生物技术与基因改造

Biotechnology 生物技术 uses living things (or their enzymes 酶) to make useful products. Bacteria 细菌 are especially useful because:

• they reproduce very fast (a high rate 速率 of reproduction).
• they can make complex molecules 分子.
• (Supplement) there are few ethical 伦理 concerns about growing them, and they contain plasmids 质粒 (small DNA rings that are easy to work with).

Yeast: biofuels and bread

Yeast 酵母 carries out anaerobic respiration 无氧呼吸 (without oxygen), making ethanol 乙醇 and carbon dioxide.

• the ethanol can be used as a biofuel 生物燃料.
• in bread-making, the carbon dioxide makes the dough rise.

Enzymes in industry

• pectinase 果胶酶 breaks down pectin in plant cell walls to release more juice from fruit, giving more and clearer juice.
• biological washing powders 洗衣粉 contain enzymes (such as proteases and lipases) that digest stains, even at lower temperatures.
• (Supplement) lactase 乳糖酶 breaks down lactose 乳糖 to make lactose-free milk, for people who cannot digest lactose.

Fermenters (Supplement)

A fermenter 发酵罐 is a large tank used to grow bacteria or fungi to make useful products, such as insulin 胰岛素, penicillin 青霉素 and mycoprotein 真菌蛋白. The conditions inside must be carefully controlled: temperature 温度, pH, oxygen 氧气, the supply of nutrients, and the removal of waste products.

Genetic modification 基因改造 means changing an organism's genetic material 遗传物质 by removing, changing or inserting individual genes 基因.

Making a human protein in bacteria (Supplement)

For example, to make a human protein 蛋白质 (such as insulin) in bacteria:

1. cut the human gene out of human DNA using restriction enzymes 限制酶, which leave sticky ends 黏性末端.
2. cut open a bacterial plasmid with the same restriction enzymes, giving matching sticky ends.
3. join the human gene into the plasmid using DNA ligase 连接酶, making a recombinant plasmid 重组质粒.
4. put the recombinant plasmid into a bacterium.
5. the bacteria multiply and express 表达 the human gene, making the human protein.

Worked example. Put the steps of making human insulin in bacteria in order. Restriction enzymes cut the human insulin gene out of human DNA, leaving sticky ends. The same restriction enzyme cuts open a bacterial plasmid, so its sticky ends match the gene's. Ligase joins the gene into the plasmid, making a recombinant plasmid. The plasmid is put into a bacterium, and the bacteria multiply in a fermenter, each one expressing the gene and making human insulin. Two marks hang on precision: the same enzyme must cut both the gene and the plasmid, which is what makes the sticky ends fit; and ligase joins - it does not cut.

Examples of genetic modification

Genetic modification is used to:

• put human genes into bacteria to make human proteins (such as insulin).
• put genes into crop plants 农作物 to give resistance 抗性 to herbicides 除草剂.
• put genes into crops to give resistance to insect pests.
• put genes into crops to improve their food value.

(Supplement) GM crops such as soya, maize and rice can give higher yields and better nutrition, but some people worry about effects on health, on wild species, and about the cost of GM seeds.

Common mistakes.

• Saying the alcohol makes bread rise. The carbon dioxide from yeast makes the dough rise; the small amount of ethanol evaporates when the bread is baked.
• Saying yeast makes ethanol by aerobic respiration. Ethanol comes from anaerobic respiration; with plenty of oxygen yeast makes carbon dioxide and water instead.
• Saying the enzymes in washing powder "kill" stains. Enzymes digest the large stain molecules (proteins, fats) into small soluble ones that wash away.
• Using two different restriction enzymes in genetic modification. The gene and the plasmid are cut with the same restriction enzyme, so their sticky ends match; DNA ligase then joins them.
• Saying the bacteria make insulin because they are "told to". The bacteria express the human gene: they read it like their own genes and make the human protein.
• Forgetting why a fermenter is cooled. The microorganisms' respiration releases heat; without cooling the temperature rises and their enzymes denature.

• Bacteria are used because they reproduce fast, make complex molecules, and contain plasmids.
• Yeast (anaerobic respiration) → ethanol (biofuel) and carbon dioxide (bread). Enzymes: pectinase (fruit juice), proteases and lipases (washing powders), lactase (lactose-free milk).
• Fermenters make insulin, penicillin and mycoprotein; control temperature, pH, oxygen, nutrients and waste.
• Genetic modification: cut a gene with restriction enzymes (sticky ends) → join it into a plasmid with ligase (a recombinant plasmid) → put into bacteria → express the gene to make the protein.
• GM crops can resist herbicides or insect pests, or have better nutrition.