Two enzymes: restriction cuts, ligase joins
Genetic engineering uses two enzymes. A restriction enzyme cuts DNA at a specific site, leaving short single-stranded sticky ends; a ligase enzyme joins pieces of DNA by sealing the sugar-phosphate backbone. Sequence: a restriction enzyme cuts the wanted gene from the donor DNA and cuts the plasmid open; the complementary sticky ends base-pair; ligase seals the joins, forming recombinant DNA. Hook: Restriction Rips, Ligase Links.
Plasmid vector and human insulin
A vector carries foreign DNA into a host cell. In 4BI1 the main vector is a plasmid, a small circular bacterial DNA molecule (a virus can also act as one). A recombinant plasmid carrying the human insulin gene enters a bacterium such as E. coli, which reads the gene and makes human insulin. The bacteria multiply in a fermenter, so large amounts are made; this insulin matches human insulin, so it causes fewer immune reactions than pig or cattle insulin.
GM crops and the term transgenic
GM crops carry a foreign gene for a useful trait. Herbicide-resistant crops survive a spray that kills competing weeds, so the crop yields more. Pest-resistant crops make a toxin (e.g. Bt toxin from Bacillus thuringiensis) that kills insect pests, cutting pesticide use. Transgenic means transfer of genetic material from one species to a DIFFERENT species. Risks include harm to non-target species, gene spread to wild relatives and lower biodiversity.
Drawn from real examiner reports.
Restriction cuts, ligase joins: do not swap
The most-penalised 5c error is reversing the two enzymes: Restriction Rips (cuts DNA at a specific site), Ligase Links (seals the joins). Correct chain: the restriction enzyme cuts the gene out, the same enzyme opens the plasmid, the sticky ends anneal, then ligase seals. Writing "restriction enzyme joins" or "ligase cuts" loses that mark.
June 2024 Paper 1BR Q9(a): candidates swapped the two enzyme roles.
Transgenic needs "different species"
Transgenic means transfer of genetic material from one species to a DIFFERENT species. Writing "from one organism to another" scores nothing because the cross-species element is missing. Changing a gene within one species is not transgenic. Human insulin made in E. coli is a perfect example: a human gene placed into a bacterium of a different species.
June 2023 Paper 1BR Q9(a)(i): "one organism to another" without "different species" lost the mark.
Herbicide kills weeds, not pests
A herbicide kills plants (weeds); a pesticide kills animal pests, so do not call it a pesticide. Do not stop at "kills weeds" either: the marks are in the mechanism. Weeds compete with the crop for light, water and mineral ions, so removing them cuts competition and the crop yields more. Name at least one resource the weeds took.
June 2023 Paper 1BR Q9(a)(ii): candidates confused herbicide with pesticide and omitted what weeds compete for.
Name the enzymes and the plasmid step
Vague answers cost marks. "An enzyme cuts and joins the DNA" earns little; name restriction AND ligase separately. Many also forget the plasmid: the gene is joined into a plasmid (the vector), not "fused with" the whole organism. Say the gene joins DNA in the plasmid, which carries it into the host cell. This plasmid-as-vector step is frequently missed.
June 2024 Paper 1BR Q9(a): fewer candidates knew the plasmid vector role; some wrote of fusing genes with the organism instead of with DNA.
Evaluate GM data, do not just describe
On "evaluate" GM questions, use the data both ways and judge the conclusion, do not just restate figures. For GM fish that grow fast and waste less feed, less waste means less decomposition and eutrophication; but there is no evidence of lower disease, genetic uniformity can raise disease risk, and no sample size is given. State support, counter-evidence and the limitation.
June 2024 Paper 2B Q6(b)(iii)/(v): weak answers described the GM tilapia data without evaluating the conclusion or noting sample size.
GM inserts a gene; cloning copies a genome
Genetic modification and cloning are different. GM inserts a new gene from another species using restriction and ligase enzymes and a vector, changing the genome. Cloning (topic 5d) makes genetically identical copies of an existing organism, adding no new gene. On a GM question do not describe nuclear transfer or micropropagation; describe the enzyme-and-vector method.
Balance an evaluate: benefit, risk, verdict
For discuss or evaluate questions, give a benefit with its mechanism, then a risk with its mechanism, then a conclusion that weighs the evidence and notes missing data like sample size. Points with no verdict cannot get full marks.
Write the GM process as a numbered sequence
For a describe-the-process question, list ordered steps: identify the gene; restriction enzyme cuts it out; the same enzyme opens the plasmid; sticky ends anneal; ligase seals; the plasmid enters the bacterium, which makes the protein. Missing or scrambled steps lose marks.
Explain the mechanism, do not just describe
4BI1 examiners separate describe from explain on every paper. To explain, give the mechanism and a direction of change, e.g. fewer weeds means less competition so the crop grows more. State the biological link, not the observation alone.
Genetic engineering (also called genetic modification, GM) is the process of taking a gene from one organism and inserting it into the deoxyribonucleic acid (DNA) of another organism so that the receiving organism gains a new, desired characteristic.
Two essential enzymes (statement 5.12):
| Enzyme | Role | Memory hook |
|---|---|---|
| Restriction enzyme | Cuts DNA at a specific recognition sequence, leaving "sticky ends" | Restriction = Rips / cuts |
| Ligase enzyme | Joins pieces of DNA together (seals the phosphodiester backbone) | Ligase = Links / joins |
Key term — Transgenic (statement 5.16): the transfer of genetic material from one species to a different species. Both elements are required for full marks: (1) genetic material is transferred, and (2) it crosses a species boundary. Writing "from one organism to another" WITHOUT "different species" scores zero.
Vector (statement 5.13): a DNA molecule used to carry recombinant DNA into a host cell. In 4BI1 the main vectors are:
Define the term transgenic.
Name the TWO enzymes used in genetic engineering and state the role of each. [2 marks]
