A gene is not useful simply because it is a piece of DNA. It is useful because its nucleotide sequence carries instructions for making a particular polypeptide, which may become a functional protein. Genetic engineering takes advantage of this relationship.
If scientists place a selected gene into another cell, that cell may be able to use the new genetic instructions and produce the corresponding protein. This makes it possible to give organisms useful characteristics that they did not previously possess.
Bacteria can be modified to produce human insulin, while plants and animals can be engineered to develop economically useful characteristics. The important ideas are therefore what gene is transferred, how it is transferred and what effect the transferred gene has.
Try these five questions before reading the chapter. They test the essential ideas behind gene transfer, plasmids and the enzymes used in genetic engineering.
- 5 diagnostic questions
- Core genetic engineering vocabulary
- Instant explanation for every answer option
What is genetic engineering?
Every gene is a sequence of nucleotides within DNA. The order of these nucleotides carries genetic information that can ultimately determine the sequence of amino acids in a polypeptide.
Genetic engineering deliberately alters genetic material. One important approach is to transfer a selected gene from one cell into another. The recipient cell then contains genetic information that it did not originally possess.
Gene → protein → characteristic
If a transferred gene is successfully expressed, the recipient cell can make the protein encoded by that gene. The new protein may then give the organism a new characteristic.
Genes can be transferred between organisms
Genetic engineering can transfer genes between cells from organisms of the same species or between organisms of different species.
When genetic material from another organism has been introduced into an organism, the recipient may be described as transgenic.
For example, a crop plant may receive a gene that allows it to produce a protein protecting it against a particular insect pest. The plant now contains genetic information that it did not originally possess.
This is possible because living organisms use the same basic genetic code. The nucleotide sequence of the transferred gene still carries instructions for making a particular polypeptide.
The main tools used in genetic engineering
Genetic engineering becomes much easier to understand when each tool is given a clear job. Three ideas are especially important: restriction enzymes, DNA ligase and vectors such as plasmids.
| Tool or structure | What it does | Why it is useful |
|---|---|---|
| Restriction enzyme | Cuts DNA at particular nucleotide sequences. | Allows the required gene to be obtained and the plasmid to be cut open. |
| DNA ligase | Joins DNA fragments together. | Joins the selected gene to the plasmid DNA. |
| Plasmid | A small circular DNA molecule found in bacteria. | Can act as a vector that carries an inserted gene into a bacterial cell. |
| Bacterium | Acts as a host cell containing the recombinant plasmid. | Can reproduce rapidly and produce the protein encoded by the inserted gene. |
Restriction enzymes cut DNA
Restriction enzymes recognise particular nucleotide sequences within DNA and cut the DNA at or near those sequences.
In genetic engineering, a suitable restriction enzyme can be used to cut out a DNA fragment containing the required gene. The same restriction enzyme can also be used to cut open a plasmid.
Using the same restriction enzyme can produce compatible DNA ends, allowing the required DNA fragment to fit into the opened plasmid.
DNA ligase joins DNA
Cutting DNA is only the first part of the process. Once the required gene is placed into the opened plasmid, DNA ligase joins the DNA fragments together.
The resulting DNA contains genetic material from different sources. This is called recombinant DNA.
A plasmid containing an inserted gene may therefore be called a recombinant plasmid.
The plasmid acts as a vector
A vector carries genetic material into another cell. Bacterial plasmids are useful vectors because they can be removed from bacteria, modified and then introduced back into bacterial cells.
Keep the three jobs separate:
Restriction enzyme = cuts DNA
DNA ligase = joins DNA
Plasmid = carries the gene
How genetically modified bacteria produce human insulin
One of the most important medical applications of genetic engineering is the production of human insulin.
Insulin is a hormone involved in regulating blood glucose concentration. People whose bodies do not produce enough effective insulin may require insulin as part of their treatment.
Instead of memorising the process as an isolated list, follow the journey of the insulin gene: obtain it, place it into a plasmid, introduce the plasmid into a bacterium and then allow the genetically modified bacteria to produce insulin.
1. Obtain the insulin gene
The gene containing the genetic information needed for the production of human insulin is identified and obtained.
The important point is that the gene carries the nucleotide sequence containing instructions for making the required protein.
2. Cut out the insulin gene
A suitable restriction enzyme is used to cut the DNA and obtain the fragment containing the insulin gene.
Because restriction enzymes recognise particular DNA sequences, the required region can be cut at specific positions.
3. Prepare the bacterial plasmid
A plasmid is removed from a bacterium. Remember that a plasmid is a small circular DNA molecule that is separate from the bacterium's main chromosome.
The plasmid is cut open using the same restriction enzyme used to obtain the insulin gene.
This can produce compatible DNA ends on the insulin gene and the opened plasmid.
4. Insert the insulin gene into the plasmid
The DNA fragment containing the insulin gene is placed into the opened plasmid. The compatible ends allow the two pieces of DNA to fit together.
5. Join the DNA using DNA ligase
DNA ligase joins the insulin gene to the plasmid DNA.
The result is a recombinant plasmid containing DNA from different sources.
This is why restriction enzymes and DNA ligase must not be confused: restriction enzymes perform the cutting, while DNA ligase performs the joining.
6. Introduce the recombinant plasmid into a bacterium
The recombinant plasmid is introduced into a bacterial cell.
The plasmid therefore acts as a vector because it carries the insulin gene into the bacterium.
The bacterium now contains introduced genetic information allowing it to produce human insulin.
7. Allow the genetically modified bacteria to multiply
Under suitable conditions, bacteria reproduce rapidly by binary fission.
As the genetically modified bacteria multiply, many bacterial cells containing the recombinant plasmid are produced.
8. Produce, collect and purify human insulin
The bacteria express the inserted genetic information and produce human insulin.
The bacteria can be cultured on a large scale so that large quantities of insulin are produced. The insulin is then collected and purified before being prepared for medical use.
The insulin sequence to remember:
obtain insulin gene → cut gene → cut plasmid with the same restriction enzyme → insert gene → join with DNA ligase → introduce recombinant plasmid into bacterium → grow bacteria → collect and purify human insulin
Why can bacteria produce a human protein?
At first, it may seem strange that a bacterium can produce a human protein such as insulin.
The reason is that DNA carries genetic information using the same basic genetic code across living organisms. The transferred gene still contains the nucleotide sequence that specifies the required polypeptide.
If the inserted genetic information can be expressed by the bacterial cell, the bacterium can use that information to produce the corresponding protein.
The bacterium does not become human. It remains a bacterium. It has simply gained additional genetic information that allows it to manufacture a particular human protein.
Benefits of genetic engineering
Genetic engineering can be useful in medicine and in the production of economically important plants and animals. The benefit depends on the particular gene transferred and the purpose of the modification.
Benefits in medicine
Human insulin can be produced by genetically modified bacteria. Once suitable bacterial cells have been produced, large populations can be cultured under controlled conditions.
This allows human insulin to be produced in large quantities and with consistent quality.
Genetic engineering can also be used to produce other medically useful biological substances.
Benefits in crop plants
Genes may be introduced into economically important crop plants to give useful characteristics.
- Resistance to particular insect pests can reduce crop damage and may reduce the amount of insecticide required.
- Resistance to certain diseases can reduce losses in agricultural production.
- Improved nutritional characteristics may increase the quantity of a useful nutrient or nutrient precursor in the crop.
- Improved agricultural productivity may result when fewer plants are lost to pests or disease.
Benefits in economically important animals
Genetic engineering may also be used to alter characteristics of animals used in agriculture or other industries.
Possible aims include improved disease resistance, faster growth or increased production of economically useful materials.
In an examination, do not simply state a benefit such as "higher yield". Explain how the engineered characteristic leads to that benefit.
Ethical considerations and possible risks
A technology can have useful applications while still raising questions about how it should be used. Genetic engineering therefore needs to be considered from several viewpoints.
| Issue | Possible benefit | Consideration or concern |
|---|---|---|
| Medicine | Genetically modified microorganisms can produce useful substances such as insulin. | Production must be controlled so that the final medical product is safe and pure. |
| Pest-resistant crops | Less crop may be destroyed by particular insect pests. | Effects on other organisms and the possible evolution of resistant pests need to be considered. |
| Gene escape | An introduced gene may give a useful characteristic to the crop. | Pollen may transfer the gene to closely related plants outside the intended crop population. |
| Animal production | Genetic changes may improve economically useful traits. | Animal health and welfare must also be considered. |
| Cost and access | Genetically engineered crops may improve agricultural productivity. | Patents and seed costs may affect farmers' access to the technology. |
Possible effects on human health
A genetically engineered food should not automatically be assumed to be either safe or unsafe simply because it has been genetically modified.
The relevant question is whether the particular genetic modification has produced an unintended harmful effect.
For example, a newly produced protein could potentially cause an allergic response in some individuals.
Each genetically engineered product therefore needs to be assessed according to the specific modification involved.
Possible environmental effects
Organisms interact with their surroundings, so an introduced gene may have effects beyond the genetically modified organism itself.
For example, pollen from a genetically modified crop could transfer an introduced gene to a closely related plant.
A pest-resistant crop may also place selection pressure on the pest population. Individuals that happen to be more resistant may survive and reproduce, increasing the frequency of resistance within the population.
These possibilities do not mean that every genetically modified crop will cause environmental harm. They are factors that should be investigated for the particular organism, gene and environment involved.
Animal welfare
When genetic engineering is applied to animals, increased productivity should not be considered in isolation.
A modification may be economically useful but could also affect the health or quality of life of the animal. Animal welfare therefore needs to be considered.
Social and economic considerations
Genetic technologies can also affect people economically.
For example, patented genetically engineered seeds may provide useful agricultural characteristics but may cost more or require farmers to purchase new seed under particular agreements.
Ethical discussion can therefore involve questions about who gains the benefits, who carries the risks and whether access to the technology is fair.
How to answer genetic engineering questions
Genetic engineering questions often test whether you understand the function of each component rather than whether you can simply reproduce a memorised list.
If the question asks how human insulin is produced using bacteria, make sure your explanation links together:
insulin gene → restriction enzyme → plasmid → DNA ligase → recombinant plasmid → bacterium → insulin production
For questions about benefits and ethical considerations, develop the biological reasoning.
Instead of writing only "higher yield", explain why the introduced characteristic could lead to a higher yield. Instead of writing only "bad for the environment", identify a specific possible environmental effect.
Common Mistakes
| Common mistake | Better understanding |
|---|---|
| Saying that restriction enzymes join DNA. | Restriction enzymes cut DNA. DNA ligase joins DNA. |
| Saying that DNA ligase cuts the plasmid. | A restriction enzyme cuts the plasmid open. DNA ligase later joins the inserted gene to the plasmid. |
| Forgetting why the same restriction enzyme is used. | Using the same restriction enzyme can produce compatible DNA ends on the desired gene and the plasmid. |
| Saying that the plasmid produces insulin. | The plasmid carries the insulin gene. The bacterial cell expresses the genetic information and produces insulin. |
| Saying that bacteria become human after receiving a human gene. | The bacteria remain bacteria. They simply gain genetic information allowing them to produce a particular human protein. |
| Saying that recombinant DNA means DNA that has reproduced. | Recombinant DNA contains DNA that has been combined from different sources. |
| Treating every genetically modified organism as automatically dangerous. | Benefits and possible risks depend on the particular gene, organism, application and available evidence. |
| Giving an ethical concern without explaining it. | State the concern and explain the biological, environmental or social consequence that makes it important. |
What You Should Be Able to Do
Core ideas
- Explain what genetic engineering involves.
- Explain that genes can be transferred between cells.
- Describe what is meant by a transgenic organism.
- Relate a transferred gene to the production of a particular protein.
Tools and processes
- State the role of restriction enzymes.
- State the role of DNA ligase.
- Explain why bacterial plasmids can act as vectors.
- Explain what is meant by recombinant DNA.
Human insulin
- Outline how the human insulin gene is inserted into a bacterial plasmid.
- Explain how genetically modified bacteria produce human insulin.
- Put the major stages of insulin production in the correct order.
- Distinguish the roles of enzymes, plasmids and bacterial cells.
Applications and evaluation
- Explain possible medical benefits of genetic engineering.
- Explain possible benefits in economically important plants and animals.
- Discuss possible environmental, health and welfare concerns.
- Explain social and economic considerations where appropriate.
Now apply the ideas from the chapter. These questions test the sequence of gene transfer, the roles of the genetic engineering tools and the evaluation of benefits and possible concerns.
- 10 application questions
- Human insulin and gene transfer
- Every answer option is explained
Frequently asked questions
Genetic Engineering FAQ
Once the basic sequence is clear, most genetic engineering questions come back to a few recurring ideas: what is transferred, what each enzyme does, how a plasmid carries the gene and what consequences the resulting organism may have.
What is the difference between genetic engineering and selective breeding?
Selective breeding involves choosing organisms with desirable characteristics and breeding them over successive generations. Genetic engineering directly changes genetic material, such as by transferring a selected gene into another cell.
What is a transgenic organism?
A transgenic organism contains genetic material that has been artificially introduced from another organism. The introduced gene may allow the recipient to produce a new protein and therefore show a new characteristic.
Can genes be transferred only between different species?
No. Genes may be transferred between cells from organisms of the same species or from organisms of different species. The important idea is that genetic information is deliberately introduced into a recipient cell.
Is a plasmid the same as a bacterial chromosome?
No. A plasmid is a small circular DNA molecule that is separate from the bacterium's main chromosome. Plasmids can be modified to carry selected genes.
Why are restriction enzymes sometimes described as molecular scissors?
Restriction enzymes cut DNA at particular nucleotide sequences. They can therefore be used to obtain a required DNA fragment and to cut open a plasmid.
Why is the same restriction enzyme used on the required DNA and the plasmid?
Using the same restriction enzyme can produce compatible DNA ends on the desired gene and the opened plasmid. This allows the gene fragment to fit into the plasmid before DNA ligase joins the DNA.
What is the difference between a restriction enzyme and DNA ligase?
A restriction enzyme cuts DNA at particular sequences, while DNA ligase joins DNA fragments together.
What is recombinant DNA?
Recombinant DNA is DNA formed by combining genetic material from different sources. A bacterial plasmid containing an inserted human insulin gene is an example.
Why can bacteria produce human insulin without becoming human cells?
The transferred insulin gene contains instructions for making the required protein. A bacterial cell can use the inserted genetic information while remaining a bacterial cell.
Why are bacteria useful for producing insulin?
Bacteria reproduce rapidly under suitable conditions. Once genetically modified bacteria containing the insulin gene have been obtained, large populations can be cultured to produce substantial quantities of insulin.
Does genetic engineering always make an organism better?
No. A modification may produce a useful characteristic, but possible health, environmental, welfare and economic effects also need to be considered.
Are all genetically modified foods dangerous?
No conclusion should be based only on the label "genetically modified". The safety of a particular food depends on the specific modification and its effects. Evidence from appropriate testing is needed.
How should I discuss the ethics of genetic engineering in an examination?
Give a specific benefit or concern and explain its consequence. For example, a pest-resistant crop may suffer less insect damage and therefore give a greater usable yield, but the possible transfer of the introduced gene to related wild plants may also need to be considered.
Genetic engineering becomes much easier once you follow the gene. A useful gene is identified, DNA is cut, the gene is carried by a vector such as a plasmid, DNA ligase joins the required pieces and the recipient cell gains new genetic information. Understanding that sequence makes it much easier to explain both the applications of genetic engineering and the questions that arise from its use.