Most recombinant DNA technology involves the insertion of foreign genes into the plasmids of common laboratory strains of bacteria. Plasmids are small rings of DNA; they are not part of the bacterium’s chromosome (the main repository of the organism’s genetic information). Nonetheless, they are capable of directing protein synthesis, since the nucleotide sequence of DNA encodes instructions that are transcribed into RNA and translated into proteins, and, like chromosomal DNA, they are reproduced and passed on to the bacterium’s progeny. Thus, by incorporating foreign DNA (for example, a mammalian gene) into a bacterium, researchers can obtain an almost limitless number of copies of the inserted gene. Furthermore, if the inserted gene is operative (i.e., if it directs protein synthesis), the modified bacterium will produce the protein specified by the foreign DNA, which may alter the organism’s observable traits depending on the protein’s function.

A subsequent generation of genetic engineering techniques that emerged in the early 21st century centred on gene editing. Gene editing, based on a technology known as CRISPR-Cas9, allows researchers to customize a living organism’s genetic sequence by making very specific changes to its DNA. This typically is accomplished by targeting particular nucleotide sequences and introducing, removing, or replacing segments of genetic material; such changes can modify how a gene is expressed or alter the structure of the protein it encodes, which in turn may affect the organism’s traits. Gene editing has a wide array of applications, being used, for instance, in the genetic modification of crop plants and livestock and in the modification of laboratory model organisms (e.g., mice).

The correction of genetic errors associated with disease in animals suggests that gene editing has potential applications in gene therapy for humans. Gene therapy is the introduction of a normal gene into an individual’s genome in order to repair a mutation that causes a genetic disease. When a normal gene is inserted into a mutant nucleus, it most likely will integrate into a chromosomal site different from the defective allele; although this may repair the mutation, a new mutation may result if the normal gene integrates into another functional gene. If the normal gene replaces the mutant allele, there is a chance that the transformed cells will proliferate and produce enough normal gene product for the entire body to be restored to the undiseased phenotype.

Applications

Genetic engineering has advanced the understanding of many theoretical and practical aspects of gene function and organization. Through recombinant DNA techniques, scientists have been able to engineer bacteria and other host cells to produce important human proteins in a controlled and scalable way. By inserting human genes into microbial DNA, these organisms can function as biological factories, synthesizing substances such as insulin, human growth hormone, interferons, and vaccine components. This approach has largely replaced earlier methods that relied on extraction from animal tissues, improving safety, purity, and availability. It has also enabled the large-scale production of life-saving therapies for conditions such as diabetes, growth disorders, viral infections, and certain cancers, marking a major advance in pharmaceutical development and personalized medicine.

Plants can be genetically engineered to acquire traits that are not naturally present or are inefficiently expressed. For example, researchers have explored introducing or enhancing genes involved in nitrogen fixation, enabling crops to utilize atmospheric nitrogen more effectively and reducing dependence on synthetic fertilizers. Other modifications have focused on pest and weed control, such as inserting genes that produce insecticidal toxins (as in some varieties of corn and cotton) or genes that confer resistance to herbicides. Similarly, genetic techniques can be used to correct or compensate for defective genes that limit growth, yield, or resistance to stress. By replacing or modifying dysfunctional genes, scientists can improve traits such as disease resistance, drought tolerance, and nutrient use efficiency. These approaches not only increase crop productivity but also support more sustainable agricultural practices by lowering input requirements and minimizing environmental impact.

Controversy and ethical issues

In 1980 the “new” microorganisms created by recombinant DNA research were deemed patentable, and in 1986 the U.S. Department of Agriculture approved the sale of the first living genetically altered organism—a virus, used as a pseudorabies vaccine, from which a single gene had been cut. In the decades that followed, patents were granted for numerous genetically engineered bacteria, plants, and other organisms.

Advances in biotechnology, particularly the development of precise gene-editing tools such as CRISPR-Cas9 in the 21st century, have further expanded the range of patentable innovations. At the same time, patents on GMOs—especially crops—have remained controversial, raising ongoing questions about corporate control of seeds, farmers’ rights, food security, and access to biotechnology. These debates continue to shape regulatory policies and public perception worldwide.

Concerns about genetic engineering have focused on the potential introduction of harmful traits, such as antibiotic resistance or toxin production, particularly in microorganisms. While these risks are taken seriously, modern biotechnology is subject to extensive regulatory oversight and safety testing. Debate continues over the environmental and health impacts of GMOs, including crops, though many have been widely adopted after rigorous evaluation. For example, biofortified crops such as golden rice have been developed to address nutritional deficiencies, with varying levels of implementation worldwide. The release of GMOs, including insects such as mosquitoes, remains an area of active research and public discussion, particularly with the advent of gene-editing technologies such as CRISPR.

Advances in gene-editing technologies notably have also intensified long-standing ethical and social debates surrounding the genetic modification of humans. Concerns have focused especially on germline editing, in which changes to sperm, eggs, or embryos could be inherited by future generations, raising questions about unintended consequences and the potential for altering traits such as disease susceptibility, cognition, or physical appearance. While research has explored the possibility of reducing the risk of inherited conditions, the long-term effects of such interventions remain incompletely understood. In response, many countries and international organizations have established guidelines or restrictions governing human gene editing, particularly for heritable applications, though global consensus on its appropriate use continues to evolve.