OCR A Level Biology A: Genetic engineering and gene therapy (H420)

This guide follows the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), and covers module 6.1.3 (Manipulating genomes) learning outcomes (f), (g) and (h): genetic engineering, the ethical issues it raises, and gene therapy. Outcomes (a) to (e) of the same module are not taught here. Condensed recall is in the revision notes and exam-style questions are in the practice set. Other topics sit on the OCR A Level Biology hub and the printable checklist, and the free diagnostics help you choose where to start.

What the unit covers

Outcome	Skills to demonstrate
6.1.3(f)(i)	Explain the principles of genetic engineering, including isolating a gene from one organism and placing it in another using a suitable vector
6.1.3(f)(ii)	Describe the techniques: restriction enzymes, plasmids and DNA ligase to make recombinant DNA, and electroporation
6.1.3(g)	Discuss the positive and negative ethical issues of manipulating animals (including humans), plants and microorganisms; includes insect-resistant soya, modified pathogens for research, pharming, patenting and technology transfer
6.1.3(h)	Explain the principles and potential of gene therapy, including somatic cell and germ line cell gene therapy

Principles of genetic engineering (6.1.3(f)(i))

Genetic engineering is the deliberate transfer of a gene from one organism into another so that the recipient makes the gene’s product. This works because the genetic code is universal: a bacterium’s ribosomes read a human gene using the same codons. The organism that now carries the foreign gene is transgenic, and DNA made by joining sequences from different sources is recombinant DNA.

Every project has the same four stages.

1. Isolate the gene of interest.
2. Insert it into a vector, a carrier that can enter a host cell and is copied or expressed there.
3. Transfer the recombinant vector into host cells.
4. Identify the cells that took it up, and grow them.

Isolating the gene

• Restriction enzymes cut the gene out of the donor’s DNA, though it must then be found among many fragments.
• Reverse transcriptase can make complementary DNA (cDNA) from mRNA. Cells that make a lot of a protein hold a lot of its mRNA. For example, pancreatic beta cells hold insulin mRNA. The cDNA is a copy of the coding sequence with no introns, which suits a bacterial host that cannot splice them out.
• A gene machine can build a DNA sequence from known bases.

Vectors

A plasmid is a small circular loop of DNA in bacteria, separate from the main chromosome, which copies itself and can carry useful genes. Other vectors include viruses and, in plants, the Ti plasmid of Agrobacterium tumefaciens. A useful plasmid vector has an origin of replication, a restriction site and a marker gene.

The inserted gene also needs a promoter at its start, so RNA polymerase can begin transcription in the host, and a terminator at its end. Without a promoter that works in the host, the gene is present but not expressed.

Techniques (6.1.3(f)(ii))

Restriction enzymes

A restriction enzyme (restriction endonuclease) cuts DNA by hydrolysing phosphodiester bonds, but only where it finds its specific recognition sequence. The shape of the active site is complementary to that sequence only. EcoRI recognises 5′-GAATTC-3′ and cuts each strand after the G. The sequence reads the same 5′ to 3′ on both strands (palindromic), so the cuts are staggered. This leaves short single-stranded overhangs called sticky ends. Some enzymes cut straight across and leave blunt ends.

If you cut the gene and the plasmid with the same enzyme, the sticky ends are complementary and will pair by hydrogen bonds. That pairing is called annealing.

Worked example 1: fragments from a digest. A linear piece of DNA is 7,400 base pairs (bp) long. EcoRI cuts it at positions 1,900 and 5,200 bp from the left end.

1. Two cuts in a linear molecule give three pieces.
2. Left piece: 1,900 bp.
3. Middle piece: 5,200 − 1,900 = 3,300 bp.
4. Right piece: 7,400 − 5,200 = 2,200 bp.
5. Check: the three pieces sum to 7,400 bp.

A circular plasmid cut at two sites gives two pieces, not three.

Worked example 2: how often does a site occur? A six-base site has 4⁶ = 4,096 possible sequences, so in random DNA it appears about once per 4,096 bp. A 4,700 bp plasmid might have about one, which is why vectors are chosen to have a single useful site.

DNA ligase

Annealed sticky ends are held only by hydrogen bonds and can separate. DNA ligase makes phosphodiester bonds where the gene and plasmid backbones meet, closing the nicks permanently.

Worked example 3: checking the product. A vector of 4,700 bp is opened with one EcoRI cut and a 1,150 bp gene, released by the same enzyme, is ligated in.

• Recombinant plasmid = 4,700 + 1,150 = 5,850 bp.
• Cutting it again with EcoRI should give 4,700 bp and 1,150 bp pieces, confirming the insert.

Getting DNA into the host cell

Method	How it works	Used for
Electroporation	A brief high-voltage pulse creates temporary pores in the cell surface membrane (and wall) so DNA can pass through	Bacteria, yeast, some plant and animal cells
Heat shock with calcium ions	Chilled cells in calcium chloride take up plasmid DNA when briefly warmed	Bacteria
Agrobacterium	The Ti plasmid carrying the gene transfers it into plant DNA	Plants
Viral vector	A modified virus carries the gene into the cell	Animal cells, gene therapy

Most cells take up no plasmid.

Identifying transformed cells

A marker gene is put in the vector. Two common types are:

• Antibiotic resistance. Only cells with the plasmid grow on agar containing the antibiotic.
• Green fluorescent protein (GFP). Transformed cells glow under ultraviolet light.

Insertional inactivation adds a second check. If the gene is inserted into the middle of one resistance gene, that resistance is lost. Colonies that grew on antibiotic A but fail on antibiotic B carry a recombinant plasmid.

Worked example 4: counting recombinants. A vector has one gene giving resistance to spectinomycin and another giving resistance to chloramphenicol. The gene of interest is placed in the middle of the chloramphenicol gene. Treated bacteria are spread on spectinomycin agar and 248 colonies grow. Replica plating copies them onto chloramphenicol agar, where 59 colonies grow.

1. All 248 hold a plasmid, because they resist spectinomycin.
2. The 59 that also resist chloramphenicol have an intact gene, so no insert.
3. Recombinants: 248 − 59 = 189.
4. Percentage: 189 ÷ 248 × 100 = 76.2%.

Worked example 5: transformation efficiency. A tube receives 0.05 µg of plasmid. After recovery the cells are in 2.0 cm³ of broth, and 0.2 cm³ is plated, giving 130 colonies.

1. Colonies in the whole tube: 130 × (2.0 ÷ 0.2) = 1,300.
2. Efficiency: 1,300 ÷ 0.05 = 26,000 colonies per µg of DNA.

Ethical issues (6.1.3(g))

Give both sides and a reasoned judgement: who benefits, who may be harmed, and what is uncertain. Ethical issues for humans are covered under gene therapy below.

Plants: insect-resistant soya

A gene from the bacterium Bacillus thuringiensis codes for a protein toxic to certain insect larvae, and it can be put into soya plants.

Positive	Negative
Less crop lost to pests, so higher yield and income	Non-target insects may be harmed
Less insecticide sprayed, which saves money and reduces pollution	Pests may evolve resistance to the toxin through selection
Food supply is more secure	Genes could spread to wild relatives through pollen, and long-term effects are uncertain

Worked example 6: judging a trial. In a trial, conventional soya gave a mean yield of 2.9 tonnes per hectare and the modified variety gave 3.4. The modified plots were sprayed 2 times in the season; the conventional plots 6 times.

• Yield increase: (3.4 − 2.9) ÷ 2.9 × 100 = 17.2%.
• Fall in spraying: (6 − 2) ÷ 6 × 100 = 66.7%.
• Judgement: both measures favour the modified crop, but one trial says nothing about wild insects or resistance building up.

Microorganisms: modified pathogens for research

Researchers alter bacteria and viruses, for example adding a fluorescent protein gene so infection can be followed, or removing a gene to find its role. This improves understanding of disease. The negative case is biosecurity: a modified pathogen could escape or be misused, so work must be tightly controlled.

Animals: pharming

Pharming means genetically modified animals that produce pharmaceuticals. A human gene is joined to a promoter active only in mammary gland cells, so the protein is secreted in milk and purified. For example, a goat giving 600 dm³ of milk a year with 3.5 g of the protein in each dm³ gives 600 × 3.5 = 2,100 g, or 2.1 kg. Positive: complex human proteins are made correctly and cheaply. Negative: animal welfare, low success rates in producing transgenic animals, and objections to treating animals as production systems.

Patenting and technology transfer

A company can patent a modified organism or gene. A patent rewards invention, but it can make seed expensive and give a few companies control of food production. Technology transfer asks whether the benefits reach poorer farmers, for example by making modified seed available at low cost. Sharing could reduce hunger, but farmers may become dependent on suppliers, and a variety bred for one region may not suit another.

Gene therapy (6.1.3(h))

Gene therapy treats a genetic disorder by altering the genes in a patient’s cells. It works best for disorders caused by a single faulty gene, especially recessive ones, where supplying one working copy can restore the missing protein. Examples include severe combined immunodeficiency (SCID) and cystic fibrosis.

Principles

1. Identify the gene responsible and obtain a normal copy.
2. Put the normal allele into a vector. Common vectors are modified viruses (the viral genes that cause disease are removed) and liposomes (lipid spheres that fuse with the cell membrane).
3. Deliver it to the target cells, either by treating cells outside the body and returning them (ex vivo) or by delivering the vector directly (in vivo).
4. The cells make the functional protein.

Somatic and germ line therapy

Feature	Somatic cell gene therapy	Germ line cell gene therapy
Cells treated	Body cells, such as bone marrow, lung or retina cells	Gametes, a zygote or early embryo
Inherited by children?	No	Yes, because every cell of the person, including their gametes, carries the change
Who is affected?	The patient only, who can consent	Future people, who cannot consent
Effect	Often temporary, because treated cells die and are replaced, so treatment may need repeating	Potentially permanent in the family line
Status	Used in medicine	Heavily restricted, and ethically controversial

Potential and problems

• Potential: it can treat the cause rather than relieve symptoms.
• Problems: the gene may insert at random and disrupt another gene, which can cause cancer; the immune system may attack the vector; the effect may wear off; and cost is high.

Worked example 7: reading a gene therapy result. A patient with an enzyme deficiency has blood enzyme activity of 2 units before treatment. After somatic gene therapy it is 11 units at 1 month, 7 units at 6 months and 4 units at 12 months.

• At 1 month: 11 ÷ 2 = 5.5 times the starting activity.
• At 12 months: 4 ÷ 2 = 2 times the starting activity, so still above baseline.
• Interpretation: the treatment worked, but activity falls. Treated cells may die and be replaced by untreated ones, so repeat treatment may be needed.

Common errors

• Writing that ligase “joins the bases”. It forms phosphodiester bonds in the sugar-phosphate backbone.
• Saying a restriction enzyme cuts at any sequence. Its active site fits one recognition sequence.
• Calling cDNA a gene cut from the chromosome. It is made from mRNA, so it has no introns.
• Giving ethics answers with one side only, or with no conclusion.

Official syllabus

This guide follows OCR Level 3 Advanced GCE in Biology A (H420), Version 4.1 (April 2026), for first assessment in 2025, published by OCR (Cambridge University Press & Assessment), module 6.1.3(f) to (h).