CRISPER Cas-9; The molecular ✂️ of the body

7 min readFeb 23, 2023

Imagine having the ability to Crtl+X your DNA, eliminating medical conditions right from their very core.

Picture this… 💭

You’re at a doctors office waiting for results from a NHS genetic test, a test which will tell you’ve inherited a cancer risk gene.

You test positive for a gene mutation, giving you high odds of developing cancer later in your life.

Immediately, your heart races as your palms start to sweat.

This changes your life; if only you had some control over it.

If only there way a way where you could erase this unwanted sequence from your DNA, cutting out the possibility for cancer/other genetic conditions.

What if I told you that there was 😱

Though we can’t yet completely cut disease out of our DNA, we’re getting closer to eradicating genetic illnesses such as some cancers, sickle cell disease, muscular dystrophy and more.

How you ask?

CRISPER Cas-9.

And no, despite it’s encryption-like name, CRISPER Cas-9 is a simple yet efficient method aspiring in the field of healthcare.

But first, what is a genetic disorder?🧬

In all simplicity, a genetic disorder is a harmful mutation (a change to a gene, AKA a pathogenic variant) which effects your overall genetic material. But, that’s just scratching the surface.

To go deeper, genes are made of DNA (deoxyribonucleic acid), which contain instructions for cell functioning and the characteristics that make you unique.

Each and every human is born thanks to a process called meiosis- the fertilization of male and female gametes (sex cells) in order to form a zygote. The zygote will ultimately grow into a human over the course of 9 months. During this process, both parents provide 23 chromosomes (structures that carry genes), resulting in them both being responsible for their baby’s genetic code. This is why genetic disorders may be inherited by a gene mutation from either one parent or both.

A test called Karyotyping used to detect genetic disorders using images of an individuals chromosomes. This particular imagen shows a female with Turner’s syndrome.

In theory, more than a mutated gene can caused potential harm 👎

Before birth, genes can be altered in the womb.

After birth, chromosomes damaged by either changes in the number or structure produce genetic irregularities. A combination environmental factors (such as smoke) have also been proven to play a part in the overall formation.

Regardless of how it was produced, all genetic disorders can be categorized in 1 of 3 ways:

Monogenetic: A single-gene disorder where a mutation affects one gene.
Chromosomal: This type affects the structures that hold your genes/DNA within each cell (chromosomes). Individuals with such conditions either lack or have duplicated chromosome material.
Multifactorial: This occurs when a mutation occurs in 2+ genes.

Knowing the above, it’s much easier to comprehend how CRISPR Cas-9 works, let’s dive into it! 🏊‍♀️

Think of our DNA as a long line of code (genetic code) on a computer, unique to each one of us. CRISPER Cas-9 acts as the mouse having the power to to edit it.

Just like all forms of genetic modification, CRISPR has the ability to make specific changes to the DNA of a cell, by “cutting” the strands at a specific location. The “CRISPR” part of the name refers to a specific sequence of DNA, and the “Cas9" part refers to an enzyme that acts like a pair of scissors.

CRISPR ( Short for…get ready for it; Clustered Regularly Interspaced Short Palindromic Repeats!!) refers to sequences of DNA found in the genomes in most prokaryotic organisms such as bacteria and archaea. In such organisms, sequences are used to store information about viral DNA that they carry and have encountered in the past in order to recognize and defend against future diseases.

The Cas-9 (CRISPR-associated protein 9) is an enzyme which exists in the genome in the form of a specialized protein which is able to recognize and bind to specific sequences of DNA.

Altogether, this technique works by guiding the Cas-9 protein to the needed location in the genome via RNA molecule AKA a guide RNA (gRNA). The Cas9 enzyme is then given the ability to create a cut producing a double-stranded break in the DNA. Through this cut, material can either be added, removed or altered.

It’s worth noting that there are different types of Cas enzymes that can be used in the CRISPR system, each with its own unique properties and abilities. For example, Cas12a and Cas13a are more specific than Cas9 and they can target RNA instead of DNA. DNA is made up of intricate chemicals but right now, we’re only interested in the nucleobases. Each genetic code is made up of a combination of 4 nucleotides/bases; Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). These chemical bonds act like ladder steps, holding strands of DNA together.

Ladder-like representation of nucleotide bases in DNA (as well as RNA) strands).

Lets get more scientific 🥼

As CRISPR is natural defense mechanism found in bacteria, it acts similarly to a lysogenic virus. This allows it to recognize and destroy invading viral DNA. It is composed of two main components: the CRISPR array, which contains short, repetitive sequences of DNA, and the Cas (CRISPR-associated) genes, which encode for Cas proteins. In addition to this, much like the human immune system, the CRISPR sequencing “memorizes” any sequences needed by storing a spacer sequence AKA viral DNA. This is essential as when a virus with the same sequence attacks again, CRISPR is ready to find the DNA and replace, renew or cut it out.

The Cas9 protein is the most commonly used protein in CRISPR gene editing. It is a type of endonuclease, which means that it can cut DNA at specific locations. The CRISPR-Cas9 system works by first identifying the specific DNA sequence that needs to be cut. This is achieved by using a small guide RNA (gRNA) fitting to the desired target DNA sequence.

The gRNA used in CRISPR-Cas9 gene editing is typically 20 nucleotides in length, and it is designed to be complementary to the target DNA sequence. The specificity of the gRNA is essential for precise gene editing because if it binds to an unintended location in the genome, it can lead to off-target effects that can cause unintended mutations.

With the suitable gRNA in place, it can later binds to the Cas9 protein, forming a complex that can recognize and bind to the specific DNA sequence. A diagram depicting the regions where the Cas enzymes are located alongside the gRNA.

A diagram depicting the regions where the Cas enzymes are located alongside the gRNA.

Once the complex has bound to the target DNA, the Cas9 protein cuts both strands of the DNA, creating a double-stranded break. This break activates the cell’s natural DNA repair mechanisms, which can result in one of two outcomes;

1. The first outcome is non-homologous end joining where the broken ends are simply rejoined. This process can cause small insertions or deletions in the DNA sequence, resulting in a frameshift mutation. This type of mutation can lead to the disruption of the gene’s function.

2. The second outcome is homology-directed repair where a repair template is provided along with the gRNA and Cas9 protein. The repair template contains the desired genetic sequence that will replace the cut DNA. HDR is a precise mechanism for repairing the DNA and results in the replacement of the cut sequence with the desired sequence.

A complete diagram depicting a simplified process of gene sequence mutation. The aboce showcases both deleted (disrupted) sequences as well as altered ones with newly inserted material.

As a whole, CRISPR moreover acts as molecular scissors allowing us to mainly cut but also edit and add sequences to the body!

Knowing this, what does the future hold? 🤔

Through the realm of genetic editing, especially CRISPR-Cas9 is fairly new, countless uses, techniques and implementation are being developed. Concerns of ethicality often come into consideration but CRISPR definitely hold a promising future. Below are some of my favorites;

1. Engineering modified organisms – > CRISPR can be used to engineer organisms with desirable traits, a strand which many times comes under fire on an ethical standpoint. An revolutionary way this technique can be used is giving crops qualities such as drought or disease resistance in order to produce higher crop yields. This technology can also be used to create sustainable products such as bacteria that produce biofuels or enzymes that break down plastic.

2. Biomedical/pathobiological research – > CRISPR can be used to study the function of specific genes or cells through turning them on/off or modifying them in a specific way. This can be imminent in terms of neural networks for example, allowing us to uncover the brain.

3. Gene drives – > CRISPR can be used to create gene drives, which are genetic modifications that spread rapidly through a population. This technology could be used to control or eradicate disease-carrying insects, such as mosquitoes that transmit malaria or dengue fever. Furthermore, this can lead to the eradication of countless genetic diseases in a multitude of organisms including humans.

4. The detection/treatment of genetic diseases. – > CRISPR can be used to treat genetic diseases by correcting mutations in the DNA that cause the disease. By repairing or replacing the defective genes, scientists hope to cure diseases such as sickle cell anemia, Huntington’s disease, and cystic fibrosis.