0610 · 21.3Genetic modification

Genes and genetic modification

0:00How can a bacterium produce a human protein, even though it is not human? And how can changing a gene help a crop survive insect attack or improve its nutritional value? These examples have the same starting point, which is changing the genetic instructions inside an organism. By the end of this video, you will understand how those instructions can be transferred, why they work in a different organism, and why genetically modified crops have both benefits and disadvantages.

0:26First, we need a short reminder about genes. A gene is a length of D N A that codes for a protein. Proteins have many roles, including acting as enzymes, so changing which proteins an organism makes can change its characteristics. Think of a gene as an instruction, rather than the finished product. Transferring a gene transfers information that the receiving cell may use to make a protein. Genetic modification means changing the genetic material of an organism by removing, changing or inserting individual genes.

0:57Notice that inserting genes is only one possibility, although it is the method used in our main examples. Genetic modification does not mean simply feeding an organism a useful substance. It means altering its genetic instructions, so that its cells can behave differently. This is different from selective breeding, where organisms with desirable characteristics are chosen as parents. Here, a particular gene can be inserted directly, and it can come from a different species.

1:22A human gene can work in a bacterium because the genetic code is almost universal. In other words, cells in different organisms generally interpret the same genetic instructions in the same way.

Making a recombinant plasmid

1:33Let us work through bacterial production of human insulin as our main example. Insulin is a human protein used to treat some people with diabetes, but our focus here is how bacteria can make it. Bacteria contain their main genetic material and may also contain small, circular pieces of genetic material called plasmids. A plasmid can act as a carrier that takes an inserted human gene into a bacterial cell.

1:58Imagine our task as an instruction-transfer problem. We need the human gene for insulin, a bacterial plasmid to carry it, and a way to join them together. We then need bacteria that contain the altered plasmid and can use its new instruction. Keep that goal in mind, because each stage solves one part of this problem rather than being an isolated fact to memorise. First, restriction enzymes are used to isolate the D N A making up the human gene.

2:22These enzymes cut at particular sequences, rather than cutting randomly anywhere along the genetic material. In this example, their cuts leave short sections of one strand projecting beyond the other strand. These projections are called sticky ends, because they can pair with ends that have complementary base sequences. The word sticky does not mean that the ends are coated with glue. It means that the short single-stranded ends have complementary bases that can pair with matching ends.

2:48This makes the ends useful for bringing two pieces of genetic material together. The important idea is matching. The human gene needs ends that can pair with the ends of the opened plasmid. Next, the bacterial plasmid is cut using the same restriction enzyme used to isolate the human gene. Using the same enzyme produces complementary sticky ends on the plasmid and the human genetic material. The matching ends can pair, positioning the human gene inside the opened plasmid.

3:15Using just any cutting enzyme would not guarantee compatible ends, which is why the choice of enzyme matters. Pairing the sticky ends is not the same as permanently joining the genetic material. An enzyme called D N A ligase inserts the human D N A into the bacterial plasmid D N A, joining them permanently. The result is a recombinant plasmid. Recombinant means that genetic material from different sources has been combined, here a bacterial plasmid and a human gene.

Bacterial insulin production

3:44The recombinant plasmid is then inserted into a bacterium. You do not need the specific insertion technique at this level, but you do need to understand what has moved, which is the plasmid carrying the human gene. The bacterium has now received an additional genetic instruction. It has not become a human cell, and it has not received a complete set of human genes. Bacteria containing recombinant plasmids are allowed to multiply, producing a large population carrying the inserted gene.

4:11In suitable conditions, the bacteria express the human gene, meaning that they use its information to make the human protein. Multiplication increases the number of cells available to produce insulin. Expression is the step that actually produces insulin, so copying the gene and using the gene are different processes. Notice the division of labour in this process. Restriction enzymes cut, complementary sticky ends help the pieces match, and ligase joins the pieces securely.

4:39The plasmid carries the gene, while the bacteria provide the living cells that make the protein. The useful product is the human protein, not the gene itself, and the protein can then be collected and purified for use.

Herbicide-resistant soya

4:51Now let us apply the same central idea to crops. Instead of giving a bacterium instructions for a human protein, scientists insert genes into a crop to give it a useful characteristic. Our three examples are resistance to herbicides, resistance to insect pests, and improved nutritional qualities. The key question each time is how the new characteristic changes what happens to the plant or to the person eating it. Consider herbicide-resistant soya.

5:18A herbicide is a chemical used to kill unwanted plants, which we call weeds. Weeds compete with crops for light, water and mineral ions, so controlling them can help crops grow. Inserting a suitable gene gives the soya resistance to a particular herbicide, allowing the farmer to kill susceptible weeds without killing that resistant crop. The advantage is easier weed control and potentially a higher yield because the crop faces less competition.

5:43However, resistance does not mean that the crop is protected against every herbicide. It also does not mean that the crop kills weeds itself. The inserted gene protects the crop from the relevant herbicide, while the herbicide is what kills susceptible weeds. There are also possible environmental disadvantages. Herbicide use can remove wild plants that provide food or habitats for other organisms, reducing biodiversity. A resistance gene could spread through cross-pollination if a modified crop breeds with a compatible wild relative.

6:12Repeated herbicide use can also favour resistant weeds, making future weed control more difficult.

Insect-resistant maize and golden rice

6:18Insect-resistant maize illustrates a different modification. An inserted gene allows the maize to produce a protein that is harmful to certain insect pests feeding on it. Less insect damage can mean that more of the crop survives to be harvested. It may also reduce the need to spray insecticides, which are chemicals used to kill insects. However, the benefit does not mean that every insect is affected or that insecticides will never be needed.

6:43Possible disadvantages include effects on non-target organisms, which need assessment for the particular crop and protein involved. Insect pests may also evolve resistance over generations, reducing the protection provided by the crop. An advantage today does not automatically remain equally effective indefinitely. For improved nutrition, consider golden rice. Inserted genes enable the edible part of the rice grain to produce beta-carotene, which the human body can convert into vitamin A.

7:11This can help improve vitamin A intake where rice forms a major part of the diet. Be precise here. The rice provides beta-carotene, rather than directly providing ready-made vitamin A. Improved nutritional quality is a different benefit from resistance to pests or herbicides. A nutritionally improved crop may provide more of a useful nutrient without necessarily producing a larger harvest. It may help reduce a deficiency, but it does not replace the need for a balanced diet.

7:38The usefulness depends on whether the crop reaches people who need it and supplies enough of the relevant nutrient. Across soya, maize and rice, economic considerations also matter. Some genetically modified seeds may be expensive, and licensing arrangements may require farmers to buy new seed rather than save it. A larger harvest or lower spending on pesticides might outweigh those costs, but that is not guaranteed. Farmers need to consider the particular crop, local conditions and the costs and benefits together.

8:07Health and environmental concerns should also be considered carefully, rather than treated as proof that all modified crops are harmful. New proteins need assessment for possible harmful effects, including allergic reactions. Equally, saying that every modified crop is completely harmless simply because it has a useful trait is not a balanced conclusion. Genetic modification describes a method, while the consequences depend on the actual modification and how it is used.

Exam answers and recap

8:33Let us try a worked crop example. A farmer grows maize, and an insect pest is badly damaging the crop. The farmer wonders whether to grow an insect-resistant variety instead. Begin with the problem, because the insects are eating tissue that would otherwise add to the harvest. Then link the modification to that problem. The inserted gene enables production of a protein harmful to the pest, so less damage may lead to a higher yield.

8:58Now consider the other side before reaching a conclusion. Reduced insecticide spraying could be another benefit, while seed costs and the possibility of pests evolving resistance are disadvantages. The farmer should compare these factors under local conditions rather than assuming that a modified variety is always better. That is how you discuss a crop, connecting each advantage or disadvantage to a biological or economic reason. Watch out for three common process mistakes.

9:25Restriction enzymes do not join the gene into the plasmid, ligase performs that joining role. The plasmid is the carrier, not the insulin-producing protein. Finally, multiplying bacteria does not by itself explain protein production, because the human gene must also be expressed. For an exam-style question, suppose you are asked to outline the process of genetic modification using bacteria to produce human insulin. The command word outline asks for the main stages, rather than detailed laboratory methods.

9:55Start by obtaining the human gene. Then describe how it is put into a plasmid, and how the plasmid is put into bacteria. End with the bacteria multiplying and making the protein, and give each enzyme its correct job. Restriction enzymes isolate the D N A making up the human insulin gene, forming sticky ends. The bacterial plasmid is cut using the same restriction enzymes, forming complementary sticky ends. D N A ligase inserts the human D N A into the bacterial plasmid D N A to form a recombinant plasmid.

10:26The recombinant plasmids are inserted into bacteria. Bacteria containing the recombinant plasmids multiply. The human gene is expressed in the bacteria to make human insulin. Those six stages explain how the instruction is obtained, transferred and used, rather than merely naming equipment. If the question asks you to discuss genetically modifying crops, give relevant advantages and disadvantages. For example, insect-resistant maize may have a higher yield because insect pests cause less damage.

10:55It may require less insecticide spraying, but insect pests may evolve resistance to the protective protein. Use the named crop and modification in your reasoning, rather than claiming that all modified crops have the same benefits and risks. Let us bring everything together. Genetic modification changes an organism’s genetic material by removing, changing or inserting individual genes. In our bacterial example, restriction enzymes cut, sticky ends match, ligase joins, and a recombinant plasmid carries the human gene into bacteria.

11:27The bacteria multiply and express the gene to make the human protein. In crops, inserted genes can provide herbicide resistance, insect resistance or improved nutritional qualities. Those changes can improve weed control, reduce pest damage or improve nutrient intake, but environmental and economic disadvantages also need consideration. The central connection is between a changed genetic instruction and a changed characteristic. Once you understand that connection, both the process and its applications become much easier to explain.