Edexcel IGCSE Biology · Spec 5.12-5.16
Genetic Modification
What genetic modification is, how it is carried out, and examples such as bacteria making insulin.
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Genetic Modification
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Explained
Genetic modification, and what a transferred gene actually does
Genetic modification means transferring a gene from one organism into another, so that the second organism makes a protein it could not make before.
The reason it works at all is that the genetic code is universal. The same three bases specify the same amino acid in a bacterium as in a human, so a human gene placed inside a bacterium is read correctly and produces the human protein. Without that shared code, none of this would be possible, and it is the single fact most worth understanding on this page.
An organism containing a gene from a different species is described as transgenic. That word is sometimes worth a mark on its own.
The outline of the process
Identify and locate the gene you want in the DNA of the donor organism.
Cut it out using a restriction enzyme, and cut open a bacterial plasmid with the same enzyme so the two ends match.
Join the gene into the plasmid using DNA ligase, producing a recombinant plasmid.
Insert the plasmid into a bacterium, which is then described as transformed, and grow that bacterium in a fermenter so that it multiplies and makes the protein in quantity.
The plasmid is doing the job of a vector, meaning the thing that carries the gene in. A virus can be used as a vector instead, and for plant cells a bacterium that naturally infects plants is often used.
Insulin, the example to know
People with type 1 diabetes cannot make insulin, so they inject it. Before genetic modification, that insulin was extracted from the pancreases of pigs and cattle.
The human insulin gene is now inserted into bacteria, which are grown in fermenters and make human insulin continuously. The advantages follow from that directly. The insulin is human, so it works better and causes fewer allergic reactions than animal insulin. It can be produced in unlimited quantities rather than being limited by the supply of animal pancreases. It is cheaper. And it is acceptable to people whose religion or beliefs would not allow them to use a product taken from a pig or a cow.
That last advantage is a real one and is frequently credited, so it is worth writing rather than leaving out as somehow not scientific.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme gives one mark simply for the word transgenic to describe an organism carrying a gene from another species. One mark, one word, and no explanation required.
The same paper then asks about a rice crop modified to contain vitamin A, and credits two of the following: less night blindness, fewer eye problems or better vision; that rice is a common food, part of many diets, a common crop or cheap to buy; that it is cheap or easy to grow or gives large yields; and that less health care is needed or there is less damage to the economy. Its notes accept staple food and part of the daily diet.
Beside that list sits a rejection: reject scurvy, rickets and anaemia. Those are deficiency diseases of vitamin C, vitamin D and iron. Naming the wrong deficiency disease loses the mark outright, and this is a question where a candidate who knows the topic can still be caught by naming a disease from memory rather than working out which vitamin is involved.
Notice how many of the credited points are not biological at all. Rice is widely eaten, it is cheap, it is easy to grow. A question asking why a modification is useful is asking about people as much as about the gene.
The next part asks for concerns, and credits that the plants may grow wild and affect food chains, webs or ecosystems, that they may compete with other species, that they may interbreed with wild plants so that genes transfer into them, and that people may think they are a risk to health.
That last one is carefully worded. The mark is for the perception of risk, not for a claim that the food is dangerous. Writing that GM food causes harm states something the mark scheme does not; writing that people are concerned it might states something it does.
Other applications
Crops are modified to resist insect pests, so that less pesticide is needed, and to tolerate herbicides, so that weeds can be sprayed without harming the crop. Others are modified for longer shelf life or for improved nutritional content.
Bacteria are modified to produce human growth hormone, blood clotting factors and enzymes used in industry, all by the same route as insulin.
Weighing it up
An evaluation question wants both sides and a conclusion, and the conclusion is what most answers forget.
On one side: higher yields, less pesticide, better nutrition, medicines that could not otherwise be produced. On the other: modified genes spreading into wild populations, effects on other species and food webs, the cost of seed to small farmers, and public concern about safety.
Then commit to a position and say why, referring back to the evidence you have just given. Two lists with no verdict answers a question that was not asked.
Spec 5.12-5.16
What you need to know
- Explain what genetic modification is
- Describe how a gene is transferred
- Give examples, including making insulin
Active recall
Quick check
Answer each question before opening the answer.
What is genetic modification?
Transferring a gene from one organism into another to give it a desired characteristic.
Give an example of genetic modification.
Producing human insulin from GM bacteria, or making pest-resistant crops.
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