S4.2.8 Chapter 8

Mutation and genetic engineering

Bacteria can make human insulin. How can a tiny bacterium make a human protein?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A People put the human gene for insulin into the bacterium, and the bacterium reads it.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The bacterium eats human cells and keeps their insulin.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The bacterium turns into a human cell.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how a mutation can change a gene and its protein
Weigh up the benefits and risks of genetically modified crops

A gene is a code of bases. What happens if the code changes? A change in the order of the bases in DNA is called a mutation.

Mutations happen by chance. A cell can make a mistake when it copies its DNA. Ultraviolet light, other radiation and some chemicals can also damage DNA. Cells repair most of the damage, but not all.

A mutation of one base. The fourth base changed from G to C.

Remember that a cell reads bases three at a time. Each group of three codes for one amino acid. In the picture the groups are TAC GGA before and TAC CGA after. One group changed.

Worked example

Compare the strands TACGGA and TACCGA. In which position do they first differ, and in which group of three bases is the change?

Compare the two strands base by base.
They first differ at position 4.
Positions 1 to 3 are group 1, and positions 4 to 6 are group 2.
The change is in group 2.

Answer: Position 4, in group 2

What does a changed group do? That depends on the mutation, and there are three main cases.

First, most mutations have no effect. Several groups of three code for the same amino acid, so the protein stays the same. Only a small part of human DNA codes for proteins at all, so many mutations fall outside genes.

Second, some mutations change an amino acid. A protein folds into a shape, and its shape gives it its job. A different amino acid can change the shape, so the protein may do its job badly or not at all. In sickle cell disease, one base change in the haemoglobin gene means that one amino acid is different.

Third, a few mutations help. A changed protein may work better, or do something new. New alleles begin as mutations, and helpful ones can spread through a species over many generations.

How a mutation can change a protein. Each step may not happen.

A mutation in a body cell, such as a skin cell, stays in that cell and the cells that come from it. It is not passed to children. A mutation in an egg or a sperm can be passed on, and then every cell of the child has it. An extra or a missing base shifts every group after it, and this often has a bigger effect than a swap.

Try it

Change a code written in threes

You need: an old sheet of paper, a pencil.

Write THE CAT SAT. Each word has three letters, like a group of three bases.
Change one letter: write THE CAR SAT. Read it. One word changed, and the rest is still clear.
Now write THECATSAT, with no gaps. Put an extra letter X after THE. Write the letters again, in groups of three, from the start.
Read the groups. How many words are wrong now?

What you will see: A change of one letter damaged one word. An extra letter shifted every group after it, so every word after the change was wrong. This is why an extra or missing base can be so serious.

If you cannot try it, this is what happens.

People can also change the DNA of a living thing on purpose. Genetic engineering means taking a gene from one living thing and putting it into another. It works because every living thing reads the DNA code in the same way.

Insulin is a protein that helps to control the level of sugar in the blood. Some people's bodies do not make enough of it. Today, most insulin that people use is made by engineered bacteria or yeast. Here we follow the bacteria. The human gene for insulin is cut out of human DNA with enzymes that act as scissors. It is put into a bacterium, and the bacterium reads it and makes insulin.

Cut, insert, grow. The bacteria divide quickly, so one engineered cell becomes very many.

Crops can be engineered too. A gene may be added to a plant for a new feature. The plant may resist a pest insect, survive with little water, or have more vitamins in its grain. Such a plant is genetically modified, or GM.

Worked example

In one example trial, pests destroyed 25 of every 100 plants of an ordinary crop. They destroyed 5 of every 100 plants of a modified crop. How many fewer plants out of 100 were lost with the modified crop?

Of every 100 plants, the ordinary crop lost 25.
Of every 100 plants, the modified crop lost 5.
The difference is 25 − 5 = 20 plants.
Check: the modified crop kept 100 − 5 = 95 plants. The ordinary crop kept 100 − 25 = 75. So 95 − 75 = 20.

Answer: Out of every 100 plants, 20 fewer are lost.

Example numbers for one trial of one modified crop. Another trial or another crop may give a different result.

Is a modified crop a good idea? A fair answer weighs the benefits against the risks. The result can differ for each new crop and each place, so each one is judged on its own.

Read across the rows. Each one is explained in the text. Drought means a crop that lasts through a dry spell, and unknowns means long term effects that are not yet known.

The benefits can be large. A crop that resists a pest can give more food from the same land, and farmers need fewer sprays against insects. A crop with more vitamins can help people whose diet lacks them. A crop that lasts in drought can still grow when rain is short.

The risks need care. Pollen can carry a new gene to wild plants of the same kind. The effect on wild plants and on insects may not be known. Some seed may be controlled by its maker. A farmer may not be allowed to save seed and plant it again. Some long term effects may not show for many years.

Pests can also become resistant to a crop over many generations. Large reviews by science bodies have found that the GM foods now sold are as safe to eat as other foods. Each new crop is tested before it is grown.

To weigh a case, ask questions. Who gains, and who could be harmed? What does the evidence show? Has it been tested, and by whom? Can the risks be reduced? Two careful people can weigh the same facts and reach different answers.

A wrong idea many people have

Every mutation is harmful.

Why it feels right: The word sounds frightening, and we hear most about mutations that cause disease.

The test that breaks it: Compare the strands before and after a base changes. If the new group codes for the same amino acid, the protein is the same. Many mutations are like this, or fall outside genes.

What is true: Most mutations have no effect. Some are harmful and a few help. Mutations are also the source of new alleles, and so of the variation that we see in living things.

Key words

mutation: a change in the order of the bases in DNA.
genetic engineering: moving a gene from one living thing into another.
genetically modified: having a gene that was added by genetic engineering. GM is short for it.

Back to your guess

The answer is A: People put the human gene for insulin into the bacterium, and the bacterium reads it.

People cut out the human gene for insulin and put it into bacteria. All living things read the DNA code in the same way, so the bacteria make insulin from it.

Keep it

Copy this into your exercise book.

A mutation changes the order of bases. Most do nothing, some change a protein, a few help. Genetic engineering moves a gene between species: cut, insert, grow. Weigh benefits against risks.

Draw: Three boxes: cut the gene, insert it, grow the cells, with one benefit and one risk written beside them.