explain that genetic engineering may help to solve the global demand for food by improving the quality and productivity of farmed animals and crop plants, using the examples of GM salmon, herbicide resistance in soybean and insect resistance in cotto

Cambridge International AS & A Level Biology – Syllabus‑Aligned Revision Notes

Learning Objective

Explain how genetic engineering can help meet the global demand for food by improving the quality and productivity of farmed animals and crop plants, using the examples of GM salmon, herbicide‑resistant soybean and insect‑resistant cotton. In addition, provide concise, exam‑focused coverage of all required syllabus topics (1‑19) and the practical skills needed for Papers 3 and 5.

1. Syllabus Coverage Overview

2. Core AS Topics (1‑11)

2.1. Cell Structure & Microscopy

• Prokaryotes: no nucleus, circular DNA, 70 nm ribosomes, simple membrane.
• Eukaryotes: nucleus, membrane‑bound organelles, 80–100 nm ribosomes, cytoskeleton.
• Microscopy:

  • Light microscope – up to 2 000×; bright‑field, phase‑contrast.
  • Electron microscope – TEM/SEM up to 10⁶×; reveals ultrastructure.

• Staining techniques: Gram (cell‑wall), iodine (starch), methylene blue (nuclei).

2.2. Biological Molecules

• Carbohydrates: monosaccharides → disaccharides → polysaccharides (starch, glycogen, cellulose).
• Lipids: fatty acids, triglycerides, phospholipids; hydrophobic barrier, energy dense (≈ 9 kcal g⁻¹).
• Proteins: amino‑acid sequence → primary, secondary, tertiary, quaternary structure; enzymes, structural, transport.
• Nucleic acids: DNA (double helix, deoxyribose, thymine) vs RNA (single‑strand, ribose, uracil).
• Water: polarity, hydrogen bonding, cohesion, adhesion, high specific heat.

2.3. Enzymes

• Active site specificity, induced fit.
• Lower activation energy (Eₐ).
• Factors: temperature, pH, substrate concentration, inhibitors (competitive, non‑competitive).
• Regulation: allosteric control, covalent modification, feedback inhibition.

2.4. Cell Membranes & Transport

• Phospholipid bilayer with embedded proteins (channels, carriers, pumps).
• Passive transport: simple diffusion, facilitated diffusion, osmosis.
• Active transport: primary (ATP‑driven) and secondary (co‑transport).
• Bulk transport: endocytosis (phagocytosis, pinocytosis) and exocytosis.

2.5. The Cell Cycle

• Interphase (G₁, S, G₂) – DNA synthesis and growth.
• Mitosis: prophase, metaphase, anaphase, telophase.
• Cytokinesis – cleavage furrow (animal) or cell plate (plant).
• Checkpoints (G₁‑S, G₂‑M) regulated by cyclins and CDKs.

2.6. DNA, RNA & Protein Synthesis

• DNA replication: semi‑conservative, origin of replication, DNA polymerase, ligase.
• Transcription: RNA polymerase, promoter, terminator, mRNA processing (capping, poly‑A tail, splicing).
• Translation: ribosome (large + small subunit), tRNA anticodon, peptide bond formation, start/stop codons.
• Gene regulation: operons (lac, trp), transcription factors, epigenetic DNA methylation.

2.7. Plant Transport

• Xylem: dead tracheids & vessels, cohesion‑tension theory, transpiration pull.
• Phloem: living sieve‑tube elements, pressure‑flow hypothesis (source → sink).
• Water potential (Ψ) = Ψₛ + Ψₚ; role of root pressure and capillarity.

2.8. Animal Transport

• Blood components: plasma, RBCs (haemoglobin), WBCs, platelets.
• Heart: chambers, valves, cardiac cycle (systole/diastole).
• Systemic & pulmonary circuits; capillary exchange (diffusion, bulk flow).
• Lymphatic system – returns interstitial fluid, immune surveillance.

2.9. Gas Exchange

• Respiratory surfaces: alveoli (human), gills (fish), tracheae (insects).
• Diffusion driven by partial pressure gradients (O₂, CO₂).
• Ventilation: diaphragm & intercostal muscles; tidal volume, respiratory rate.

2.10. Infectious Disease & Immunity

• Pathogen types: viruses, bacteria, fungi, protozoa, prions.
• Routes of infection: direct contact, airborne, vector‑borne, faecal‑oral.
• Innate immunity:

  • Physical barriers (skin, mucous), chemical barriers (lysozyme, low pH).
  • Cellular: phagocytes (macrophages, neutrophils), natural killer cells.
  • Inflammatory response – vasodilation, increased permeability.

• Adaptive immunity:

  • B‑cells → antibodies (IgM, IgG, IgA, IgE); clonal expansion.
  • T‑cells → cytotoxic (CD8⁺) and helper (CD4⁺) functions.
  • Memory cells → faster secondary response.

• Vaccines: live‑attenuated, inactivated, subunit, toxoid, conjugate – stimulate primary response without disease.
• Antibiotics & Antivirals: target specific microbial processes; importance of resistance management.

2.11. Homeostasis & Control (New)

• Negative feedback – the most common control mechanism (e.g., blood glucose regulation).

  • Sensor (pancreatic β‑cells) → Integrator (brain) → Effector (insulin release).

• Positive feedback – rapid amplification (e.g., oxytocin during labour).
• Nervous control: rapid, short‑term; action potentials, synaptic transmission.
• Hormonal control: slower, long‑term; endocrine glands, target‑cell receptors.
• Thermoregulation: vasodilation/vasoconstriction, sweating, shivering.

3. A‑Level Extension Topics (12‑16)

3.1. Energy & Respiration (Topic 12)

• Overall aerobic yield ≈ 30–38 ATP per glucose.
• Anaerobic pathways: alcoholic fermentation (yeast) – 2 ATP, lactic fermentation (muscle) – 2 ATP.
• Respiratory quotient (RQ) = CO₂ produced / O₂ consumed. RQ ≈ 1 for carbohydrates, 0.7 for fats.

3.2. Photosynthesis (Topic 13)

• Light‑dependent reactions (thylakoid membranes):

  • Photosystem II → H₂O → O₂ + e⁻; water splitting releases O₂.
  • Electron transport chain creates a proton gradient.
  • ATP synthase → ATP (photophosphorylation).
  • Photosystem I → NADP⁺ → NADPH.

• Calvin cycle (light‑independent) (stroma):

  1. Carbon fixation – Rubisco adds CO₂ to ribulose‑1,5‑bisphosphate.
  2. Reduction – ATP & NADPH convert 3‑PGA to G3P.
  3. Regeneration – G3P used to regenerate RuBP.

  Net: 3 CO₂ + 6 ATP + 2 NADPH → G3P + 3 ADP + 2 NADP⁺ + 3 Pi.

• C₃ vs C₄ plants: C₄ (e.g., maize) concentrates CO₂ in bundle‑sheath cells, reducing photorespiration.
• Limiting‑factor investigations: light intensity, CO₂ concentration, temperature, water availability.

3.3. Homeostasis & Control (Topic 14 – already covered in 2.11)

Refer to Section 2.11 for the concise summary.

3.4. Inheritance (Topic 15)

• Mendelian genetics: monohybrid & dihybrid crosses, law of segregation & independent assortment.
• Gene linkage: recombination frequency, map units (cM), test‑cross analysis.
• Sex‑linked traits: X‑linked recessive (e.g., haemophilia), Y‑linked.
• Polygenic inheritance: quantitative traits (height, skin colour); normal distribution.
• Pedigree analysis: autosomal dominant/recessive, X‑linked, mitochondrial inheritance.

3.5. Evolution & Natural Selection (Topic 16)

• Population genetics: allele frequencies, Hardy‑Weinberg equation (p² + 2pq + q² = 1).
• Mechanisms of evolution: natural selection, genetic drift, gene flow, mutation.
• Speciation: allopatric (geographic isolation), sympatric (polyploidy, niche differentiation).
• Adaptive radiation: rapid diversification when new niches become available (e.g., Darwin’s finches).

3.6. Classification, Biodiversity & Conservation (Topic 17)

• Taxonomic hierarchy: Domain → Species; binomial nomenclature.
• Molecular phylogeny: DNA sequencing (e.g., 16S rRNA) to infer evolutionary relationships.
• Threat categories (IUCN): Extinct, Extinct in the Wild, Critically Endangered, Endangered, Vulnerable, Near‑threatened, Least Concern.
• Conservation strategies:

  • In‑situ (protected areas, habitat restoration).
  • Ex‑situ (seed banks, captive breeding).
  • Legislation (CITES, Biodiversity Convention).

4. Topic 19 – Genetic Technology (Core & A‑Level Extension)

4.1. Gene Identification & Cloning

• Identification: genetic maps, BLAST searches, functional assays (e.g., reporter gene activity).
• Cloning steps:

  1. Isolation of target DNA (restriction enzymes).
  2. Insertion into a vector (plasmid, BAC, viral vector).
  3. Transformation of a host cell (commonly E. coli).
  4. Selection of recombinant colonies (antibiotic resistance, blue‑white screening).

• Common vectors:

  • Plasmids – multiple cloning site, selectable marker.
  • Agrobacterium tumefaciens – natural plant‑transformation system (Ti plasmid).
  • Viral vectors – retrovirus, adenovirus for animal cells.

4.2. Transformation Techniques

4.3. Selection & Screening of Transformants

• Selectable markers: antibiotic resistance (ampicillin, kanamycin), herbicide resistance (phosphinothricin).
• Screenable markers: reporter genes (GUS, GFP, β‑galactosidase) for visual confirmation.
• Confirmation techniques: PCR, Southern blot, sequencing, restriction‑digest analysis.

4.4. Molecular Techniques Required for Paper 5

1. Polymerase Chain Reaction (PCR)

   • Denaturation (94‑98 °C), annealing (50‑65 °C), extension (72 °C).
   • Components: template DNA, primers, Taq polymerase, dNTPs, Mg²⁺, buffer.
   • Applications: gene detection, genotyping, cloning.

2. Gel Electrophoresis

   • Agarose concentration (0.8‑2 %) determines resolution.
   • DNA fragments separate by size; visualised with ethidium bromide or SYBR Safe.

3. Sanger DNA Sequencing

   • Chain‑termination using dideoxynucleotides.
   • Interpret electropherograms for base‑calling.

4. Quantitative PCR (qPCR) – measures gene expression (Ct values, standard curves).

4.5. Case Studies Demonstrating Food‑Security Benefits

4.5.1. GM Salmon – AquAdvantage®

• Gene construct: Chinook salmon growth‑hormone gene driven by ocean‑pout antifreeze‑protein promoter.
• Phenotypic effect:

  • Market size (~4 kg) reached in ≈ 18 months vs 30‑36 months for conventional Atlantic salmon.
  • Feed conversion ratio improved by ~25 % (less feed per kg of fish).
  • Reduced pressure on wild stocks and lower carbon footprint of aquaculture.

• Containment measures: All‑female production, land‑based recirculating systems, physical barriers, sterile triploid fish.

4.5.2. Herbicide‑Resistant Soybean – “Roundup Ready”

• Introduced gene: Modified 5‑enolpyruvylshikimate‑3‑phosphate synthase (EPSPS) insensitive to glyphosate.
• Benefits:

  • Glyphosate can be sprayed to control weeds without harming the crop.
  • Reduced mechanical tillage → less soil erosion, lower fuel use.
  • Enables earlier planting and, in some regions, double‑cropping.

• Environmental considerations: Evolution of glyphosate‑resistant weeds; need for integrated weed‑management.

4.5.3. Insect‑Resistant Cotton – Bt Cotton

• Gene introduced: Cry1Ac toxin gene from Bacillus thuringiensis.
• Mode of action: Cry protein binds to mid‑gut receptors of lepidopteran larvae, forms pores → cell lysis.
• Outcomes:

  • Pesticide applications reduced by up to 70 %.
  • Yield stability increased, especially in pest‑prone regions.
  • Economic savings for farmers and lower environmental pesticide load.

• Resistance management: Refuge strategy (non‑Bt buffer zones) to delay evolution of resistant insects.

4.6. Linking GM Technology to Global Food Security

Yield increase from a GM crop can be expressed as:

ΔP = A × (Y_GM – Y_conv)

where A = cultivated area (ha) and Y = yield (ton ha⁻¹). Example: a 15 % yield boost on 10 Mha of soybean gives an extra 0.15 × Y_conv × 10 Mha of food without expanding farmland.

4.7. Ethical, Legal & Social Implications (ELSI)

• Food safety: Allergenicity testing, toxicology, long‑term feeding studies.
• Environmental impact: Gene flow to wild relatives, non‑target effects, biodiversity.
• Socio‑economic issues: Patent ownership, farmer dependency, market access, labeling debates.
• Regulatory frameworks: Cartagena Protocol on Biosafety, national GMO legislation, pre‑market risk assessment, post‑release monitoring.

4.8. Practical Skills Required for Papers 3 & 5

5. Quick Revision Summary

• Memorise the flow of energy in cellular respiration and the light‑dependent/independent phases of photosynthesis – diagrams are worth marks.
• Contrast innate vs adaptive immunity; know the three vaccine types most commonly examined.
• Be able to write the equation for a negative feedback loop and give a real‑world example.
• For genetic technology, recall the steps: isolate → vector → transform → select → confirm.
• Case‑study facts (GM salmon, Roundup Ready soybean, Bt cotton) should be at hand – focus on gene introduced, phenotypic effect, and a key advantage/disadvantage.
• Practice interpreting PCR gel images and calculating RQ or ATP yield – these are frequent Paper 5 questions.