B6 Global challenges — OCR Gateway Biology (J257/02)

Communicable disease

Communicable diseases are caused by pathogens (microorganisms) and can be transmitted between individuals.

Types of pathogen:

Transmission routes:

• Direct contact (skin-to-skin, sexual contact).
• Droplets (coughing, sneezing — influenza, TB).
• Contaminated food/water (cholera, Salmonella).
• Vectors — organisms carrying pathogens without being affected (mosquito → malaria; black fly → river blindness).

Defence mechanisms:

Non-specific (innate) immunity:

• Skin (physical barrier).
• Mucus and cilia (trap and remove pathogens from respiratory tract).
• Phagocytosis: phagocytes (neutrophils, macrophages) engulf and destroy pathogens.
• Fever: raised temperature denatures pathogen enzymes and speeds up immune response.

Specific (adaptive) immunity:

• Lymphocytes detect specific antigens on pathogens.
• B-lymphocytes → produce antibodies specific to that antigen → mark pathogen for destruction (agglutination) or neutralise toxins.
• T-lymphocytes → T-helper (activate B-cells); T-killer (destroy infected cells).
• Memory cells persist after infection → faster, stronger response on re-exposure → immunity.

Vaccination: introduces dead/weakened pathogens or antigens → triggers primary immune response → memory cells formed → immunity without disease.

• Herd immunity: if enough of the population is vaccinated, the disease cannot spread (even unvaccinated individuals are protected).

Antibiotics: kill bacteria by disrupting cell wall synthesis (e.g. penicillin) or protein synthesis. Have NO effect on viruses. Overuse drives antibiotic resistance (natural selection — see B5).

Non-communicable diseases

Caused by lifestyle/environmental factors or genetic predisposition. Cannot be transmitted.

Cardiovascular disease (CVD):

• Atherosclerosis: cholesterol-rich plaque builds up in arteries → narrows lumen → increases blood pressure.
• Atherosclerosis in coronary arteries → angina or heart attack (myocardial infarction).
• Risk factors: high blood cholesterol, smoking (damages endothelium), high blood pressure, obesity, lack of exercise, Type 2 diabetes.

Cancer:

• Uncontrolled mitosis due to mutations in tumour suppressor genes (p53) or proto-oncogenes.
• Benign tumour: does not invade surrounding tissue; can be removed.
• Malignant tumour: invades surrounding tissue; can metastasise (spread via blood/lymph).
• Risk factors: UV radiation, carcinogens (tobacco smoke, asbestos), ionising radiation, some viruses (HPV → cervical cancer).

Obesity and Type 2 diabetes (risk factors for CVD, linked to diet and physical activity — see B3).

Biotechnology

Genetic engineering

1. Identify and extract the gene of interest using restriction enzymes (cut at specific restriction sites).
2. Cut open a vector (usually a bacterial plasmid) with the same restriction enzyme → complementary "sticky ends."
3. Insert gene into vector using DNA ligase to join sticky ends.
4. Introduce vector into host cell (e.g. bacterium) → cell now carries recombinant DNA.
5. Host cells reproduce → clone of cells all carrying and expressing the gene → protein produced at scale.

Example: human insulin — insulin gene inserted into E. coli → bacteria produce human insulin (Humulin) for Type 1 diabetics. Advantages over pig insulin: identical to human protein, no allergic reactions, can be produced in large quantities.

GM crops (genetically modified)

Examples:

• Golden Rice: rice engineered with genes from daffodil and bacterium to produce β-carotene (pro-vitamin A). Addresses vitamin A deficiency in developing countries.
• Herbicide-resistant crops (Roundup Ready): engineered to survive herbicide → farmers spray fields → weeds die, crop survives → higher yields.
• Bt crops: gene from Bacillus thuringiensis inserted → crop produces insecticide protein toxic to specific pests.

Arguments for GM crops:

• Higher yields; drought/pest resistance → food security.
• Reduced pesticide use; nutritional improvements (Golden Rice).

Arguments against:

• Ethical concerns about modifying nature; gene flow to wild plants.
• Reduced biodiversity (if herbicide kills all wild plants).
• Corporatisation of food supply (patented seeds).
• Long-term health effects unknown.

Monoclonal antibodies

Production:

1. Inject mouse with antigen → B-lymphocytes produce antibodies.
2. Fuse B-lymphocytes with tumour cells → hybridoma cells (immortal + antibody-producing).
3. Clone hybridoma cells → all produce identical antibodies specific to that antigen.
4. Harvest monoclonal antibodies.

Uses:

• Pregnancy tests: antibodies specific to hCG hormone in urine. If hCG present → antibody-antigen complex forms → colour change on test strip.
• Cancer therapy: antibodies carry chemotherapy drugs directly to cancer cells (targeted drug delivery) — fewer side effects than systemic chemotherapy.
• ELISA tests: detect specific proteins in blood samples (e.g. HIV diagnosis, COVID-19 lateral flow).
• Imaging: attach radioactive label to antibody → locates tumours in body.

Gene therapy

Somatic gene therapy: insert a working copy of a gene into body cells to treat a genetic disease. Only affects the patient — not inherited by offspring.

• Example: SCID (severe combined immunodeficiency) — viral vector delivers correct ADA gene into patient's T-cells.

Germline gene therapy: alter reproductive cells → change passed on to all offspring. Currently illegal in most countries due to ethical concerns and unknown long-term effects.

Drug development and testing

Drug discovery pipeline:

1. Discovery: identify compound (from plant extracts, computer modelling, or screening chemical libraries).
2. Pre-clinical testing: laboratory tests on cell cultures and animal models to assess toxicity, efficacy, mechanism.
3. Clinical trials:
   • Phase I: safety — small group of healthy volunteers; assess dosing, side effects.
   • Phase II: efficacy — small group of patients; does it work?
   • Phase III: large-scale randomised controlled trial (RCT); compare with existing treatment or placebo; double-blind design (neither patient nor doctor knows who gets drug).
4. Regulatory approval: data submitted to MHRA (UK) for review.
5. Phase IV (post-marketing): continued monitoring for rare side effects.

Double-blind placebo-controlled trial: controls for the placebo effect (psychological benefit of believing you're being treated). Both patient and clinician are unaware of who receives the drug vs placebo.

Thalidomide: developed as a sedative; given to pregnant women for morning sickness; caused severe limb deformities in ~10,000 babies. Now used for leprosy and some cancers under strict controls. Drove the development of modern clinical trial regulations.

Common OCR examiner traps

1. Antibiotics kill bacteria, not viruses — antibiotics are ineffective against influenza, COVID-19, the common cold.
2. Vaccination creates memory cells — not antibodies directly. The antibodies produced during the primary response are short-lived; it's the memory cells that give lasting immunity.
3. Monoclonal = one type of antibody targeting one specific antigen. Not a general-purpose treatment.
4. Gene therapy treats the patient; it does NOT alter germline (currently).
5. Double-blind means neither patient NOR doctor knows who receives drug/placebo — not just the patient.