that make up DNA. They are called adenine (A), cytosine (C), guanine (G), and thymine (T). adenine (A), guanine (G), cytosine (C), and thymine (T). The sequence of these four building blocks is a code for the instructions to create the organism, similar to the way sequences of letters form words. In some places the code forms palindromic sequences, which can

PALINDROMIC SEQUENCE
A specific order of nucleotides that reads the same, forward or backward. For example, TAGCGAT is a palindromic sequence. be read the same way forward or backward. The word "kayak" is an example of a palindrome. A DNA palindrome could look like TAGCGAT. Just like the word kayak, TAGCGAT reads the same both forward and backward. Now, imagine there were several copies of the TAGCGAT sequence in the DNA. You could call them short palindromic "repeats" because they are repeated. If those palindromic repeats are found in groups throughout the DNA code, then we call them "clustered." Since those palindromic repeats are separated by other DNA sequences, they are called "interspaced." Put everything together, and you just discovered Clustered Regularly Interspaced Short Palindromic Repeats, or CRISPR! Last, Cas-is included in the name because it is an important protein used in the CRISPR system, which will be described below.

HOW DOES CRISPR/CAS HELP SCIENTISTS?
You may be wondering why the defense system of bacteria is so important to scientists. Scientists figured out how to use the kids.frontiersin.org October | Volume | Article |  CRISPR/Cas system to edit, or change the sequence of, DNA. This discovery of how to use CRISPR/Cas in the laboratory was extremely important for the scientific community-so important that, in , two researchers who helped discovered it (Emmanuelle Charpentier and Jennifer A. Doudna) received the most important scientific award-The Nobel Prize in Chemistry.
As we mentioned, the nucleotides that make up DNA form a code that holds the instructions to make the entire organism. Sections of DNA that code for a specific trait, for example the color of a dog's fur, are called genes. Let's say we know which gene controls the color of a dog's fur. If the order of the nucleotides in that gene is changed, the dog's fur color would change. Why? Because we have changed the instructions for making the dog's fur color ( Figure ).
To understand how we use the CRISPR/Cas system to edit genes, you must first understand the parts of this system and how they work. First, scientists must know the order of nucleotides, or the sequence, of the gene they are interested in editing. Luckily, there are websites with this information. Once the gene sequence is known, the first piece of the CRISPR/Cas system that we must use is a special protein called Cas .

CAS
A protein that acts like scissors to cut DNA. This protein is a required part of CRISPR/Ca which is why it is included in the name! Cas acts like scissors to cut the DNA. Cas can cut DNA anywhere, but we only want it to cut the gene we are interested in. Getting the Cas to the right location requires a component called guide RNA, which

GUIDE RNA
A small RNA strand that leads Cas to the location it needs to cut the DNA. acts like GPS coordinates directing Cas to the sequence we want to cut. We design the guide RNA from the sequence of the gene, so that Cas will only cut the DNA at that exact location ( Figure A). Near the guide RNA is a PAM site, which is a three-nucleotide sequence that  on humans yet. If CRISPR/Cas gene therapy is shown to be safe for humans, it may be a promising way to help people su ering from diseases caused by mistakes in the DNA.
How cool would it be to see a wooly mammoth or a dinosaur walking around? Scientists believe they can use CRISPR/Cas to bring back extinct animals! Recently, scientists have successfully used CRISPR/Cas to change elephant cells to contain the DNA of the extinct wooly mammoth. These changes may help give elephants wooly mammoth traits, like wooly fur, which would help them survive in colder weather.
Companies can also use CRISPR/Cas to create new products. Imagine how tasty a spicy tomato would be! Researchers are exploring how to change tomato DNA so that it can produce capsaicin, the substance that makes chili peppers spicy. Salsas will never be the same! Other scientists are using CRISPR/Cas to help develop new varieties of plants like corn. These new varieties are able to grow even during extreme weather events like drought and heat waves caused by climate change.

GREAT POWER CARRIES GREAT RESPONSIBILITY
The applications for CRISPR/Cas are exciting, but the scientific community wants international rules to make sure that this technology is used in a safe and ethical manner.

SUMMARY
The CRISPR/Cas system keeps bacteria safe from viruses. Now, it is also being used by scientists to edit the DNA of organisms in the laboratory. The ability to change DNA in the laboratory allows us to learn more about how DNA works, to treat diseases, and to develop new products. There are many benefits of CRISPR/Cas technology if it is used in a responsible way. However, we must follow ethical rules to ensure no one is hurt in the process. Sometimes, new technological developments occur so fast that neither scientists nor policy makers can fully prepare. As scientists, we must always think about the risks of what we do as we strive to improve human lives.

CONFLICT OF INTEREST:
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
COPYRIGHT © Marnik, Bautista, Drangowska-Way, Simopoulos and Merritt. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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AUTHORS ELISABETH A. MARNIK
Elisabeth is an assistant professor of molecular biochemistry at Husson University. She uses small worms, called C. elegans, to understand how cells become other cells. She does this by using CRISPR to delete parts of the worm's DNA and studying the changes that result! In her free time, she loves running while pushing her toddler in the stroller, reading, and hiking. *marnike@husson.edu; † orcid.org/ --- She is interested in creating and applying new computational tools for analyzing biological data. Currently, she is researching how the collection of bacteria, viruses, archaea, and fungi that live in our intestines can influence our health. Specifically, she is interested in how these microbial communities change when we take di erent medications. In her spare time, Caitlin loves to cycle around the beautiful Ottawa-Gatineau region, experiment with new recipes, and hang out with her two rescue cats. csimopou@uottawa.ca; † orcid.org/ ---

THOMAS J. S. MERRITT
Thomas is a professor of chemistry and biochemistry at Laurentian University, in Northern Ontario, Canada. He is interested in how changes in DNA lead to di erences in an organism's traits and how DNA changes a ect individuals di erently. Much of his research is done using fruit flies, but some projects involve bacteria, and the lab is occasionally SNOLAB, a particle physics laboratory located km underground. Outside of the lab, he spends as much time as he can on the water, paddling, rowing, and organizing a rowing program for people with disabilities. tmerritt@laurentian.ca