The Genetic Engineering Lab

The Genetic Engineering Lab

Every living thing reads DNA the same way. That means a human gene, dropped into a bacterium, still works: the bacterium starts making a human protein. That single fact is the whole basis of genetic engineering, and it is why insulin for diabetes now comes out of a fermenter rather than an animal pancreas.

The toolkit

Genetic engineering (or genetic modification) means moving a gene from one organism into another, so the second organism makes the protein that gene codes for. Five things make that possible:

Label the gene transfer

This diagram shows a gene being moved into a bacterium. Drag each label onto the correct part.

The engineering process

Put the steps of genetic engineering into order, from the donor DNA to a protein being made.

• A restriction enzyme cuts the useful gene out of the donor DNA
• The same enzyme cuts open a plasmid, leaving matching sticky ends
• Ligase joins the gene into the plasmid to make the vector
• The plasmid carries the gene into a host cell, such as a bacterium
• The host cell reproduces, and the modified cells make the protein

What if it were missing?

Why the same enzyme twice?

The gene is cut out of the donor DNA, and the plasmid is cut open, using the SAME restriction enzyme. Why does that matter?

• Each enzyme cuts at its own particular sequence, so only the same enzyme leaves sticky ends on the gene and the plasmid that are complementary and can base-pair together
• Using one enzyme for both cuts is faster and cheaper than using two
• Ligase can only work on DNA that has been cut by a restriction enzyme it recognises
• A second enzyme would damage the gene as it was being transferred

The insulin case

Before 1982, insulin for people with diabetes was extracted from the pancreases of pigs and cattle. It worked, but the supply depended on slaughterhouses, and animal insulin is not quite the same protein: pig insulin differs from human insulin by one amino acid, cattle insulin by three. Then the human insulin gene was engineered into bacteria. The bacteria read it normally and produced genuine human insulin, and because bacteria reproduce every twenty minutes or so, they could be grown in fermenters and produce it in bulk. It was the first genetically engineered medicine approved for human use, and it took the supply of a life-saving drug away from the meat industry entirely.

Why a bacterium?

Bacteria are the usual host for making a protein like insulin. Which combination of reasons best explains that?

• They reproduce very quickly, they already carry plasmids that can act as vectors, and they can be grown cheaply in large fermenters
• They are the only organisms whose cells can read a human gene
• They have no DNA of their own, so the inserted gene has nothing to compete with
• They have a nucleus, which protects the inserted gene

The GM crop argument

Genetically modified crops are the part of this topic people actually argue about, and the argument is a real one on both sides:

Approve it or not?

You advise a government deciding whether to approve a pest-resistant GM maize. Work through it.

• The developer says the crop needs far less insecticide. Why is that an environmental argument in its favour, not just a financial one?
• Objectors point out that a wild grass growing at the field margins can cross-pollinate with maize. What is their actual concern?
• A farmer asks what happens if the pest evolves resistance to the crop's built-in defence. What should you tell them?
• The final report has to weigh a serious benefit against a serious risk. Which recommendation is best supported by the biology?

Which are true?

Select ALL THREE statements that are TRUE.

• A human gene works in a bacterium because the genetic code is shared by all living things, not because the gene is altered to suit its new host
• A pest-resistant GM crop applies a selection pressure to the pest, so resistant pests may become more common over time
• Golden rice was engineered to produce beta-carotene, which the human body then converts into vitamin A
• Genetic engineering has no risks or drawbacks at all
• Genetic engineering creates a brand-new gene that did not exist before
• Genetic engineering always makes organisms dangerous

Explain the route

Exam practice. In about 50 words, explain how a human gene ends up producing human insulin inside a bacterium. Include:

• which enzyme cuts the gene out, and why the plasmid is cut with the same one
• which enzyme joins the gene into the plasmid, and what the plasmid then does
• why the bacterium is able to read a human gene at all

Engineering summary

In genetic engineering, a _____ enzyme cuts the useful gene out of the donor DNA, leaving short single-stranded overhangs called _____ ends. The enzyme _____ then joins the gene into a plasmid, which carries it into a host cell. Engineered bacteria are used to make human _____ for people with diabetes.