Genetic modification: cut, join, carry
Genetic modification (genetic engineering) transfers a gene from one organism to another. A restriction enzyme cuts DNA at a specific place, leaving sticky ends; the SAME enzyme cuts the gene from the donor and cuts open the plasmid so their sticky ends match. Ligase then joins the gene into the plasmid. A plasmid (a small loop of bacterial DNA) or a virus is the vector carrying the gene into the host cell. The plasmid now holds DNA from two sources — recombinant DNA.
GM bacteria make human insulin
Human insulin is the standard example; the steps run in a fixed order: a restriction enzyme cuts the insulin gene from human DNA (sticky ends); a plasmid is cut open with the same enzyme; ligase joins the gene into the plasmid, making recombinant DNA (the plasmid is the vector); it is put into a bacterium, now transgenic; the bacteria are grown in a fermenter, where they multiply and the gene makes them produce insulin, which is extracted in large amounts.
GM plants for food; transgenic means a different species
GM plants improve food production: a herbicide-resistance gene lets a farmer spray a herbicide that kills weeds but not the crop, so the crop has less competition and a higher yield (a herbicide, not a pesticide); a pest-resistance gene means fewer pests eat the crop; other genes add nutrients (e.g. vitamin A in golden rice) or a longer shelf life. Transgenic means an organism containing genetic material transferred from a DIFFERENT species, not just from another organism.
Drawn from real examiner reports.
Restriction cuts, ligase joins
The single most common error is swapping the two enzymes. Restriction enzymes CUT the DNA, leaving sticky ends; ligase JOINS the pieces of DNA together. Writing an enzyme cuts and an enzyme joins without naming either scores nothing. Learn the pair: restriction = cut, ligase = join, and name both in any technique answer.
Name the plasmid as the vector
The plasmid role is the step fewest candidates know. The plasmid (or a virus) is the vector that carries the gene into the host cell. Also, the gene is joined INTO the plasmid's DNA — answers that say the gene is fused with the bacterium or the tomato, rather than joined into its DNA, miss the point. Always name the vector.
Transgenic means a DIFFERENT species
The mark-scheme definition of transgenic has two elements: genetic material that has been transferred, AND from a different species. Writing an organism with a gene from another organism is too vague and loses the mark, because another organism could be the same species. The point is that the DNA has crossed a species boundary — e.g. a human gene put into a bacterium.
Herbicide kills weeds, not a pesticide
Be exact about the chemical. A herbicide kills weeds (plants); a pesticide or insecticide kills pests (insects). Candidates who wrote that the crop kills the pests confused the two and lost marks. The crop is resistant to the herbicide, so the herbicide kills the weeds but not the crop — the GM crop itself does not kill anything.
Name what the weeds compete for
The competition mark is not earned by the word competition alone. State what the weeds compete with the crop for: light, water, mineral ions (nutrients) and space. Removing the weeds gives the crop more of these, so it grows better and yields more. Name at least two of the four resources.
Insulin is injected, not swallowed
Insulin must be injected, not taken as a tablet, because it is a protein: swallowed insulin would be digested by protease enzymes (and denatured by stomach acid) in the gut before it could act. This is a common applied-question point — link it to insulin being a protein and to protease in the digestive system.
Use the 5-step GM scaffold
For "describe how a GM organism is made", use one chain: restriction enzyme cuts the gene from the donor DNA; the same enzyme cuts open the plasmid; ligase joins the gene into the plasmid; insert it into the host bacterium; grow in a fermenter. The method marks fall out.
Same restriction enzyme on both
Say the SAME restriction enzyme is used to cut the gene and to cut open the plasmid. This is why their sticky ends are complementary and can be joined by ligase — a point that separates full-mark answers from vague ones.
Herbicide chain for GM-yield questions
For "explain how a GM crop raises yield", give the chain: herbicide-resistance gene, spray herbicide, weeds die but crop survives, less competition for light, water, mineral ions and space, higher yield. Name the resources, not just competition.
Evaluate a GM claim: for, against, judge
For "evaluate a GM claim", give points for and against and reach a judgement, and use the data. Comment on the trial design too — a small, one-season trial with no stated sample size or repeats gives results that may not be reliable.
Genetic modification (genetic engineering) is the transfer of a gene from one organism to another. It uses a small, fixed tool-kit - learn the two-element, mark-scheme definitions below exactly.
| Term | Precise definition |
|---|---|
| Genetic modification / engineering | The transfer of a gene (genetic material) from one organism to another |
| Restriction enzyme | An enzyme that cuts a DNA molecule at a specific place, leaving short single-stranded "sticky ends" |
| Ligase enzyme | An enzyme that joins two pieces of DNA together (seals the join) |
| Vector | Something used to carry the gene into the host cell - a plasmid or a virus |
| Plasmid | A small circular loop of DNA found in a bacterium, separate from its main chromosome |
| Recombinant DNA | DNA made by joining together DNA from two different sources (organisms) |
| Transgenic | An organism containing genetic material transferred from a DIFFERENT species |
The same restriction enzyme must be used to cut out the gene AND to cut open the plasmid, so their sticky ends are complementary and can be joined by ligase.
Define a restriction enzyme and state its role in genetic engineering.
Scientists use genetic engineering to move a gene from one organism into another.
(a) Name the type of enzyme used to cut DNA. (1) (b) Name the type of enzyme used to join pieces of DNA together. (1) (c) Explain what is meant by the term transgenic. (1)
