Cambridge International AS & A Level Biology 9700: Genetic technology – Study Guide

This guide teaches topic 19, Genetic technology, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers every learning outcome in sections 19.1 to 19.3. This is A Level content, examined on Paper 4 (A Level Structured Questions). The syllabus says the topic relies on your knowledge of DNA and RNA structure and protein synthesis from topic 6, so revise that first if base pairing, transcription or translation feel shaky.

Use the revision notes for final recall and the practice questions to test yourself. The 9700 Biology hub lists every topic, the printable checklist tracks the outcomes, and the A Level diagnostic shows your gaps.

What this topic covers

Section	What you must be able to do
19.1 Principles of genetic technology	Define recombinant DNA; explain genetic engineering and gene editing; three sources of genes; roles of restriction endonucleases, ligase, plasmids, DNA polymerase and reverse transcriptase; promoters; fluorescent marker genes; PCR with Taq polymerase; gel electrophoresis; microarrays; sequence databases
19.2 Genetic technology applied to medicine	Advantages of recombinant insulin, factor VIII and adenosine deaminase; genetic screening for BRCA1/BRCA2, Huntington’s disease and cystic fibrosis; gene therapy for SCID and inherited eye diseases; social and ethical issues
19.3 GMOs in agriculture	How GM salmon, herbicide-resistant soybean and insect-resistant cotton help meet food demand; ethical and social implications

19.1 Principles of genetic technology

Key definitions

• Recombinant DNA: DNA made by joining pieces of DNA from two or more different sources, often different species.
• Genetic engineering: the deliberate manipulation of genetic material to modify specific characteristics of an organism. It may involve transferring a gene into an organism so that the gene is expressed (transcribed and translated into its protein).
• Gene editing: a form of genetic engineering in which DNA is inserted, deleted or replaced at specific sites in the genome. The change is targeted, not random.

Three sources of a gene

1. Extracted from the donor’s DNA, by cutting it out with restriction enzymes. In eukaryotes this gene contains introns, which bacteria cannot remove.
2. Synthesised from mRNA using reverse transcriptase. Cells that make the protein contain many copies of its mRNA, and the complementary DNA (cDNA) made from it has no introns.
3. Synthesised chemically from nucleotides, following a known base sequence. This needs no donor cells, and the sequence can be adjusted.

The enzymes and vectors

Tool	Role
Restriction endonuclease	Cuts DNA at a specific recognition sequence, often a palindrome. Many make staggered cuts that leave single-stranded sticky ends
DNA ligase	Forms phosphodiester bonds in the sugar–phosphate backbone, joining the gene into the vector
Plasmid	Small circle of bacterial DNA that replicates independently; used as a vector to carry the gene into a host cell
Reverse transcriptase	Makes single-stranded cDNA using mRNA as a template
DNA polymerase	Makes the second strand to give double-stranded cDNA; also copies DNA in PCR

Cutting the plasmid and the gene with the same restriction enzyme gives complementary sticky ends. These pair by hydrogen bonding, and ligase then seals the backbone.

Why transfer a promoter?

A promoter is the DNA sequence where RNA polymerase binds to start transcription. A gene taken from a eukaryote comes with a promoter that bacterial RNA polymerase may not recognise. Without a suitable promoter the gene is not transcribed, so no protein is made. A promoter can also be chosen so the gene is expressed only in a particular tissue, such as seeds or milk.

Confirming expression with fluorescent markers

A marker gene coding for a fluorescent product, such as green fluorescent protein (GFP), is inserted alongside the desired gene, under the same promoter. Cells that glow under ultraviolet or blue light have taken up the DNA and are expressing it. You can pick those cells out without harming them.

The polymerase chain reaction (PCR)

PCR clones and amplifies a chosen length of DNA. The mixture contains template DNA, two primers (short single-stranded DNA complementary to the ends of the target), free DNA nucleotides and Taq polymerase, in a buffer. Each cycle has three steps:

1. Denaturation (about 95 °C): hydrogen bonds between the strands break, giving single strands.
2. Annealing (about 55–65 °C): primers bind by complementary base pairing to the 3′ ends of the target on each strand.
3. Extension (about 72 °C): Taq polymerase adds nucleotides to the primers, building new complementary strands.

Taq polymerase comes from Thermus aquaticus, a bacterium from hot springs. It is not denatured at 95 °C and works best near 72 °C, so it does not need replacing each cycle.

Each cycle doubles the number of DNA molecules. After n cycles, starting number × 2ⁿ.

Worked example. A forensic sample contains 50 copies of a target sequence. Assuming every molecule is copied each cycle, how many copies are there after 25 cycles?

copies = 50 × 2²⁵
       = 50 × 33 554 432
       = 1 677 721 600
       ≈ 1.68 × 10⁹ (3 s.f.)

Gel electrophoresis

1. DNA fragments (often cut with restriction enzymes) are loaded into wells at one end of an agarose gel in a buffer.
2. A voltage is applied. DNA is negatively charged because of its phosphate groups, so it moves towards the positive electrode (anode).
3. The gel acts as a sieve. Shorter fragments move further in the same time.
4. A DNA ladder of known fragment lengths runs alongside for comparison.
5. The DNA is stained or labelled so the bands can be seen.

Worked example. A linear piece of DNA 6 000 base pairs (bp) long is cut by a restriction enzyme that recognises sites at positions 1 200 and 4 500.

fragments: 1 200 − 0     = 1 200 bp
           4 500 − 1 200 = 3 300 bp
           6 000 − 4 500 = 1 500 bp
check:     1 200 + 3 300 + 1 500 = 6 000 bp

Three bands. The 1 200 bp band travels furthest; the 3 300 bp band stays closest to the well. If the same two sites were on a 5 000 bp circular plasmid (at 800 and 3 100), two cuts give only two fragments: 2 300 bp and 2 700 bp. A circle cut n times gives n fragments; a line cut n times gives n + 1.

Microarrays

A microarray is a small slide or chip carrying thousands of different single-stranded DNA probes, each in a known position.

• Genome analysis: DNA from a sample is cut, made single-stranded and labelled with a fluorescent tag. Where it is complementary to a probe it hybridises, and that spot fluoresces. This shows which genes or alleles are present, or compares the genomes of two species or individuals.
• Gene expression: mRNA is extracted from cells and reverse transcribed into cDNA, which is labelled. Spots that fluoresce show which genes were being transcribed. The brightness indicates how active each gene was. Samples from two conditions, such as healthy and cancerous tissue, can be labelled with different colours and compared on one array.

Databases

Online databases hold nucleotide sequences of genes and genomes, and amino acid sequences and structures of proteins. Benefits:

• Free, quick access for researchers worldwide; no need to repeat sequencing work.
• New sequences can be compared with known ones to find similar genes, identify organisms or study evolutionary relationships.
• Primers for PCR can be designed from known sequences.
• Protein structures help explain function and support drug design.

19.2 Genetic technology applied to medicine

Recombinant human proteins

Protein	Condition	Advantages of the recombinant version
Insulin	Type 1 diabetes	Identical to human insulin, so fewer immune reactions than insulin from pigs or cattle; large, reliable supply; avoids religious and ethical objections to animal sources
Factor VIII	Haemophilia (a blood-clotting factor is missing)	No risk of infection such as HIV or hepatitis that came with factor VIII purified from donated blood; reliable supply
Adenosine deaminase (ADA)	ADA deficiency, a form of SCID	Produced in quantity for regular injection, replacing the missing enzyme

Genetic screening

Genetic screening tests DNA for particular alleles.

• BRCA1 and BRCA2: certain alleles raise the risk of breast cancer. A positive result allows more frequent checks, earlier diagnosis, or a choice of preventive surgery.
• Huntington’s disease: caused by a dominant allele, with symptoms usually starting in adulthood. Testing lets people plan their lives and make informed choices about having children. There is currently no cure.
• Cystic fibrosis: a recessive condition. Screening can identify carriers, test embryos or newborns, and allow early treatment.

Gene therapy

Gene therapy adds a normal allele to cells of a person with a genetic disease.

• SCID: white blood cells or bone-marrow stem cells are removed, a normal ADA allele is inserted using a viral vector, and the cells are returned. In some early trials the vector inserted next to a gene that controls cell division, and some patients developed leukaemia.
• Inherited eye diseases: a vector carrying the normal allele is injected into the retina. The eye is small and enclosed, so only a small dose is needed, and the untreated eye can act as a comparison.

Treating body (somatic) cells does not change the alleles passed to children.

Social and ethical considerations

• A positive test for an incurable condition, such as Huntington’s disease, can cause great anxiety.
• Results affect relatives, who may not want to know.
• Risk of discrimination by employers or insurers.
• Embryo testing raises questions about which embryos are chosen.
• Gene therapy is expensive, and vectors can trigger immune responses or cause cancer.
• Changing germ cells would affect future generations who cannot consent.

19.3 GMOs in agriculture

Genetic engineering can raise the quality and productivity of farmed animals and crop plants, helping to meet the global demand for food.

• GM salmon: carries a growth hormone gene from another salmon species with a promoter that keeps it switched on all year. The fish reach market size much sooner, so less feed and time are needed per fish. They are farmed in land-based tanks and the females are sterile, lowering the risk of breeding with wild fish.
• Herbicide-resistant soybean: contains a gene giving resistance to a herbicide. Farmers can spray to kill weeds without harming the crop, so the crop faces less competition and yields rise.
• Insect-resistant (Bt) cotton: contains a gene from the bacterium Bacillus thuringiensis coding for a protein toxic to insect larvae that eat the plant. Less insecticide spraying is needed and yields are higher.

Ethical and social implications

Possible benefits: higher yields; less insecticide; food security. Concerns: genes may spread to wild relatives, creating herbicide-resistant weeds; insects may evolve resistance to the Bt toxin; non-target insects may be harmed; seed is expensive and may need buying each year, which is hard for poorer farmers; a few companies control the seed supply; some consumers distrust GM food and want clear labelling; escaped GM fish could harm wild populations.

Common errors

• Saying restriction enzymes “cut DNA randomly”.
• Saying ligase forms hydrogen bonds; it forms phosphodiester bonds.
• Forgetting that cDNA has no introns, which is why it is used for bacteria.
• Saying Taq polymerase “works at high temperatures” without saying it is not denatured at 95 °C.
• Stating that larger fragments move further in electrophoresis.
• Describing gene therapy as curing the patient’s children.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus, for examination in 2025, 2026 and 2027 (version 1), published by Cambridge University Press & Assessment. Topic 19, sections 19.1 to 19.3.