The big idea: Genetic modification means deliberately changing an organism's DNA — usually by adding a gene from another species, or by editing a gene that is already there.
An organism whose DNA has been changed this way is called a genetically modified (GM) organism, or a transgenic organism if it now carries a gene from a different species.
To move a gene from one organism into another, the cell's own molecular tools are borrowed: enzymes that cut DNA, an enzyme that joins DNA, and a small carrier of DNA called a vector.
- Genetic modification (genetic engineering)
- Deliberately changing an organism's DNA — for example by inserting a gene from another organism, or editing an existing gene.
- Genetically modified (GM) organism
- An organism whose DNA has been deliberately altered by genetic engineering.
- Transgenic organism
- A GM organism that carries a gene transferred from a different species.
- Restriction enzyme (restriction endonuclease)
- An enzyme that cuts DNA at a specific recognition sequence, often leaving short single-stranded 'sticky ends'.
- DNA ligase
- An enzyme that joins two pieces of DNA together by re-forming the sugar–phosphate backbone.
- Vector (plasmid)
- A small loop of DNA (often a bacterial plasmid) used to carry a gene into a host cell.
- Recombinant DNA
- A single DNA molecule made by joining DNA from two different sources (for example a plasmid with a new gene added).
| Tool | What it does | Why it is needed |
|---|---|---|
| Restriction enzyme (restriction endonuclease) | Cuts DNA at a specific recognition sequence, often leaving short single-stranded 'sticky ends' | Cuts the wanted gene out AND opens the vector at the same site, so their ends match |
| DNA ligase | Joins two pieces of DNA by re-forming the sugar–phosphate backbone | Seals the gene into the vector to make one continuous recombinant DNA molecule |
| Vector (usually a plasmid) | A small loop of DNA that can enter a host cell and be copied there | Carries the new gene into the host cell so the cell can use it |
| Host cell (e.g. a bacterium) | Takes up the recombinant vector and expresses the gene | Makes the product of the gene (a GM organism is produced) |
Two roles to never mix up: A restriction enzyme CUTS DNA (it is the 'scissors').
DNA ligase JOINS DNA (it is the 'glue').
Both names hide a clue: 'restriction' enzymes restrict by cutting; 'ligase' comes from ligate, meaning to tie together.
To make a GM bacterium or crop, a single wanted gene has to be moved into the host and made to work.
The classic method joins the gene to a plasmid vector to make recombinant DNA, then puts that plasmid into a host cell. The whole process relies on the two enzymes above acting in the right order.
Why the SAME restriction enzyme is used twice: A restriction enzyme cuts DNA wherever it finds one specific recognition sequence, leaving short single-stranded sticky ends.
If the same enzyme is used to cut out the gene and to open the plasmid, both pieces are left with the same sticky ends.
Because the sticky ends are complementary, the gene and the plasmid can pair up by base-pairing — and then DNA ligase seals the join into one continuous recombinant molecule.
| Step | What happens | Tool used |
|---|---|---|
| 1. Cut out the gene | The wanted gene is cut from the source DNA | Restriction enzyme |
| 2. Open the vector | The SAME restriction enzyme opens the plasmid, leaving matching sticky ends | Restriction enzyme |
| 3. Join them | The gene's sticky ends pair with the plasmid's, and the join is sealed | DNA ligase → recombinant DNA |
| 4. Insert into host | The recombinant plasmid is taken up by the host cell (transformation) | Vector / plasmid |
| 5. Express the gene | The host cell copies and uses the gene, making the GM organism | Host cell |
Then: insert and express: The recombinant plasmid is taken up by a host cell (a process called transformation).
Inside the host, the gene is copied and expressed — the cell now makes the gene's product.
For example, GM bacteria carrying the human insulin gene make insulin; a crop given a herbicide-resistance gene can survive being sprayed while the weeds around it die.
CRISPR-Cas9 — editing instead of adding: Older methods mainly add a gene. CRISPR-Cas9 lets scientists edit a gene already in the cell — far more precisely.
A short guide RNA is made to match one chosen DNA sequence. It leads the Cas9 protein to exactly that spot, where Cas9 cuts the DNA.
The cell then repairs the cut — and during repair a gene can be disabled (knocked out), corrected, or have a new piece inserted. This is why CRISPR is called a tool for gene editing, not just gene transfer.
| Feature | Recombinant DNA (gene transfer) | CRISPR-Cas9 (gene editing) |
|---|---|---|
| Main aim | ADD a new gene from another organism | EDIT or switch off a gene already in the cell |
| What targets the DNA | A restriction enzyme cuts at its fixed recognition site | A guide RNA leads Cas9 to one chosen sequence |
| What cuts the DNA | Restriction enzyme | The Cas9 protein (an enzyme) |
| Typical use | Insulin-making bacteria; herbicide-resistant crops | Knocking out a faulty gene; precise crop improvement |
Restriction enzyme — the scissors
- Cuts DNA at a specific recognition sequence
- Leaves matching sticky ends
- The same enzyme cuts the gene AND the vector
- So their ends are complementary and can pair up
DNA ligase — the glue
- Joins the gene into the vector
- Re-forms the sugar–phosphate backbone
- Makes one continuous recombinant DNA molecule
- Seals the join so the plasmid is whole again
A memory hook: Cut → join → carry → express. Restriction enzyme cuts the gene and the vector with matching ends; ligase joins them into recombinant DNA; the vector carries it into the host; the host expresses the gene.
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How this is tested: Paper 3 likes a 3-mark Describe question on how a gene (often glyphosate / herbicide resistance) is engineered into a crop — the marks are for the ordered steps: restriction enzyme cuts gene and vector, ligase joins them, the vector carries the gene into the cell.
It often follows up with a 2-mark Explain on an advantage of the GM crop, and a 1-mark Suggest on a limitation (such as why uptake never reaches 100 %).
On Paper 1 a 1-mark question can ask you to state the role of DNA ligase, and on Paper 2 a short Explain can ask how restriction endonucleases are used in gene transfer.
IB-style question — describe how a gene is inserted into a crop
A herbicide-resistance gene is to be transferred from a bacterium into a crop plant so the crop survives spraying. Describe how the gene is cut out, joined to a vector and inserted. [3]
How to score all three marks
- Cut with a restriction enzyme. A restriction enzyme cuts the resistance gene out of the source DNA, and the same enzyme opens the plasmid vector, so both have matching sticky ends.
- Join with ligase. The gene's sticky ends base-pair with the plasmid's, and DNA ligase joins (seals) them into one recombinant DNA molecule.
- Insert into the plant cells. The recombinant vector carries the gene into the crop's cells (transformation), where the gene is expressed so the plant becomes herbicide-resistant. (Award 1 mark for the cut, 1 for the ligase join, 1 for inserting/expressing via the vector.)
Final answer
A restriction enzyme cuts out the gene and opens the plasmid at the same site (matching sticky ends); DNA ligase joins them into recombinant DNA; the vector carries the gene into the crop cells, where it is expressed.
✓ Why this scores full marks: Each mark is a distinct, ordered step — cut, join, insert — with the right tool named at each stage.
The most common slip is to say 'restriction enzyme joins the gene' (it cuts) or to leave out the vector that actually carries the gene in.
| Step | What happens | Tool used |
|---|---|---|
| 1. Cut out the gene | The wanted gene is cut from the source DNA | Restriction enzyme |
| 2. Open the vector | The SAME restriction enzyme opens the plasmid, leaving matching sticky ends | Restriction enzyme |
| 3. Join them | The gene's sticky ends pair with the plasmid's, and the join is sealed | DNA ligase → recombinant DNA |
| 4. Insert into host | The recombinant plasmid is taken up by the host cell (transformation) | Vector / plasmid |
| 5. Express the gene | The host cell copies and uses the gene, making the GM organism | Host cell |