5c Genetic Modification (Genetic Engineering)¶
Part of 5 Use of Biological Resources.
Genetic modification changes an organism by inserting a gene from another source, so that it makes a protein it could not make before. Three ideas carry the topic: the enzymes that cut and join DNA, the vector that carries the gene into a host cell, and the useful products that result.
What You Need to Learn¶
Further detail: Pearson Edexcel International GCSE Biology specification.
How restriction enzymes and ligase are used to make recombinant DNA, how vectors carry genes into host cells, and the uses of genetic modification in making insulin and in crops.
Recombinant DNA Tools¶
Genetic engineering, or genetic modification, means changing the genome of an organism by introducing a gene from another organism to give a desired characteristic. Two kinds of enzyme do the work:
| Enzyme | What it does |
|---|---|
| Restriction enzyme | Cuts DNA at a specific base sequence, leaving "sticky ends": short, unpaired, single-stranded stretches at each cut. Different restriction enzymes recognise different sequences |
| Ligase | Joins pieces of DNA together by forming covalent bonds between them |
To make recombinant DNA:
- Cut the wanted gene out of the donor's DNA with a restriction enzyme, which leaves sticky ends.
- Cut a plasmid, which will be the vector, with the same restriction enzyme, which leaves matching sticky ends.
- Mix the gene and the plasmid. The sticky ends are complementary, so they join by base pairing.
- Ligase seals the joins, so the gene becomes part of the plasmid.
Exam technique
Use the same restriction enzyme for the gene and the plasmid, and say why: it leaves complementary sticky ends that can pair. State the jobs exactly: restriction enzymes cut, and ligase joins.
Explore Recombinant Plasmid Formation¶
Use the interactive below to keep the restriction-enzyme, plasmid and ligase sequence in order. It focuses on the standard plasmid route that sits behind insulin production and the broader idea of recombinant DNA in this course. Open full interactive.
How to read the model
This is a simplified process map. It keeps the focus on sticky ends, ligase and the plasmid as a vector rather than on every later industrial step after the host cell has taken up the recombinant DNA.
Vectors and Gene Transfer¶
A vector is the vehicle that carries recombinant DNA into a host cell. Two are used:
- Plasmids are small circular loops of DNA found in bacteria. A bacterium can take one up and copy it along with its own chromosome.
- Viruses can inject DNA into host cells.
Once the recombinant DNA is inside a suitable host cell, such as a bacterium, the host copies the inserted gene each time it divides. The cells can then be grown on a large scale, and they make the protein that the gene codes for.
Uses of Genetic Modification¶
Human insulin. Insulin used to treat diabetes was once extracted from pigs and cattle. It is now made by genetically modified bacteria:
- The human insulin gene is cut out with a restriction enzyme, leaving sticky ends.
- A bacterial plasmid is cut with the same restriction enzyme.
- The insulin gene is inserted into the plasmid, and ligase joins the sticky ends.
- The recombinant plasmid (the vector) is put into a bacterial cell, the host.
- The bacteria are grown in large fermenters. They reproduce and express the human insulin gene, producing large amounts of human insulin.
Genetically modified crops. Plant cells can be engineered to express genes from other organisms. Current uses include resistance to insects, which reduces the use of pesticides, resistance to a herbicide, which lets farmers kill weeds without harming the crop, and improved yield or nutritional content.
A transgenic organism contains genes transferred from another species. Transgenic bacteria make human insulin, and transgenic plants may carry pest-resistance genes from other species.
| Benefits | Concerns |
|---|---|
| Large amounts of a human protein, such as insulin, can be made reliably | Some people worry about eating food from GM crops, or about the effects on wildlife |
| Crops can resist pests or herbicides, so yields rise and fewer pesticides may be needed | A gene could spread to wild relatives, and the seeds can be expensive for farmers |
| Crops can be made more nutritious | Long-term effects on ecosystems are hard to predict |
Common Confusions¶
- Vector vs gene: The gene is the DNA being transferred. The vector, a plasmid or virus, is the carrier that takes it into the host cell.
- Vector vs host: The plasmid is the vector. The bacterium that receives it is the host cell.
- Restriction enzyme vs ligase: Restriction enzymes cut DNA, and ligase joins it. Both are needed to make recombinant DNA.
- Selective breeding vs genetic modification: Selective breeding works within a species using existing alleles. Genetic modification can introduce genes from entirely different species.
Check Yourself¶
- State the job of a restriction enzyme and the job of ligase.
- Explain why the same restriction enzyme must be used to cut the plasmid and the gene.
- Name the vector and the host cell in the production of human insulin.
- Describe how genetically modified bacteria are used to produce insulin on a large scale.
- Give one benefit and one concern about a genetically modified crop.
Answers
- A restriction enzyme cuts DNA at a specific base sequence, leaving sticky ends. Ligase joins pieces of DNA together.
- The same enzyme cuts both at the same sequence, so the sticky ends are complementary and can pair with each other.
- The vector is the plasmid, and the host cell is the bacterium.
- The human insulin gene is cut out and inserted into a plasmid, which is put into bacteria. The bacteria are grown in large fermenters, where they reproduce and make the insulin.
- Benefit: the crop can resist pests, so yields rise and less pesticide is needed. Concern: the gene may spread to wild plants, or the effects on ecosystems may be hard to predict.
Key Terms¶
- Genetic modification (genetic engineering): altering the genome of an organism by inserting a gene from another organism.
- Restriction enzyme: an enzyme that cuts DNA at a specific base sequence, leaving sticky ends.
- Sticky ends: short single-stranded DNA overhangs made by restriction enzymes.
- Ligase: an enzyme that joins DNA fragments by forming bonds between nucleotides.
- Recombinant DNA: DNA formed by combining genetic material from two or more different sources.
- Vector: a carrier used to introduce recombinant DNA into a host cell.
- Plasmid: a small circular DNA molecule that can act as a genetic engineering vector.
- Transgenic: describing an organism that has had genetic material from another species inserted into its genome.