An Introduction to Genetic Engineering: Once Science Fiction, Now a Reality
- ecure
- Aug 13, 2020
- 5 min read
Updated: Sep 3, 2020
Few advances have rocked the realm of biology as immensely as those of genetic engineering. Here, we will cover an overview of the definition, implications, and benefits of genetic engineering.
Written by: Shreya
In 2018, humanity forever changed the course of its evolutionary trajectory by surmounting natural selection and actively interfering with its own genetic code. Chinese scientist Dr. He Jiankui and his team used an up-and-coming gene editing tool known as CRISPR-Cas9 in an attempt to edit HIV-resistance into the embryos of two twin girls, and their work took the world by storm, raising questions about the ethical boundaries of human genome editing as well as the limits of its transformative potential.
So what exactly is genetic engineering, and how does it differ from the often contentious term “genetic modification”? Genetically-modified organisms (GMOs) are individuals whose DNA has been altered in a way that does not occur naturally (1), and they can be derived through a multitude of methods, including selective breeding, induced mutagenesis, protoplast fusion, and gene editing/recombinant DNA. Genetic engineering, on the other hand, is a sub-branch of genetic modification that involves any procedure where technology is used to directly alter the genome of an organism. Let’s take a look at what exactly constitutes genetic modification.
Selective Breeding: Also known as artificial selection, selective breeding is the act of breeding organisms that express a desired trait in order to produce offspring that possess that characteristic. Examples include breeding fast dogs for herding and protection, heat resistant cows for ranching, and blemish-free tomatoes for aesthetic crops.
Induced Mutagenesis: This is a technique to introduce new variants of a species to a gene pool by exposing organisms to mutagens, or substances that can cause mutations (ie. UV light). The mutation can render the gene nonfunctional (which is known as gene “knockout”) or create mutant strains with useful properties and/or improved characteristics. In order to make a very specific alteration to the DNA, the DNA fragment of interest can be genetically engineered to contain the desired mutation and then incorporated as a replacement to the original DNA of the organism, which is known as site-directed mutagenesis.
Protoplast Fusion: Protoplasts are plant cells whose cell walls have been removed, enabling them to fuse together to form what’s called a somatic hybrid. This form of genetic modification yields new hybrid offspring that can carry desirable traits from each “parent”. Since fusion does not involve any sexual component, the protoplasts do not need to be from the same species, allowing the hybrid to contain beneficial traits from each species (5). Fusion can be induced under polyethylene glycol or through an electric field (electrofusion).
Recombinant DNA/Genetic Engineering: Genetic engineering is the direct alteration of DNA through technology. When introducing foreign DNA into an organism, the foreign sequence must be incorporated into the current genome, resulting in recombinant DNA. In order to do so, a restriction enzyme cleaves the individual’s gene fragment at the region on the organism’s genome where new DNA will be implanted (known as the restriction site). The foreign DNA is then introduced to the cleaved fragment and joined at its ends by the enzyme ligase, yielding a transgenic organism with a new sequence of DNA. The process is similar to the “cut/paste” mechanism on digital documents. Restriction enzymes “cut” or remove old DNA and “paste” or replace it with foreign DNA at the site where the change is needed.
Restriction enzymes are site-specific, meaning that they only cleave DNA at a particular sequence; their regard for sequence means that the applicability of each individual enzyme is extremely limited (6). Researchers found a solution to this constraint by artificially engineering a restriction enzyme with an affinity to multiple domain sites– zinc finger nucleases (ZFNs). Zinc fingers are proteins found in eukaryotes that can bind to DNA, and when attached to domains that are artificially produced to target specific DNA sites, form a ZFN complex that can cleave DNA at specific locations. Similarly, TALENs, or transcription activator-like effector nucleases, are restriction enzymes that can be engineered to cut specific DNA sequences. The Tal protein is combined with an endonuclease specific to a particular DNA sequence, and the complex combines to cleave DNA at desired locations.
As novel as it may seem, genetic engineering has already woven itself into multiple domains of today’s society, from the food we consume to medications in our prescriptions. For instance, 82% of corn currently produced in the United States is Bt corn (2), or corn with genetically-engineered insect resistance, and that number is only growing by the year. Bt, or Bacillus thuringiensis, is a bacterium that produces crystals of proteinaceous insecticidal δ-endotoxins (abbreviated Cry proteins) that are highly toxic to certain pestilent insect species. In order to create a pest-resistant variety of corn, scientists selected the genes responsible for Cry proteins in Bt and incorporated them into embryonic corn cells, resulting in mature corn plants that expressed Cry proteins in their leaves (3). As a safe, efficacious, and natural alternative to chemical insecticides, Bt-corn provides health benefits to consumers and growers, yield protection, and environmental relief. Similarly, genetic engineering has assisted in amplifying insulin production for diabetics. In order to produce insulin treatments for patients whose bodies are unable to generate their own supply of the hormone, Dr. Herbert Boyer ligated the human gene for insulin to a plasmid present in E. Coli bacteria in 1978 (4). The transgenic cell rapidly divided, resulting in millions of bacterial cells producing human insulin that could be purified and distributed, and his insulin treatment was approved for public use by the FDA by 1982.
In addition to its many developed applications, genetic engineering is in the midst of establishing several new frontiers, the most notable of which is in gene therapy, or the treatment of genetic disorders through removing and replacing aberrant DNA. There are over 10,000 disorders that could be treated through genetic engineering, and a potential medium was just recently discovered in 2012: the CRISPR-Cas9 genome editing complex.
The complex’s potential in the field of genome editing was unearthed by researchers Dr. Jennifer Doudna, Emmanuelle Charpentier and Feng Zhang, who co-invented the first CRISPR/Cas9 gene-editing technology, but its original purpose is actually as a defense mechanism against viral attacks in bacteria. If a bacterial cell survives a viral invasion, it cuts off a snippet of the virus’s genome and stores it as a CRISPR array. These arrays enable the bacterium to remember the viral strain; if the virus attacks again, the bacterium transcribes the CRISPR array into an RNA segment that then binds to its complementary sequence on the viral DNA. An enzyme called Cas9 that is bound to the RNA then cleaves the viral DNA, rendering the virus nonfunctional. In the lab, scientists can manufacture CRISPR arrays that coincide with any complementary DNA sequence of interest and use Cas9 to target, cut, and replace both strands of that sequence.
A diagram of the RNA segment (yellow) bound to its complementary sequence on the viral DNA and Cas9 (represented by scissors) conducting a double-stranded cut on the DNA
As a fast, precise, and cheap mode of genome editing, CRISPR-Cas9 presents the potential to remedy thousands of genetic disorders, improve crops, increase food production, mitigate climate change, and much more. However, it also raises several ethical questions on where the boundaries of our control over genetics should lie. Throughout this series of articles, we will dive into the benefits of and concerns around CRISPR in future articles, considering both the potential for betterment offered by genetic engineering and the moral complexities laced within the questions of whose betterment are we working towards and whether those benefits will be sustained in the future.
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