The Heart

The heart is a double pump with four chambers. The right side pumps blood to the lungs (pulmonary circulation); the left side pumps blood to the body (systemic circulation).

• Right atrium — receives deoxygenated blood from the body via the vena cava
• Right ventricle — pumps blood to the lungs via the pulmonary artery
• Left atrium — receives oxygenated blood from the lungs via the pulmonary vein
• Left ventricle — pumps blood to the body via the aorta; has thicker walls (needs more force)

Valves prevent backflow of blood. The septum separates left and right sides.

Blood Vessels

Blood Components

• Red blood cells — carry oxygen bound to haemoglobin; no nucleus; biconcave shape
• White blood cells — defend against pathogens (phagocytes engulf, lymphocytes produce antibodies)
• Platelets — cell fragments that help blood clot at wound sites
• Plasma — liquid that transports dissolved substances (glucose, CO₂, urea, hormones)

Heart Disease

Coronary heart disease (CHD) occurs when coronary arteries become narrowed by fatty deposits (atheroma), reducing blood flow and oxygen supply to the heart muscle.

Treatments: stents (keep arteries open), statins (reduce cholesterol), bypass surgery.

Risk factors: high-fat diet, smoking, lack of exercise, stress, genetic factors.

Structure of the Respiratory System

Air enters through the nose/mouth → trachea (windpipe, reinforced with C-shaped cartilage rings) → bronchi → bronchioles → alveoli (tiny air sacs where gas exchange occurs).

Gas Exchange in the Alveoli

Oxygen diffuses from the alveoli into the blood. Carbon dioxide diffuses from the blood into the alveoli.

Alveoli are adapted for efficient gas exchange:

• Large surface area (millions of alveoli)
• Thin walls (one cell thick) — short diffusion distance
• Rich blood supply — maintains concentration gradient
• Moist lining — gases dissolve before diffusing

Breathing Mechanism

Respiratory Diseases

• Asthma — bronchioles become inflamed and constricted, making breathing difficult; treated with inhalers
• Lung cancer — uncontrolled cell division in lung tissue; strongly linked to smoking
• COPD — chronic obstructive pulmonary disease; includes emphysema (alveoli walls break down, reducing surface area) and chronic bronchitis

The Central Nervous System (CNS)

The CNS consists of the brain and spinal cord. It processes information from receptors and coordinates a response via effectors (muscles or glands).

Pathway: Stimulus → Receptor → Sensory neuron → CNS → Motor neuron → Effector → Response

Types of Neurons

• Sensory neurons — carry impulses from receptors to the CNS
• Relay neurons — connect sensory and motor neurons within the CNS
• Motor neurons — carry impulses from the CNS to effectors

Synapses

A synapse is the tiny gap between two neurons. The electrical impulse triggers the release of neurotransmitter chemicals from vesicles. These diffuse across the gap and bind to receptors on the next neuron, triggering a new impulse.

Reflex Arcs

Reflexes are rapid, automatic, involuntary responses to protect the body. The reflex arc pathway:

Receptor → Sensory neuron → Relay neuron (in spinal cord) → Motor neuron → Effector

Example: touching a hot surface — receptor in skin detects heat, impulse travels via sensory neuron to spinal cord, relay neuron passes to motor neuron, muscles in arm contract to pull hand away.

The Eye

• Cornea — refracts (bends) light entering the eye
• Iris — controls the amount of light entering through the pupil
• Lens — focuses light onto the retina (changes shape for near/far objects — accommodation)
• Retina — contains light-sensitive receptor cells (rods and cones)
• Optic nerve — carries impulses from the retina to the brain

Neuron Structure

All neurons share common features but have structural differences related to their function:

• Sensory neurons — have a long dendron connecting the receptor to the cell body, and a shorter axon leading to the CNS. The cell body is located off to the side of the main fibre.
• Relay neurons — short neurons found entirely within the CNS (brain or spinal cord). They have many short dendrites and a short axon, connecting sensory to motor neurons.
• Motor neurons — have a large cell body with many dendrites at one end, and a long axon running to the effector (muscle or gland). The axon is often covered in a myelin sheath (fatty insulation) to speed up impulse transmission.

Key structures of a typical neuron: dendrites (receive impulses), cell body (contains the nucleus), axon (carries impulses away from cell body), myelin sheath (insulates and speeds up transmission), synaptic knob (terminal end, releases neurotransmitters).

Synapse Structure and Transmission

At a synapse, the electrical impulse cannot cross the gap directly. Instead:

1. The impulse arrives at the synaptic knob (end of the presynaptic neuron)
2. Vesicles containing neurotransmitter chemicals (e.g., acetylcholine) fuse with the membrane
3. Neurotransmitter diffuses across the synaptic cleft (the tiny gap)
4. Neurotransmitter binds to specific receptors on the postsynaptic neuron membrane
5. This triggers a new electrical impulse in the next neuron
6. The neurotransmitter is then broken down by enzymes or reabsorbed

Synapses ensure impulses travel in one direction only (vesicles are only on the presynaptic side).

Reaction Time

Reaction time can be measured using a ruler drop test. Factors affecting it include age, caffeine, tiredness, and practice.

Accommodation (Near and Far Vision)

Accommodation is the process by which the eye changes the shape of the lens to focus on objects at different distances.

Eye Defects and Correction

The Endocrine System

The endocrine system is made up of glands that produce hormones. Hormones are chemical messengers carried in the blood to target organs.

Nervous vs Hormonal Communication

Insulin and Diabetes

When blood glucose rises (e.g., after a meal), the pancreas releases insulin. Insulin causes cells to take up glucose and the liver to store glucose as glycogen. When blood glucose falls, the pancreas releases glucagon, which converts glycogen back to glucose.

• Type 1 diabetes — the pancreas does not produce insulin; treated with insulin injections; usually develops in childhood
• Type 2 diabetes — body cells stop responding to insulin (insulin resistance); linked to obesity and lifestyle; treated with diet, exercise, and medication

Adrenaline: Fight or Flight

Adrenaline is released by the adrenal glands in response to stress or danger. It causes: increased heart rate, faster breathing, blood diverted to muscles, glycogen converted to glucose for energy.

The Menstrual Cycle

The menstrual cycle is approximately 28 days, controlled by four hormones:

• FSH (follicle stimulating hormone) — causes an egg to mature in the ovary
• Oestrogen — repairs the uterus lining; stimulates LH release
• LH (luteinising hormone) — triggers ovulation (release of the egg, around day 14)
• Progesterone — maintains the uterus lining; if no fertilisation, levels drop and menstruation occurs

Contraception

• Hormonal methods — the pill (contains oestrogen and progesterone to prevent ovulation), implants, injections
• Barrier methods — condoms (also prevent STI transmission), diaphragm
• Other — IUD, sterilisation, abstinence

Fertility Treatments

• Clomifene — a drug that stimulates the release of FSH and LH, causing ovulation. Used when a woman is not ovulating naturally.
• IVF (In Vitro Fertilisation):
  1. FSH and LH are given by injection to stimulate the ovaries to produce multiple eggs
  2. Eggs are collected from the ovaries
  3. Eggs are fertilised with sperm in a laboratory dish
  4. The fertilised eggs develop into embryos
  5. One or two embryos are implanted into the mother's uterus

IVF is emotionally and physically demanding, expensive, has a low success rate per cycle, and can lead to multiple pregnancies. However, it gives couples who cannot conceive naturally the chance to have children.

Plant Hormones

Plants produce hormones called auxins that control growth and responses to stimuli.

Phototropism (Response to Light)

Plant shoots grow towards light (positive phototropism). Auxin moves to the shaded side of the shoot. The higher auxin concentration on the shaded side causes cells there to elongate more, bending the shoot towards the light.

Gravitropism (Response to Gravity)

• Roots grow towards gravity (positive gravitropism) — auxin accumulates on the lower side, which inhibits cell elongation in roots, so the upper side grows more, bending the root downward
• Shoots grow away from gravity (negative gravitropism) — auxin accumulates on the lower side, which promotes cell elongation in shoots, so the lower side grows more, bending the shoot upward

What is Homeostasis?

Homeostasis is the regulation of internal conditions to maintain a stable internal environment despite changes in external conditions. This is essential for enzymes and cells to function optimally.

Conditions controlled include: body temperature, blood glucose levels, water balance, and CO₂ levels.

Homeostasis involves negative feedback: when a level deviates from the norm, mechanisms act to return it to normal.

Body Temperature Regulation

Normal body temperature is about 37°C. The thermoregulatory centre in the brain monitors blood temperature.

Blood Glucose Regulation

See the Hormonal System section for detail on insulin and glucagon working as an antagonistic pair to control blood glucose through negative feedback.

The Kidneys and Water Balance

The kidneys filter the blood and produce urine. They remove waste products (urea) and regulate water and ion levels.

Three processes in the kidney:

• Filtration — blood is filtered at high pressure in the glomerulus; small molecules (water, glucose, urea, ions) pass into the Bowman's capsule
• Reabsorption — useful substances (all glucose, some water, some ions) are reabsorbed back into the blood
• Excretion — remaining waste (urea, excess water and ions) passes to the bladder as urine

ADH (antidiuretic hormone) controls water reabsorption. When dehydrated, more ADH is released → more water reabsorbed → concentrated urine. When over-hydrated, less ADH → dilute urine.

Types of Pathogens

• Bacteria — single-celled organisms; reproduce rapidly; produce toxins (e.g., Salmonella, MRSA)
• Viruses — much smaller than bacteria; invade cells and use them to replicate; damage cells (e.g., influenza, HIV)
• Fungi — e.g., athlete's foot
• Protists — e.g., Plasmodium (causes malaria)

The Body's Defences

Non-specific defences (first line)

• Skin — physical barrier
• Mucus and cilia in airways — trap and remove pathogens
• Stomach acid — kills pathogens in food
• Tears — contain lysozyme enzyme

White blood cells (immune response)

• Phagocytes — engulf and digest pathogens (phagocytosis)
• Lymphocytes — produce antibodies specific to antigens on the pathogen surface; also produce antitoxins to neutralise toxins

Vaccination

A vaccine contains a dead or weakened form of a pathogen. It stimulates the lymphocytes to produce antibodies. Memory cells are created, so if the real pathogen enters the body later, antibodies are produced much faster and in larger quantities — preventing illness.

Antibiotics and Antibiotic Resistance

Antibiotics kill or prevent the growth of bacteria (e.g., penicillin). They do NOT work against viruses.

Antibiotic resistance develops through natural selection:

1. Random mutation produces a bacterium resistant to an antibiotic
2. The antibiotic kills non-resistant bacteria, but the resistant one survives
3. The resistant bacterium reproduces and passes on the resistance gene
4. Over time, the resistant strain becomes more common (e.g., MRSA)

To reduce antibiotic resistance: complete the full course of antibiotics, only use when necessary, doctors should not overprescribe.

Key Ecology Terms

• Habitat — the place where an organism lives
• Population — all the organisms of one species in a habitat
• Community — all the different species living in a habitat
• Ecosystem — the community of living organisms (biotic) interacting with non-living (abiotic) factors
• Interdependence — organisms in a community depend on each other for food, shelter, pollination, seed dispersal, etc.

Abiotic and Biotic Factors

Food Chains and Food Webs

A food chain shows the transfer of energy from one organism to the next:

Producer → Primary consumer → Secondary consumer → Tertiary consumer

Producers (plants) make their own food via photosynthesis. Consumers eat other organisms. Decomposers (bacteria and fungi) break down dead organisms, returning nutrients to the soil.

A food web shows interconnected food chains in an ecosystem. If one organism is removed, it affects others in the web — this is interdependence.

Quadrats

A quadrat is a square frame (usually 0.5m x 0.5m) placed on the ground to count organisms in that area. Quadrats should be placed randomly to avoid bias (use random number generators for coordinates).

You can estimate the population size of a species by calculating the mean number per quadrat and scaling up to the total area.

Transects

A transect is a line placed across a habitat to measure how organisms are distributed as conditions change (e.g., from a pond to a field, or from shade to sunlight). Quadrats are placed at regular intervals along the transect line.

Capture-Recapture Method

Used to estimate the population of mobile animals:

1. Capture a sample, count and mark them, then release
2. After time, capture a second sample
3. Count how many in the second sample are marked

Assumptions: no immigration/emigration, no births/deaths, marks do not wear off, marks do not affect predation.

What is Photosynthesis?

Photosynthesis is the process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen. It takes place in the chloroplasts.

Uses of Glucose

• Respiration (to release energy)
• Converted to starch for storage
• Used to make cellulose for cell walls
• Combined with nitrate ions to make amino acids and proteins
• Converted to lipids for storage

Factors Affecting the Rate of Photosynthesis

• Light intensity — more light = faster rate (until another factor becomes limiting)
• Carbon dioxide concentration — more CO₂ = faster rate (until limiting)
• Temperature — increases rate up to an optimum (~25-30°C), then enzymes denature and rate decreases

A limiting factor is the factor in shortest supply that prevents the rate from increasing further.

Leaf Structure

• Waxy cuticle — reduces water loss
• Upper epidermis — transparent to allow light through
• Palisade mesophyll — main site of photosynthesis; tightly packed with chloroplasts
• Spongy mesophyll — air spaces allow gas exchange and diffusion
• Stomata — pores on the lower surface for gas exchange (CO₂ in, O₂ out)
• Guard cells — control opening and closing of stomata
• Xylem and phloem — transport water/minerals and sugars respectively

Aerobic Respiration

Aerobic respiration uses oxygen to release energy from glucose. It takes place in the mitochondria.

Anaerobic Respiration

Anaerobic respiration occurs without oxygen, e.g., during intense exercise when oxygen cannot be delivered fast enough.

Anaerobic respiration releases much less energy than aerobic because glucose is not fully broken down.

After exercise, you continue to breathe heavily to repay the oxygen debt — extra oxygen is needed to break down the lactic acid that has built up.

Comparing Aerobic and Anaerobic Respiration

The Carbon Cycle

Carbon is recycled through the ecosystem:

• CO₂ is removed from the atmosphere by photosynthesis in plants
• Carbon passes along food chains when organisms are eaten
• CO₂ is returned to the atmosphere by respiration (all living organisms)
• CO₂ is returned when decomposers break down dead organisms and waste
• CO₂ is released by combustion of fossil fuels (coal, oil, gas)

The Nitrogen Cycle

Nitrogen makes up 78% of the atmosphere but plants cannot use N₂ gas directly. Key processes:

• Nitrogen-fixing bacteria (in soil or root nodules of legumes) — convert N₂ into ammonia/nitrates
• Nitrifying bacteria — convert ammonia into nitrites and then nitrates
• Denitrifying bacteria — convert nitrates back to N₂ gas (returned to atmosphere)
• Decomposers — break down dead organisms, releasing ammonia
• Plants absorb nitrates from the soil to make amino acids and proteins

The Water Cycle

• Evaporation — water from oceans, rivers, lakes turns to water vapour
• Transpiration — water evaporates from plant leaves through stomata
• Condensation — water vapour cools and forms clouds
• Precipitation — water falls as rain, snow, etc.
• Water collects in rivers, lakes, and underground, flowing back to the sea

Pollution

• Air pollution — burning fossil fuels releases CO₂ (greenhouse gas) and sulfur dioxide (acid rain)
• Water pollution — fertilisers run off into rivers/lakes causing eutrophication (algal blooms block light, plants die, decomposers use up oxygen, fish die)
• Land pollution — pesticides, herbicides, landfill, toxic waste

Deforestation

Clearing forests for farming, timber, and building leads to:

• Loss of biodiversity (habitat destruction)
• Increased CO₂ (fewer trees for photosynthesis; burning trees releases CO₂)
• Soil erosion

Global Warming

Increased levels of greenhouse gases (CO₂, methane) trap more heat in the atmosphere, causing the Earth's average temperature to rise.

Consequences: melting ice caps, rising sea levels, more extreme weather, habitat loss, species extinction, changes to migration patterns.

Biodiversity and Conservation

Biodiversity is the variety of different species in an ecosystem. High biodiversity makes ecosystems more stable.

Conservation methods:

• Breeding programmes for endangered species
• Nature reserves and national parks
• Seed banks and gene banks
• Reducing deforestation and carbon emissions
• Sustainable development — meeting present needs without compromising future generations

Pyramids of Number, Biomass & Energy

Ecological pyramids represent the relative amounts at each trophic level in a food chain.

Energy is lost at each trophic level through:

• Respiration — organisms use energy for life processes, releasing heat
• Excretion — energy lost in urine and faeces
• Not all parts are eaten — bones, roots, etc. are left behind

Typically only about 10% of energy is passed from one trophic level to the next.

Food Webs and Interdependence

A food web shows multiple interconnected food chains. Changes to one population affect others:

• If a predator is removed, prey populations increase, which may cause their food source to decline
• If a prey species declines, predators may switch to alternative prey or decline themselves
• Competition occurs between species for the same resources (interspecific) or within species (intraspecific)

Bioaccumulation

Bioaccumulation is the build-up of toxic substances (e.g., pesticides, heavy metals) in organisms over time because they are absorbed faster than they are broken down or excreted.

As you move up the food chain, the concentration of the toxin increases at each trophic level because predators eat many prey organisms, accumulating higher doses. This is called biomagnification.

Estimating Population Size

Human Impacts on Biodiversity

• Habitat destruction — deforestation, urbanisation, draining wetlands
• Pollution — air, water, and land pollution kills organisms and degrades habitats
• Overexploitation — overfishing, overhunting, unsustainable farming
• Introduction of invasive species — outcompete native species for resources
• Climate change — changing temperatures and weather patterns alter habitats

Conservation Methods

• Protected areas — national parks, nature reserves, marine conservation zones
• Breeding programmes — captive breeding of endangered species for reintroduction
• Seed banks and gene banks — preserve genetic material for future use
• Hedgerow and habitat restoration — replanting native vegetation
• Legislation — laws to protect endangered species (e.g., CITES)
• Sustainable resource management — fishing quotas, replanting forests, recycling

DNA (Deoxyribonucleic Acid)

DNA is a double helix molecule made of two strands twisted together. Each strand is made up of nucleotides. Each nucleotide contains:

• A sugar (deoxyribose)
• A phosphate group
• One of four bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)

The bases pair up following complementary base pairing rules:

A — T and C — G

Genes, Chromosomes and the Genome

• Gene — a short section of DNA that codes for a specific protein
• Chromosome — a long, coiled molecule of DNA; humans have 23 pairs (46 total)
• Genome — the entire set of genetic material in an organism

Understanding the human genome helps us: identify genes linked to diseases, develop personalised medicines, understand human evolution and migration.

Protein Synthesis (overview)

The sequence of bases in a gene provides a code for assembling amino acids in a specific order to make a particular protein. Different proteins have different functions (enzymes, structural proteins, hormones, antibodies).

Key Genetics Terms

• Gene — a section of DNA coding for a protein
• Allele — different versions of the same gene
• Dominant allele — always expressed, even if only one copy present (shown as capital letter, e.g., B)
• Recessive allele — only expressed if two copies are present (shown as lower case, e.g., b)
• Genotype — the combination of alleles an organism has (e.g., BB, Bb, bb)
• Phenotype — the physical characteristic expressed (e.g., brown eyes)
• Homozygous — two identical alleles (BB or bb)
• Heterozygous — two different alleles (Bb)
• Carrier — heterozygous individual who carries a recessive allele but does not show the trait

Punnett Squares

Punnett squares are used to predict the possible genotypes and phenotypes of offspring.

Sex Determination

Humans have 23 pairs of chromosomes. The 23rd pair are the sex chromosomes:

• XX — female
• XY — male

There is a 50:50 chance of a child being male or female.

Sex-Linked Inheritance

Some conditions are carried on the X chromosome (e.g., colour blindness, haemophilia). Since males have only one X chromosome, they are more likely to be affected because they only need one recessive allele to show the condition.

Cystic Fibrosis (CF)

• Caused by a recessive allele (ff)
• Both parents must be carriers (Ff) or affected (ff) for a child to have CF
• Causes thick, sticky mucus in the lungs, digestive system, and reproductive system
• Symptoms: frequent chest infections, difficulty breathing, difficulty digesting food
• Treatment: physiotherapy to clear mucus, medication, lung transplant in severe cases

If two carriers have a child: Ff x Ff → 1 in 4 (25%) chance of the child having CF.

Sickle Cell Disease

• Caused by a recessive allele
• Red blood cells become sickle (crescent) shaped, especially under low oxygen
• Sickle cells can block blood vessels, causing pain and organ damage
• Carriers (heterozygous) have some resistance to malaria — this is why the sickle cell allele is more common in regions where malaria is prevalent (natural selection)

Types of Variation

Causes of Variation

• Genetic variation — caused by mutations, sexual reproduction (meiosis produces unique gametes), and random fertilisation
• Environmental variation — caused by conditions such as diet, exercise, sunlight, climate
• Most characteristics are influenced by both genes and environment (e.g., height: genes set potential, diet affects actual height)

Darwin's Theory of Natural Selection

Charles Darwin proposed that evolution occurs through natural selection:

1. There is variation within a population (caused by genetic differences)
2. Organisms compete for limited resources (food, mates, territory)
3. Those with characteristics best suited to the environment are more likely to survive and reproduce (survival of the fittest)
4. They pass on their advantageous alleles to offspring
5. Over many generations, the frequency of the advantageous allele increases in the population

Antibiotic Resistance as an Example of Natural Selection

This is a key example linking evolution to modern medicine:

1. Within a population of bacteria, random mutation produces one that is resistant to an antibiotic
2. When the antibiotic is used, non-resistant bacteria are killed
3. The resistant bacterium survives and reproduces without competition
4. The resistant gene is passed to offspring
5. The population of antibiotic-resistant bacteria increases (e.g., MRSA)

Evidence for Evolution

• Fossil record — shows how organisms have changed over time; transitional fossils show links between groups
• DNA/genetic evidence — closely related species have more similar DNA sequences
• Antibiotic resistance — observable natural selection happening today
• Comparative anatomy — similar bone structures (homologous structures) in different species suggest common ancestry

Extinction

A species becomes extinct when there are no remaining individuals. Causes include: habitat destruction, new predators, new diseases, climate change, competition from other species, catastrophic events.

Selective Breeding (Artificial Selection)

Humans choose organisms with desirable characteristics and breed them together. Over many generations, the desired trait becomes more common.

Process:

1. Choose parents with the desired characteristic
2. Breed them together
3. Select the best offspring and breed again
4. Repeat over many generations

Examples: cows that produce more milk, wheat with higher yield, dogs with specific features.

Disadvantage: reduces genetic variation (inbreeding), making the population more vulnerable to disease.

Genetic Engineering

Genetic engineering involves transferring a gene from one organism to another to give it a desired characteristic.

Process:

1. Identify and cut out the desired gene using restriction enzymes
2. Insert the gene into a vector (e.g., a bacterial plasmid)
3. Insert the vector into the target organism
4. The organism produces the desired protein

Examples:

• GM crops — crops engineered for pest resistance, herbicide resistance, or improved nutrition (e.g., Golden Rice with vitamin A)
• Insulin production — the human insulin gene inserted into bacteria, which produce insulin for diabetics

Cloning

Cloning produces genetically identical organisms.

• Plant cloning — taking cuttings; tissue culture (growing from small groups of cells on agar with hormones)
• Animal cloning — embryo transplants; adult cell cloning (e.g., Dolly the sheep: nucleus from adult cell placed into an enucleated egg cell, stimulated to divide, implanted into surrogate mother)

Ethical concerns about cloning: reduced genetic variation, animal welfare, potential misuse in humans.

Genetic Engineering: Detailed Process

The full process of genetic engineering involves several key steps and enzymes:

1. Identify the desired gene in the donor organism (e.g., the human insulin gene)
2. Cut out the gene using restriction enzymes — these act as molecular scissors, cutting DNA at specific recognition sequences, leaving sticky ends
3. Cut open a vector (usually a bacterial plasmid) using the same restriction enzyme so the sticky ends are complementary
4. Insert the gene into the vector using DNA ligase — this enzyme joins the sugar-phosphate backbones together, sealing the gene into the plasmid
5. Insert the recombinant plasmid into a host organism (e.g., a bacterium)
6. The host organism reads the new gene and produces the desired protein

GM Crops: Advantages and Disadvantages

Insulin Production by GM Bacteria

Before genetic engineering, insulin for diabetics was extracted from pig or cow pancreases. Now:

1. The human insulin gene is cut out using restriction enzymes
2. The gene is inserted into a bacterial plasmid using ligase
3. The recombinant plasmid is put into E. coli bacteria
4. Bacteria are grown in large fermenters and produce human insulin
5. The insulin is extracted and purified

Advantages: human insulin (fewer allergic reactions), produced in large quantities, no animals killed, cheaper to produce long-term.

Cloning: Detailed Methods

Plant Cloning — Tissue Culture (Micropropagation)

1. Take a small sample of tissue (explant) from the parent plant (e.g., from a shoot tip)
2. Place on sterile nutrient agar medium containing plant hormones (auxins and cytokinins)
3. Cells divide by mitosis to form a callus (mass of undifferentiated cells)
4. Transfer small groups of cells to a medium with different hormone concentrations to stimulate root and shoot growth
5. Grow plantlets into full plants — all genetically identical to the parent

Animal Cloning — Somatic Cell Nuclear Transfer (SCNT)

This is the method used to clone Dolly the sheep (1996):

1. Take a body (somatic) cell from the animal to be cloned — remove its nucleus
2. Take an egg cell from a donor and remove its nucleus (enucleation)
3. Insert the somatic cell nucleus into the enucleated egg cell
4. Stimulate the egg cell with an electric shock to begin cell division
5. The embryo develops and is implanted into a surrogate mother
6. The offspring is genetically identical to the animal that donated the somatic cell

Stem Cells: Embryonic vs Adult

Ethical Considerations

• Genetic engineering — concerns about "playing God," unforeseen consequences, GM organisms escaping into the wild
• Embryonic stem cells — involves destruction of embryos; some people believe life begins at fertilisation
• Cloning — low success rates (many failed attempts), health problems in cloned animals, reduced genetic variation
• GM crops — large corporations controlling food supply, unknown environmental effects
• Counterarguments: these technologies could save lives, treat diseases, and solve food shortages

What is a Mutation?

A mutation is a change in the DNA base sequence. Mutations occur continuously and are usually random. There are two main types: gene mutations (changes to individual genes) and chromosome mutations (changes to whole chromosomes or large sections).

Gene Mutations (Point Mutations)

These involve changes to one or a few bases in the DNA sequence:

Frameshift Mutations

Insertions and deletions are called frameshift mutations because they shift the reading frame of the genetic code. This means every triplet codon after the mutation is different, producing a completely different (and usually non-functional) protein.

Chromosome Mutations

These involve larger-scale changes:

• Non-disjunction — chromosomes fail to separate properly during meiosis, leading to gametes with extra or missing chromosomes (e.g., Down syndrome: three copies of chromosome 21)
• Translocation — a section of one chromosome breaks off and attaches to another chromosome
• Inversion — a segment of a chromosome is reversed
• Duplication — a section of chromosome is copied, giving extra genes

Causes of Mutations

• Spontaneous — random errors during DNA replication; occur naturally at a low rate
• Radiation — UV light, X-rays, and gamma rays can damage DNA and increase mutation rate (mutagens)
• Chemicals — substances such as chemicals in cigarette smoke (tar), asbestos, and certain industrial chemicals can cause mutations (carcinogens if they cause cancer)

Effects of Mutations

Mutations, Variation, and Evolution

Mutations are the ultimate source of new alleles and therefore the raw material for natural selection:

• A beneficial mutation gives an organism a survival advantage
• That organism is more likely to survive and reproduce
• The advantageous allele is passed to offspring and becomes more common over generations
• Over long periods, the accumulation of many small mutations leads to evolution and even the formation of new species

Key Examples

Sickle Cell Anaemia

• Caused by a substitution mutation in the haemoglobin gene
• A single base change (A → T) causes the amino acid glutamic acid to be replaced by valine
• This produces abnormal haemoglobin that causes red blood cells to become sickle-shaped under low oxygen
• Carriers (heterozygous, HbAHbS) have sickle cell trait and are resistant to malaria — this is why the allele is maintained in populations where malaria is common (balanced selection)

Cystic Fibrosis

• Caused by a deletion mutation in the CFTR gene on chromosome 7
• The most common mutation involves deletion of three bases coding for the amino acid phenylalanine at position 508
• This produces a non-functional CFTR protein, which normally controls chloride ion channels in cell membranes
• Without functional CFTR, thick sticky mucus builds up in the lungs, pancreas, and reproductive system

Key Formulae for Biology