Genetic Engineering
Genetic engineering is the deliberate changing of a living thing's DNA. Where traditional plant and animal breeding takes many generations to select for useful features, genetic engineering can add, remove or change genes directly in a single generation. Scientists have used genetic engineering to produce human insulin from bacteria, create disease-resistant crops, develop gene therapies for serious illnesses, and even bring back features of extinct species. In recent years a new tool called CRISPR has made gene editing easier and cheaper than anyone thought possible.
- First GMO1973A bacterium with a gene from another bacterium
- First commercial GM productInsulin1982, made by genetically modified bacteria
- CRISPR discovered as a tool2012Doudna and Charpentier
- Nobel for CRISPR2020Doudna and Charpentier shared the chemistry prize
- Common GM cropsSoya, maize, cotton, canolaGrown on millions of hectares
- First gene therapy approved1990For a rare immune deficiency in children
What is genetic engineering?
Genetic engineering means changing the genes of a living thing on purpose. Scientists can:
- Add a gene from one organism to another (like adding a glow-in-the-dark gene from a jellyfish into a mouse).
- Remove or switch off a gene that is causing a problem.
- Edit the letters of a gene to correct a mutation or add new abilities.
The result is a genetically modified organism (or GMO). The technology has been around since the early 1970s, but it has become enormously more powerful and easier to use in the last decade thanks to CRISPR.
Bacteria that make medicine
The very first big success of genetic engineering was making human insulin. Insulin is a hormone that controls blood sugar; people with type 1 diabetes need to inject extra insulin every day to stay alive. Before the early 1980s, the only way to get insulin was to extract it from the pancreas of slaughtered cows or pigs. It worked but was expensive, sometimes triggered allergic reactions, and supply was always tight.
In 1982, scientists at the company Genentech inserted the human insulin gene into common gut bacteria (E. coli). The modified bacteria started producing human insulin as if it were their own protein. By growing huge tanks of these bacteria, the company could produce purified human insulin cheaply and in any amount needed. Insulin-dependent diabetics all over the world have used this insulin ever since, and similar techniques now produce dozens of other medicines (growth hormones, clotting factors, many vaccines).
Genetically modified crops
By far the biggest use of genetic engineering is in agriculture. Major GM crops include:
- Bt cotton and Bt maize: carry a gene from a bacterium (Bacillus thuringiensis) that produces a natural insecticide, dramatically reducing the need for chemical sprays.
- Roundup-Ready soya and canola: engineered to survive a particular weedkiller, allowing farmers to spray weeds without harming the crop.
- Golden rice: engineered with vitamin A genes from carrots and bacteria, aimed at preventing vitamin A deficiency that causes blindness in poor countries.
- Bruise-resistant Arctic apples: engineered so they do not turn brown when cut.
GM crops are widely grown in the US, Brazil, Argentina, Canada and many other countries. Europe has been much more cautious and has only approved a few GM crops for cultivation. The debate over whether GM crops are safe and ethical is one of the most heated arguments in modern science.
CRISPR: rewriting genomes
The biggest revolution in genetic engineering came in 2012, when American biochemist Jennifer Doudna and French microbiologist Emmanuelle Charpentier figured out how to use a bacterial defence system called CRISPR-Cas9 as a precise gene-editing tool. CRISPR uses a piece of guide RNA to find a particular DNA sequence in a genome, then a protein scissors called Cas9 cuts the DNA at exactly that spot. The cell's own repair machinery then either heals the cut (knocking out the gene) or can be tricked into inserting a new sequence.
CRISPR turned out to be cheap, fast and easy to use compared to all previous gene-editing techniques. A single experiment that would have taken months and tens of thousands of pounds in 2010 can be done in days for under a hundred pounds today. Doudna and Charpentier shared the 2020 Nobel Prize in Chemistry.
Gene therapy
One of the most exciting medical uses is gene therapy: fixing a faulty gene directly inside a patient. The first approved gene therapy was in 1990, for children with a rare inherited immune deficiency. Recent years have brought a flood of new gene therapies:
- Luxturna (2017): restores some sight to children born with a rare form of inherited blindness.
- Zolgensma (2019): treats spinal muscular atrophy, a fatal nerve disease in babies. Often called the world's most expensive drug.
- Casgevy (2023): the first CRISPR-based gene therapy, approved for sickle cell disease.
The ethical debate
Genetic engineering raises some of the biggest ethical questions in modern science. Among the most important:
- Safety: are GM crops safe to eat? (Decades of evidence suggest yes, but some people remain unconvinced.)
- Environmental risks: could GM crops or their genes escape into wild populations?
- Designer babies: should parents be allowed to choose features of their children?
- Heritable changes: should we ever edit genes in a way that gets passed to future generations?
- Equality: if gene therapies cost millions, will only the rich get the benefits?
In 2018, a Chinese scientist called He Jiankui announced he had used CRISPR to edit the genomes of two newborn human babies. He was widely condemned and was later jailed for 3 years. The international scientific community has since agreed an informal moratorium on editing the heritable DNA of humans, but the rules are not the same in every country.
Deeper dive: how CRISPR-Cas9 actually works
The CRISPR system was originally discovered in bacteria, where it serves as a kind of immune system against viruses. When a bacterium survives a virus attack, it cuts up a small piece of the virus's DNA and stores it in a special section of its own genome called the CRISPR locus. If the same virus ever attacks again, the bacterium can use that stored memory to recognise it and destroy its DNA before it can take over the cell.
The clever bit is the recognition system. The bacterium makes short pieces of guide RNA that match exactly the stored virus sequences. These guides then bind to a protein called Cas9, which has a built-in pair of molecular scissors. The guide RNA leads Cas9 to any DNA in the cell that matches its sequence; Cas9 then cuts the DNA in two. For a real virus, this destroys the attacker. For a domestic gene editor, this lets you cut exactly the DNA you want.
To turn CRISPR into a gene editing tool, scientists design a custom guide RNA that matches whatever gene they want to edit, mix it with Cas9, and deliver them into the target cells. Cas9 cuts at exactly the right spot. If you also provide a piece of DNA with the desired new sequence, the cell's natural repair machinery can copy it into the cut and effectively edit the genome to whatever you want.
The whole system is so simple that high-school students can do it, and so accurate that it has become the gold standard for gene editing in laboratories worldwide. It is one of the most important biological discoveries of the 21st century, and it has only just begun to change medicine.
For the building blocks, see what is DNA. For the full human genome story, see the human genome.