A level-readyAll subjects · Biology · WBI15 (Unit 5)
Recombinant DNA and producing drugs with GMOs
Genes can be cut from one organism and inserted into another. Genetically modified microorganisms, plants and animals can then make medicines.
Making recombinant DNA
1. Obtain the gene: cut it out of DNA with a restriction endonuclease, or make it from mRNA using reverse transcriptase (giving complementary DNA, cDNA, with no introns).
2. Restriction endonucleases cut DNA at specific recognition sequences. Many make staggered cuts, leaving short single-stranded sticky ends.
3. Cut a vector (such as a bacterial plasmid) with the same restriction enzyme, so it has complementary sticky ends.
4. Mix the gene and vector. Sticky ends pair by complementary base pairing, and DNA ligase joins the sugar-phosphate backbones by forming phosphodiester bonds. The result is recombinant DNA.
A marker gene (for example antibiotic resistance or a fluorescent protein gene) is usually included, so cells that have taken up the recombinant DNA can be identified.
Getting DNA into cells
- Plasmids into bacteria: bacteria are treated with calcium ions and heat shock, or an electric pulse (electroporation), making membranes more permeable.
- Viruses: modified viruses can deliver genes into animal or human cells.
- Agrobacterium tumefaciens: a bacterium that infects plants; its Ti plasmid carries the gene into plant cells.
- Gene gun: tiny gold or tungsten particles coated with DNA are fired into plant cells.
- Microinjection: DNA is injected into the nucleus of a fertilised animal egg.
- Liposomes: DNA wrapped in lipid spheres fuses with cell membranes.
Producing drugs
- GM microorganisms: bacteria or yeast with the human insulin gene make human insulin in fermenters. They grow quickly and cheaply and the product is identical to human insulin.
- GM plants: crop plants can be modified to make vaccines, antibodies or other medicines in their leaves or seeds ('pharming').
- GM animals: genes for human proteins are joined to promoters active only in the mammary gland, so the protein is secreted in milk; for example goats producing human antithrombin, a protein that prevents blood clotting.
Worked example
Explain why the same restriction enzyme is used to cut the gene and the plasmid.
- Each restriction enzyme cuts at one specific recognition sequence.
- Using the same enzyme gives the gene and the plasmid complementary sticky ends.
- The ends can base-pair, and DNA ligase can then join them.
Answer: So the sticky ends are complementary and can join.
Check yourself
- What do restriction endonucleases do?
- Join DNA fragments together
- Cut DNA at specific recognition sequences
- Copy DNA from mRNA
- Add nucleotides during PCR
Show answer
Cut DNA at specific recognition sequences. They cut at specific base sequences.
- What is the role of DNA ligase?
- It cuts DNA
- It separates DNA strands
- It makes mRNA
- It forms phosphodiester bonds to join DNA fragments
Show answer
It forms phosphodiester bonds to join DNA fragments. Ligase seals the sugar-phosphate backbone.
- What are sticky ends?
- Ends of DNA coated in glue proteins
- Short single-stranded overhangs left by staggered cuts
- Double-stranded blunt ends
- The ends of chromosomes
Show answer
Short single-stranded overhangs left by staggered cuts. Overhangs can base-pair with complementary sticky ends.
- Why might reverse transcriptase be used to obtain a human gene for insertion into bacteria?
- It cuts the gene out of the chromosome
- It joins the gene to the plasmid
- It makes cDNA from mRNA, which has no introns that bacteria cannot remove
- It makes the bacteria divide
Show answer
It makes cDNA from mRNA, which has no introns that bacteria cannot remove. Bacteria cannot splice out introns, so cDNA is used.
- Why is a marker gene included with the recombinant DNA?
- To identify cells that have taken up the recombinant DNA
- To cut the plasmid
- To make the bacteria grow faster
- To stop the gene being expressed
Show answer
To identify cells that have taken up the recombinant DNA. For example, only transformed bacteria grow on an antibiotic.
- Which method is commonly used to insert genes into plant cells?
- Microinjection into fertilised eggs
- Heat shock with calcium ions only
- Agrobacterium tumefaciens carrying a modified Ti plasmid
- Injecting antibodies
Show answer
Agrobacterium tumefaciens carrying a modified Ti plasmid. Agrobacterium naturally transfers DNA into plants.
- Why are genes for human proteins in GM goats joined to a promoter active only in mammary glands?
- So the protein is secreted in the milk and easily collected
- So the protein is made in every cell
- So the goats produce more milk
- So the gene is never expressed
Show answer
So the protein is secreted in the milk and easily collected. The protein can be purified from milk without harming the animal.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.