Engineering Genes

From the 1920s on, people with diabetes who needed insulin usually injected insulin taken from the pancreases of cattle and pigs. In 1982 the FDA approved human insulin made by bacteria.

Bacteria do not have a human insulin gene. Today, human insulin products are made by bacteria or yeast that carry DNA for it. How do you get a human gene into a microbe, and how should people decide which changes to living things are worth making?

Explore: build a bacterium that makes a product

In the gene-to-product builder, you pick a restriction enzyme to cut open a plasmid and to cut out a gene. You join the pieces with ligase, put the plasmids into bacteria, and then select the bacteria that took one up. A gel panel lets you check the sizes of your DNA pieces against a ladder of known sizes.

Try this.

1. Cut the plasmid with one enzyme and the gene with a different enzyme. Predict whether ligase can join them, then test.
2. Cut both with the same enzyme. Predict what the gel will show for the joined plasmid compared with the empty one, then run it.
3. Skip the selection step. Predict how many of the bacteria on your plate will carry the gene, then check.
4. Load a short piece and a long piece on the gel. Predict which one travels farther, then run it.

Explain: tools that cut, join, copy and sort DNA

Genetic engineering is using laboratory tools to change an organism’s DNA. DNA is the same kind of molecule in every living thing, so a cell can often read a gene that came from another species. You can review how DNA is built in that lesson.

The tools work like an editing kit. A word-processing analogy helps: cut, paste and copy. The analogy breaks because DNA tools are molecules acting by chance in a tube, and each step works on only some of the DNA.

• A restriction enzyme is a protein from bacteria that cuts DNA wherever it finds one particular short sequence. Many leave short single-stranded tails, called sticky ends.
• DNA ligase is an enzyme that joins two DNA ends into one continuous strand.
• A plasmid is a small, usually circular piece of DNA in a bacterium that copies itself separately from the main chromosome. When it copies, any gene added to it is copied too.
• Recombinant DNA is DNA joined together from more than one source, such as a plasmid carrying a human gene.
• The polymerase chain reaction (PCR) makes many copies of one chosen stretch of DNA. Short DNA pieces called primers mark where copying starts. In an ideal reaction the amount doubles each cycle.
• Gel electrophoresis sorts DNA pieces by size. DNA carries a negative electric charge, so a current pulls it toward the positive end of a gel. Shorter pieces slip through the gel faster and travel farther.
• CRISPR is a gene-editing tool taken from a bacterial defense system. A guide RNA matches one DNA sequence, and a protein called Cas9 cuts both strands there.

ideal PCR: copies after n cycles = starting copies × 2 × 2 × … (n times)

For example, 1 copy after 10 ideal cycles gives 1,024 copies. Real reactions copy a bit less than perfectly.

Explain: from a human gene to insulin, crops and cures

Here is the textbook path to insulin made by bacteria. It is a simplified model, not a factory recipe.

1. Get DNA that codes for insulin. In an early experiment, reported in 1979, scientists made this DNA chemically, as two pieces, one for each of insulin’s two protein chains.
2. Cut a plasmid open with a restriction enzyme. Cutting the gene with the same enzyme gives it matching sticky ends.
3. Mix them so the ends pair, and seal them with ligase. Now you have recombinant DNA.
4. Put the plasmids into bacteria. Only some bacteria take one up. The plasmid usually also carries a gene for resistance to an antibiotic, so growing the bacteria with that antibiotic leaves mostly the ones that took up the plasmid.
5. Grow the bacteria. As they divide, the plasmid is copied. If the gene is switched on, the bacteria make the insulin chains, which are collected and purified, and the two chains are then joined into insulin.

Adding a gene does not guarantee the cell makes its protein, so scientists test for the product. You can follow how cells read a gene into a protein in that lesson.

An organism whose DNA was changed this way is a genetically modified organism. Examples come from three fields:

Synthetic biology goes a step further: it designs and builds new biological parts, devices and systems. In one example, researchers built a new chemical pathway in baker’s yeast so that it made a starting material for a malaria medicine.

Each use raises questions that data alone cannot settle. People disagree, and that is part of the work.

• Safety: Could an edit land in the wrong place? Could a crop affect insects it was not meant for? Can relying on one weed-killer select for resistant weeds, as in that lesson?
• Equity: Who can pay for a new treatment? Who can buy the seed?
• Limits: Should anyone edit sperm, eggs or embryos, when future people cannot agree to the change?

To judge a claim, ask who wrote it, what evidence it gives, whether it was checked by other scientists, and whether the writer gains from your belief.

Elaborate: how common are engineered crops?

USDA asks farmers each June what they planted. This table shows the share of planted acres grown with any genetically engineered variety.

Data are U.S. estimates from the USDA June Agricultural Survey, published by the USDA Economic Research Service (2000 to 2024 and 2025 tables). Virginia is not listed separately. NASS estimated that Virginia farmers planted about 630,000 acres of soybeans and 500,000 acres of corn in 2024.

1. Which crop reached 90 percent first, and in which year shown?
2. How many percentage points did corn rise from 2000 to 2010? Compare that with 2010 to 2025.
3. What does this table not tell you about whether engineered crops are safe or helpful?

Design problem

You put an insulin plasmid into bacteria. Plan how to show that your bacteria took up the plasmid and are making insulin, and which controls you need.

Evaluate: check your thinking

Bacteria make human insulin because scientists joined insulin DNA into a plasmid with restriction enzymes and ligase. The bacteria copy that plasmid as they divide and read the gene to make the protein. Related tools now change crops, CRISPR now edits blood stem cells, and each use is judged by its evidence and by people’s values.

Science practices you used: predicting and testing with a model, reading a data table and calculating a change, planning controls, judging the reliability of a source, and weighing benefits, risks and ethics.