Cambridge International AS & A Level Biology 9700: Genetic technology – Revision Notes

These notes condense topic 19, Genetic technology, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, sections 19.1 to 19.3. It is A Level content, examined on Paper 4. For full explanations and worked examples, read the study guide first.

Then test yourself with the practice questions and the A Level diagnostic. The 9700 Biology hub and the printable checklist cover the rest of the course.

19.1 Principles of genetic technology

Definitions

• Recombinant DNA: DNA made by joining DNA from two or more different sources.
• Genetic engineering: deliberate manipulation of genetic material to modify specific characteristics; may involve transferring a gene so that it is expressed.
• Gene editing: genetic engineering that inserts, deletes or replaces DNA at specific sites in the genome.

Sources of the gene

Source	Key point
Extracted from donor DNA	Cut out with restriction enzymes; eukaryotic genes contain introns
Synthesised from donor mRNA	Reverse transcriptase makes cDNA; mRNA is plentiful in cells making the protein; no introns
Synthesised chemically from nucleotides	Built to a known sequence; no donor cells needed

Tools

Tool	One-line role
Restriction endonuclease	Cuts at a specific recognition sequence, leaving sticky (or blunt) ends
DNA ligase	Forms phosphodiester bonds to seal the sugar–phosphate backbone
Plasmid	Vector; small circular DNA that replicates independently in bacteria
Reverse transcriptase	mRNA → single-stranded cDNA
DNA polymerase	Makes the second cDNA strand; copies DNA in PCR
Promoter	Site where RNA polymerase binds; must be one the host recognises, or the gene is not transcribed
Fluorescent marker gene (e.g. GFP)	Transferred with the gene; glowing cells are expressing the inserted DNA

Method in steps: making a GM bacterium

1. Obtain the gene (from mRNA via reverse transcriptase is common, as cDNA has no introns).
2. Add a promoter the bacterium recognises, and a fluorescent marker gene.
3. Cut the plasmid with the same restriction enzyme to give complementary sticky ends.
4. Mix gene and plasmid; sticky ends pair; DNA ligase seals them, giving recombinant DNA.
5. Put the plasmids into bacteria.
6. Select bacteria that fluoresce; culture them to make the protein.

Method in steps: PCR cycle

Step	Temperature	What happens
Denaturation	about 95 °C	Hydrogen bonds break; strands separate
Annealing	about 55–65 °C	Primers bind to complementary sequences at the 3′ ends of the target
Extension	about 72 °C	Taq polymerase adds nucleotides to the primers

• Taq polymerase: from Thermus aquaticus; not denatured at 95 °C; optimum near 72 °C.
• Copies after n cycles = starting copies × 2ⁿ (assuming perfect doubling).

Gel electrophoresis in five facts

1. DNA is negatively charged (phosphate groups).
2. It moves to the anode (positive electrode).
3. The gel acts as a sieve: shorter fragments move further.
4. A ladder of known lengths gives the scale.
5. Cuts: a linear molecule cut n times gives n + 1 fragments; a circular one gives n.

Worked reminders

PCR numbers. Starting with 3 copies, after 12 cycles: 3 × 2¹² = 3 × 4 096 = 12 288 copies. Always multiply the starting number by 2ⁿ; do not add 2n.

Fragment lengths. A linear 4 000 bp molecule cut at 700 bp and 2 600 bp gives 700, 1 900 and 1 400 bp. Check they add back to 4 000 bp. On the gel the 700 bp band is furthest from the well and the 1 900 bp band is nearest.

Sticky ends. If the gene and plasmid are cut with the same enzyme, their single-stranded ends are complementary. They pair by hydrogen bonds first; ligase then makes the permanent phosphodiester bonds.

Microarrays

• Thousands of single-stranded DNA probes in known positions.
• Genome analysis: labelled sample DNA hybridises to complementary probes; fluorescent spots show which sequences or alleles are present.
• Gene expression: mRNA → labelled cDNA; spot brightness shows how strongly each gene is transcribed; two conditions can be compared with two colours.
• Reading an array: a spot that fluoresces means hybridisation happened, so the complementary sequence was in the sample. No fluorescence means that sequence was absent, or that gene was not being transcribed.

Databases

Free global access to nucleotide sequences, amino acid sequences and protein structures; compare new sequences with known ones; identify genes and organisms; study evolutionary relationships; design primers; support drug design; avoid repeating work.

19.2 Medicine

Recombinant proteins

• Insulin: identical to human insulin, fewer allergic or immune responses, reliable large supply, no animal-source objections.
• Factor VIII (haemophilia): no risk of infection from donated blood, reliable supply.
• ADA (a form of SCID): enzyme made in quantity for replacement by injection.

Genetic screening

Condition	Allele	Advantage of screening
Breast cancer	BRCA1, BRCA2	Closer monitoring, earlier diagnosis, option of preventive surgery
Huntington’s disease	Dominant; late onset	Informed life and family planning
Cystic fibrosis	Recessive	Identify carriers; test embryos or newborns; early treatment

Gene therapy

• SCID: remove white blood cells or stem cells, insert normal ADA allele with a viral vector, return cells. Risk: vector insertion near a cell-division gene has caused leukaemia.
• Inherited eye diseases: vector with the normal allele injected into the retina; small, enclosed organ; small dose; other eye is a control.

Must-know distinctions

• Genetic screening (testing for alleles) vs gene therapy (adding a normal allele to treat).
• Somatic gene therapy (not inherited) vs germ-line (inherited; raises consent issues for future generations).
• Genetic engineering (can insert anywhere) vs gene editing (targeted site).
• cDNA (no introns) vs genomic DNA (with introns).

Ethical and social issues

Anxiety about incurable conditions; effects on relatives; discrimination by insurers or employers; embryo selection; cost and fair access; vector safety.

19.3 GMOs in agriculture

GMO	Gene added	Benefit
GM salmon	Growth hormone gene with a promoter active all year	Reaches market size sooner; less feed per fish
Soybean	Herbicide resistance	Weeds sprayed without harming crop; less competition, higher yield
Cotton	Bt toxin from Bacillus thuringiensis	Kills insect larvae that eat it; less insecticide; higher yield

Implications: gene flow to wild relatives; herbicide-resistant weeds; insects resistant to Bt toxin; harm to non-target species; cost of seed for poorer farmers; control by a few companies; consumer concern and labelling; escape of GM fish.

Quick self-test

1. Define recombinant DNA.
2. Why is cDNA, rather than the gene cut from human DNA, used to make a human protein in bacteria?
3. State the role of DNA ligase.
4. Why might a promoter need to be transferred with the gene?
5. Why does Taq polymerase not need replacing after each PCR cycle?
6. How many copies of a DNA molecule are made from one molecule after 10 cycles of PCR?
7. Starting with 4 copies, how many copies are there after 30 cycles? Give your answer to 3 s.f.
8. A linear DNA molecule is cut at three sites. How many fragments are made? How many if it is circular?
9. A protein has 150 amino acids. How many nucleotides are needed in the coding strand, including a stop codon?
10. In electrophoresis, towards which electrode does DNA move, and why?
11. State one advantage of recombinant factor VIII.
12. State one concern about herbicide-resistant soybean.

Answers

1. DNA made by joining pieces of DNA from two or more different sources.
2. cDNA has no introns; bacteria cannot remove introns, so they could not make the correct protein from the genomic gene.
3. Forms phosphodiester bonds, joining the sugar–phosphate backbones of the gene and the plasmid.
4. The host’s RNA polymerase must bind to a promoter it recognises, or the gene is not transcribed.
5. It is from a thermophilic bacterium and is not denatured at 95 °C.
6. 2¹⁰ = 1 024.
7. 4 × 2³⁰ = 4 294 967 296 ≈ 4.29 × 10⁹.
8. Linear: 4. Circular: 3.
9. 150 × 3 + 3 = 453.
10. The anode (positive), because the phosphate groups make DNA negatively charged.
11. No risk of infection from donated blood (or a reliable supply).
12. Herbicide-resistance genes could spread to wild relatives, producing resistant weeds.

Where marks are usually lost

• Writing “restriction enzymes cut DNA into pieces” without “at a specific recognition sequence”.
• Calling a plasmid a “vector” without saying what it does (carries the gene into the host and replicates).
• Saying ligase joins sticky ends by hydrogen bonds.
• Omitting why a promoter is needed (RNA polymerase binding).
• Giving PCR steps without temperatures or without what happens to the DNA at each.
• Saying Taq polymerase has a high optimum but not that it resists denaturation at 95 °C.
• Saying longer fragments move further in a gel.
• Confusing microarray genome analysis (DNA) with expression studies (mRNA → cDNA).
• Listing ethical points without linking them to the named example.

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.