CRISPR: Rewriting the Code of Life

Written by Tara Gandhi

Reviewed by Saisha Sikka 

Xu, Yuanyuan, and Zhanjun Li. “CRISPR-Cas Systems: Overview, Innovations and Applications in Human Disease Research and Gene Therapy.” Computational and Structural Biotechnology Journal, vol. 18, Sept. 2020, pp. 2401-2415, doi:10.1016/j.csbj.2020.08.031. PMCID: PMC 7508700.

Introduction:

DNA and RNA are the building blocks of life. DNA (deoxyribonucleic acid) is the molecule that carries the genetic instructions for building and maintaining all living organisms, while RNA (ribonucleic acid) helps translate those instructions into proteins by acting as a messenger and worker in the process of gene expression. A gene is a segment of DNA that codes for a specific protein or an RNA molecule. CRISPR, a revolutionary gene-editing tool that allows scientists to precisely cut, modify, or replace specific DNA sequences in living organisms is a significant breakthrough in the field of genetic engineering.

The premise of CRISPR Cas9 in Bacteria:

CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats, or more simply: short, repeating DNA sequences that are grouped together with special DNA parts between them. Initially found in bacteria, it is an essential component of the bacterial immune system that defends against viruses. 

How it works:

Step 1: When a virus invades a bacterium, the bacterial cell records a sample of the viral DNA by inserting it into the CRISPR array, so next time, the cell can recognise the invader after the sample is recorded in the CRISPR array, with a guide RNA that matches the sequence of the viral DNA. 

Step 2: A protein called Cas9, which is like a molecular scissor, binds to the RNA, and the RNA leads the Cas9 to the matching viral DNA in the cell. 

Step 3: Cas9 then cuts the viral DNA at the matching site, destroying it and stopping the virus from replicating.

Editing Genes in a Lab:

In the lab, scientists design the guide RNA to match a gene they want to edit. The guide RNA and Cas9 complex search for the target DNA sequence, and Cas9 cleaves the DNA at the target site, creating a break in the DNA. After the break, the cell tries to repair the DNA through 2 main ways:. Non-homologous end joining (NHEJ) is a way the cell repairs broken DNA by directly joining the ends back together. It works quickly but can make small mistakes that change or turn off a gene. Homology-directed repair (HDR) is a more accurate method that uses a matching DNA template to fix the break and can also be used to add or correct pieces of DNA.

Applications : 

CRISPR technology has been used in many species, from bacteria to mammals, including humans. It has been able to create replicating models of diseases like cancer, heart disease, and muscular disorders using animals, which help scientists better study the disease in a living organism, test treatments, and understand how diseases develop at the genetic and cellular levels. CRISPR can also be used in diagnostics for fatal diseases, identify and screen potentially harmful genes, and help treat blood diseases, infectious diseases, and even cancer. 

Challenges of CRISPR:

Despite its numerous benefits, CRISPR technology also opens a Pandora's box of negatives, one of which is off-target mutations. As a side effect, CRISPR can accidentally cut unintended parts of the genome or introduce new genes in unintended places, causing unwanted mutations and genome instability (the complete set of genes in a living cell). However, the use of error-detection methods such as Whole Genome Sequencing (WGS) is reducing this, along with the introduction of Cas protein variants that improve accuracy. Another issue is the efficient delivery of CRISPR components. Currently, physical methods like microinjections are used; however, they are not suited for real-world applications, and viral administration is efficient but poses safety and health risks.  Further, the body may mount an immune response against Cas proteins, and the double-stranded breaks they cause could activate the p53 pathway, which is a mechanism that stops cells from dividing or pushes them towards natural death. If cells modified by Cas proteins bypass these mechanisms, they could become cancerous; hence, monitoring this pathway is essential for safe gene editing. Overediting of genes is also a risk due to the overambitious nature of humans; however, anti-CRISPR proteins act as natural “off switches” that temporarily deactivate Cas9, improving safety. Human germline editing raises serious ethical concerns because changes would be heritable and affect future generations.

Conclusion:

Overall, CRISPR is a powerful gene-editing tool with the potential to save lives and to be applied in medicine, research, and diagnostics. It allows for precise, efficient and even curative treatments for previously incurable infections, diseases, genetic disorders, and even cancer. Nevertheless, as with any transformative technology, challenges remain, including off-target effects, delivery issues, immune responses and potential cancer risks. Hence, to harness its true potential, it must be used responsibly and under strict oversight. 

Citations

1. National Human Genome Research Institute. "CRISPR." Genetics Glossary, National Institutes of Health, 26 Oct. 2025, www.genome.gov/genetics-glossary/CRISPR.

2. Synthego. "What is CRISPR: Your Ultimate Guide." Synthego, www.synthego.com/learn/crispr.

3. National Human Genome Research Institute. "Ribonucleic Acid (RNA)." Genetics Glossary, National Institutes of Health, 26 Oct. 2025, www.genome.gov/genetics-glossary/Ribonucleic-Acid-RNA.

4. Lahalle, A. "The p53 Pathway and Metabolism: The Tree That Hides the Forest." Cancers, vol. 13, no. 1, 2021, www.ncbi.nlm.nih.gov/pmc/articles/PMC7796211/.