5cGenetic modification (genetic engineering)

Genetic modification starts with two jobs: cutting the gene you want out of one organism's DNA, and joining it into a piece of DNA that can be carried into another cell.

Restriction enzymes — the scissors

• A restriction enzyme cuts DNA at a specific sequence of bases (a specific site).
• Each restriction enzyme recognises one particular short base sequence, so it only cuts where that sequence occurs. This is why the cut is precise and the same gene is removed every time.
• Many restriction enzymes cut the two strands unevenly, leaving short lengths of unpaired bases at each end. These are called sticky ends.
• Sticky ends matter because their exposed bases will pair up (by complementary base pairing) with any other DNA cut by the same restriction enzyme.

Key exam sentence: "The same restriction enzyme is used to cut the human DNA and the plasmid, so both have complementary sticky ends."

Ligase — the glue

• Ligase joins pieces of DNA together by sealing the sugar–phosphate backbone.
• Once the sticky ends of the gene and the sticky ends of the plasmid have paired up, ligase joins them permanently.
• The result is a single piece of DNA made from two different sources — recombinant DNA.

The sequence in order

Common mistake: writing "restrictive enzymes". The word is restriction. Marks are lost for the wrong technical term. Also do not write lipase — that digests lipids and has nothing to do with genetic modification.

Examiner tip. Spell the technical terms exactly: restriction enzyme (not 'restrictive enzyme') and ligase (not 'lipase'). Examiners repeatedly report candidates losing marks for these two errors, and for naming no enzyme at all — vague phrases like 'an enzyme cuts the DNA' score nothing.

A gene on its own cannot get into a cell. It needs a vector — something that carries the DNA in.

What a vector does

A vector takes up a piece of DNA and then inserts this recombinant DNA into another cell. The two vectors you need are plasmids and viruses.

Plasmids

• A plasmid is a small circular loop of DNA found in bacteria, separate from the main bacterial chromosome.
• The plasmid is cut open with a restriction enzyme, the required gene is inserted, and ligase joins it in — the plasmid is now recombinant DNA.
• The recombinant plasmid is put back into a bacterium, which takes it up.
• When that bacterium divides, every new bacterium contains a copy of the plasmid — and therefore a copy of the gene.

Viruses

• A virus naturally inserts its own genetic material into a host cell — that is exactly what is needed.
• The required gene is inserted into the virus's genetic material, and the virus then infects the host cell and delivers the recombinant DNA into it.
• Viruses are useful vectors for getting genes into animal cells and plant cells, not just bacteria.

Transgenic

Transgenic means the transfer of genetic material from one species to a different species.

• A bacterium given the human insulin gene is transgenic.
• A maize plant given a gene from a bacterium is transgenic.
• This works because the genetic code is universal — the same base triplets code for the same amino acids in all organisms, so a human gene is read correctly by a bacterium.

Common mistake: describing a virus as "a cell with no nucleus and no organelles". That is a description of a bacterium. A virus is not a cell at all — it is genetic material surrounded by a protein coat, so the question of organelles does not arise.

Examiner tip. Do not confuse viruses with bacteria. Examiners report candidates writing that 'viruses have no nucleus or organelles' — a virus is not a cell, so this is the wrong comparison. Say a virus is genetic material in a protein coat that inserts DNA into a host cell.

People with diabetes need insulin. Large amounts can be made cheaply by putting the human insulin gene into bacteria and growing them in a fermenter.

Step by step

1. The human insulin gene is cut out of human DNA using a restriction enzyme, leaving sticky ends.
2. A bacterial plasmid is cut open with the same restriction enzyme, so it has complementary sticky ends.
3. Ligase joins the gene into the plasmid, forming recombinant DNA.
4. The recombinant plasmid (the vector) is taken up by a bacterium — the bacterium is now transgenic.
5. The bacteria are grown in a fermenter, where they reproduce very rapidly by dividing. Each new bacterium carries the insulin gene and produces human insulin.
6. The insulin is extracted and purified from the fermenter contents.

Conditions inside the fermenter

Why this method is used

• Very large quantities can be made quickly, because bacteria reproduce rapidly.
• The insulin is identical to human insulin, so it works properly and is less likely to cause a reaction than insulin extracted from animals.
• It is suitable for people who will not use animal products.

If asked what insulin does

Don't stop at "it controls blood sugar". Say how: insulin is a hormone that causes liver and muscle cells to take up glucose from the blood and convert it into glycogen for storage, which lowers the blood glucose concentration.

Common mistake: writing that high temperature "kills the enzymes" or "denatures the bacteria". Enzymes denature; bacteria (living organisms) are killed. Getting these the wrong way round loses the mark.

Examiner tip. Two things examiners flag here. First, use the right verb: enzymes denature, bacteria are killed — never 'kills enzymes' or 'denatures bacteria'. Second, if the question asks about insulin's job, explain the mechanism (glucose taken up by liver/muscle and stored as glycogen), not just 'it controls blood sugar'.

Genes can be transferred into crop plants (usually using a plasmid or virus as the vector) to improve food production. The plant is then transgenic: transgenic means genetic material has been transferred from one species into a different species.

Ways GM crops improve food production

Concerns you can mention

• The inserted gene might spread to wild plants (e.g. herbicide resistance spreading to weeds).
• Insect-resistant crops may kill harmless insects, reducing biodiversity.
• Seeds are often expensive for small farmers.
• Some people are concerned about long-term effects on health, though there is no evidence of harm.

Common mistake: confusing 'describe' and 'explain'. Describe = say what happens ('the crop is resistant to herbicide'). Explain = say why, giving a reason ('...so weeds can be killed, reducing competition for light and mineral ions, so the crop yield increases'). If the command word is explain and you only describe, you get nothing.

Examiner tip. When a question asks how a GM crop improves food production, finish the chain: gene inserted → effect on the plant or on the pest/weed → less crop lost or more crop grown → more food produced. Answers that stop at "the crop is resistant" throw away the last mark. And if you are asked what transgenic means, give the exact definition — genetic material transferred from one species into a different species — rather than a general description of genetic engineering.