Biology 9700 — chapter handouts

Eukaryote versus prokaryote

Eukaryotic cells have a nucleus and membrane-bound organelles, linear DNA wound round histones, and 80S ribosomes. Prokaryotic cells have no nucleus, circular DNA free in the cytoplasm, 70S ribosomes, a peptidoglycan cell wall, and often plasmids and flagella. Prokaryotes are typically 1–5 µm across; eukaryotes 20–100 µm.

The organelles and their jobs

Rough ER makes and transports protein; smooth ER makes lipids. The Golgi body modifies, packages and dispatches, and forms lysosomes. Lysosomes hold hydrolytic enzymes. Mitochondria make ATP by aerobic respiration. Chloroplasts capture light. Centrioles organise spindle fibres. The nucleolus makes ribosomes.

Magnification and resolution are not the same

Magnification is how much bigger the image is: magnification = image size ÷ actual size. Resolution is the smallest distance at which two points still look separate, and it is set by the wavelength used. A light microscope resolves about 200 nm; an electron microscope about 0.5 nm, because electron beams have a far shorter wavelength.

Choosing a microscope

Light microscopes are cheap, portable and can view living, coloured specimens. TEM gives the highest resolution and shows internal detail in thin sections. SEM gives a three-dimensional surface view at lower resolution. Both electron microscopes require dead, chemically fixed specimens in a vacuum.

Units matter more than you expect

1 mm = 1000 µm = 1 000 000 nm. Almost every marked calculation in this topic is lost on unit conversion rather than on biology, so convert everything to one unit before you divide.

The grown-up words

Check you have got it

Condensation and hydrolysis run everything

Monomers join by condensation, releasing a water molecule and forming a bond. Polymers are broken by hydrolysis, using a water molecule. Glycosidic bonds in carbohydrates, peptide bonds in proteins and ester bonds in lipids are all made and broken this way.

Carbohydrates: same monomer, different bonds

α-glucose polymers are starch (amylose is helical, amylopectin branched) and glycogen (highly branched, animal storage). β-glucose polymers give cellulose, where alternate molecules are flipped, producing straight chains that hydrogen-bond into strong microfibrils.

Proteins have four levels of structure

Primary is the amino acid sequence. Secondary is the α-helix or β-pleated sheet held by hydrogen bonds. Tertiary is the overall 3D fold, held by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions. Quaternary is two or more polypeptides together, as in haemoglobin.

Globular versus fibrous

Globular proteins (haemoglobin, enzymes) are compact and soluble, with hydrophilic groups outward — they do metabolic jobs. Fibrous proteins (collagen) are long, insoluble and structural. Collagen's three chains wind into a triple helix, with glycine every third residue because only glycine is small enough to fit inside.

Water's properties come from its polarity

Hydrogen bonding gives water a high specific heat capacity (temperature stability), high latent heat of vaporisation (cooling by sweating), cohesion (transpiration stream), surface tension, and its role as a near-universal solvent for polar molecules.

The grown-up words

Check you have got it

Lowering activation energy

Enzymes do not change how much energy a reaction releases; they lower the activation energy needed to start it. Binding the substrate in the active site puts strain on bonds and holds reactants in the right orientation.

Temperature and pH have different curves

Rate rises with temperature as molecules gain kinetic energy, peaks at the optimum, then falls sharply as hydrogen and ionic bonds break and the tertiary structure is lost. pH gives a symmetrical curve either side of an optimum, because extreme pH alters the charges on R-groups and disrupts the active site.

Concentration effects level off for different reasons

Raising substrate concentration increases rate until all active sites are occupied — the enzyme is the limiting factor. Raising enzyme concentration increases rate until substrate becomes limiting. Naming which factor is limiting is what earns the mark.

Inhibition, competitive and non-competitive

Competitive inhibitors resemble the substrate and bind the active site; adding more substrate overcomes them, so V_max is unchanged but K_m rises. Non-competitive inhibitors bind elsewhere and change the active site's shape; more substrate does not help, so V_max falls.

K_m measures affinity

The Michaelis–Menten constant is the substrate concentration giving half V_max. A low K_m means the enzyme reaches half speed at low substrate concentration — high affinity. Immobilised enzymes, used industrially, can be recovered and reused and are more stable, at some cost in rate.

The grown-up words

Check you have got it

The fluid mosaic model

A phospholipid bilayer with hydrophilic phosphate heads facing the water and hydrophobic tails inward. Proteins float in and through it. Cholesterol sits between the phospholipids, keeping the membrane fluid when cold and stable when warm. Glycoproteins and glycolipids act in recognition and cell signalling.

Passive transport needs no ATP

Simple diffusion: small, non-polar molecules (O₂, CO₂) move down a concentration gradient. Facilitated diffusion: polar or charged particles pass through channel or carrier proteins, still down the gradient. Osmosis: water moves from higher to lower water potential through a partially permeable membrane.

Active transport needs ATP

Carrier proteins change shape to move substances against the gradient. Bulk transport — endocytosis and exocytosis — moves large quantities in vesicles, also using ATP. Cells doing a lot of active transport are packed with mitochondria, which is a common exam observation.

Water potential explains plant and animal cells

Pure water has a water potential of 0 kPa; adding solute makes it negative. An animal cell in pure water bursts (lysis); in concentrated solution it shrinks (crenation). A plant cell in pure water becomes turgid, held by its cell wall; in concentrated solution it becomes plasmolysed as the membrane pulls away from the wall.

Surface area, distance and gradient set the rate

Fick's law in words: rate of diffusion is proportional to surface area × concentration difference, divided by diffusion distance. Every exchange surface in biology — alveoli, villi, gills, root hairs — is an answer to that equation.

The grown-up words

Check you have got it

Interphase does most of the work

G1: the cell grows and makes organelles. S: DNA replicates, so each chromosome becomes two sister chromatids joined at a centromere. G2: the cell grows again and checks the DNA. Mitosis itself occupies only a short part of the cycle.

The four stages of mitosis

Prophase: chromosomes condense and become visible, the nuclear envelope breaks down, the spindle forms. Metaphase: chromosomes line up on the equator, attached by their centromeres. Anaphase: centromeres divide and sister chromatids are pulled to opposite poles. Telophase: chromosomes decondense and two nuclear envelopes re-form. Cytokinesis then divides the cytoplasm.

The point of mitosis is identity

Mitosis produces two genetically identical daughter nuclei with the same chromosome number as the parent. It is used in growth, repair, replacement and asexual reproduction — and in cloning.

Telomeres and stem cells

Telomeres are repeated non-coding DNA at chromosome ends that stop coding DNA being lost at each replication. Stem cells retain the ability to divide and to differentiate into other cell types, which is why they matter medically.

Cancer is uncontrolled mitosis

A mutation in a gene controlling the cell cycle — an oncogene or a tumour suppressor gene — can remove the brakes on division. The resulting mass is a tumour; malignant tumours invade surrounding tissue and spread.

The grown-up words

Check you have got it

DNA structure

Two antiparallel strands of nucleotides — deoxyribose, phosphate and a base. A pairs with T by two hydrogen bonds, C pairs with G by three. The sugar–phosphate backbone is on the outside, bases inside, twisted into a double helix. Many hydrogen bonds together make the whole molecule stable.

Semi-conservative replication

DNA helicase unwinds and separates the strands; DNA polymerase builds a new strand against each template using complementary base pairing. Each daughter molecule has one original and one new strand — which is exactly what the Meselson–Stahl experiment demonstrated.

Transcription

RNA polymerase makes a single-stranded mRNA copy of one gene from the template strand, using uracil in place of thymine. The mRNA leaves the nucleus through a pore.

Translation

At the ribosome, each codon of three mRNA bases is read. A tRNA molecule with the complementary anticodon brings the matching amino acid, and peptide bonds join them into a polypeptide. The genetic code is a triplet code, universal, non-overlapping and degenerate — most amino acids have more than one codon.

Mutations change the instruction

A substitution may change one amino acid, or none at all if the code's degeneracy absorbs it. An insertion or deletion causes a frameshift, so every codon after it is misread — usually far more damaging. Sickle cell anaemia comes from a single substitution changing glutamic acid to valine in the β-globin chain.

The grown-up words

Check you have got it

Xylem and phloem do different jobs

Xylem vessels are dead, hollow, lignified tubes carrying water and mineral ions upwards only. Phloem sieve tubes are living, with perforated sieve plates and companion cells, and carry sucrose and amino acids in either direction.

The transpiration stream

Water evaporates from mesophyll cell walls and diffuses out through the stomata. This lowers the water potential in the leaf, pulling water up the xylem. Cohesion between water molecules holds the column together and adhesion to the vessel walls helps — the cohesion–tension theory.

Three routes across the root

The apoplast pathway travels through cell walls, the symplast through cytoplasm and plasmodesmata, the vacuolar through vacuoles. At the endodermis the waterproof Casparian strip blocks the apoplast route, forcing water through cell membranes — which is how the plant controls what enters.

Factors affecting transpiration rate

Light opens stomata; higher temperature raises evaporation and molecular energy; wind removes the humid boundary layer, steepening the gradient; high humidity reduces the gradient and slows it. A potometer measures water uptake, which is used as an estimate of transpiration.

Xerophytes reduce water loss

Sunken stomata, rolled leaves, thick waxy cuticles, hairs trapping humid air, reduced leaf area — every adaptation either traps water vapour near the surface or reduces the surface itself. Hydrophytes, living in water, face the opposite problem and have air spaces for buoyancy.

The grown-up words

Check you have got it

Vessels match their pressures

Arteries: thick muscular walls with elastic tissue, narrow lumen, high pressure, no valves. Capillaries: one cell thick for a short diffusion distance, huge total surface area. Veins: thin walls, wide lumen, low pressure, valves to prevent backflow, helped by skeletal muscle.

The cardiac cycle

Atrial systole — atria contract, pushing blood into the ventricles. Ventricular systole — ventricles contract, atrioventricular valves close (the first heart sound), semilunar valves open. Diastole — everything relaxes, semilunar valves close (the second sound) and the heart refills. Valves open and close because of pressure differences, nothing else.

The heart controls its own beat

The SAN in the right atrium sets the rhythm. The impulse spreads across the atria, is delayed at the AVN so the atria finish emptying, then travels down the bundle of His and Purkyne fibres so the ventricles contract from the apex upwards — squeezing blood upwards into the arteries.

Haemoglobin and the dissociation curve

The oxygen dissociation curve is S-shaped because binding one oxygen molecule makes the next bind more easily — cooperative binding. Fetal haemoglobin's curve lies to the left (higher affinity, so it takes oxygen from the mother's blood). The Bohr shift moves the curve right when CO₂ is high, releasing more oxygen exactly where respiring tissue needs it.

Carbon dioxide transport

About 85% travels as hydrogencarbonate ions, formed in red blood cells by carbonic anhydrase. The chloride shift maintains charge balance. Around 10% binds to haemoglobin as carbaminohaemoglobin and about 5% dissolves in the plasma.

The grown-up words

Check you have got it

What makes a good exchange surface

Large surface area, thin (short diffusion distance), moist, well ventilated and well supplied with blood. Alveoli deliver all five: roughly 300 million of them, walls one squamous cell thick, wrapped in capillaries and refreshed by breathing.

The airway from nose to alveolus

Trachea and bronchi have C-shaped cartilage holding them open, ciliated epithelium sweeping mucus upwards and goblet cells producing that mucus. Bronchioles have smooth muscle and no cartilage. Alveoli are lined with squamous epithelium and contain surfactant-producing cells.

Cells you must recognise

Squamous epithelium — flattened, for short diffusion distance. Ciliated epithelium — moves mucus. Goblet cells — secrete mucus trapping dust and bacteria. Smooth muscle — adjusts airway diameter. This is a common microscope-slide question.

Smoking damages every one of those defences

Tar paralyses cilia, so mucus accumulates and infection follows. Chronic bronchitis and emphysema (alveolar walls broken down, reducing surface area) leave less area for exchange. Carbon monoxide binds irreversibly to haemoglobin, reducing oxygen-carrying capacity.

Ventilation maintains the gradient

Breathing constantly replaces alveolar air, and blood flow constantly removes absorbed oxygen. Both keep the concentration difference across the alveolar wall high, which — by Fick's law — keeps the rate of diffusion high.

The grown-up words

Check you have got it

The four diseases you must know

Cholera — Vibrio cholerae, water-borne, causes severe watery diarrhoea by toxin action on the intestinal lining. Malaria — Plasmodium, spread by female Anopheles mosquitoes. TB — Mycobacterium tuberculosis, airborne droplets. HIV/AIDS — a virus in body fluids, destroying helper T-lymphocytes.

Transmission decides the control method

Cholera is controlled by clean water and sanitation. Malaria by nets, insecticide, draining breeding sites and prophylactic drugs. TB by BCG vaccination, contact tracing and completing a long course of antibiotics. HIV by education, condoms, screening blood and antiretroviral drugs.

Antibiotics act on bacteria only

Penicillin inhibits cross-linking in the peptidoglycan cell wall, so growing bacteria burst by osmosis. Viruses have no cell wall, no ribosomes of their own and no metabolism to disrupt — which is precisely why antibiotics do nothing against them.

Antibiotic resistance is natural selection

Random mutation gives a few bacteria resistance. Antibiotic use kills the rest, so the resistant ones survive, reproduce and pass the resistance on — often on plasmids, which can move between species. Misuse and not finishing courses accelerate this.

Why some diseases resist eradication

Plasmodium hides inside liver and red blood cells and changes its surface antigens. HIV mutates rapidly and destroys the very cells that would fight it. TB needs a six-month drug course that many patients cannot complete. Biology and social conditions both matter.

The grown-up words

Check you have got it

Phagocytosis is the general response

Neutrophils and macrophages engulf pathogens into a phagosome, fuse it with a lysosome and digest it. Macrophages then display fragments of the pathogen's antigens on their surface, becoming antigen-presenting cells — the link between the general and the specific response.

B and T lymphocytes divide the work

B-lymphocytes mature in bone marrow and produce antibodies — humoral immunity. T-lymphocytes mature in the thymus: helper T-cells release cytokines that activate B-cells, and killer T-cells destroy infected body cells — cell-mediated immunity.

Antibody structure follows its function

An antibody is a Y-shaped globular protein with two heavy and two light chains. The variable region at the tips has a shape complementary to one specific antigen; the constant region is the same across a class. Antibodies neutralise toxins, agglutinate pathogens and mark them for phagocytosis.

Primary and secondary responses

The primary response is slow and produces few antibodies while the right lymphocyte is found and cloned. Memory cells remain. The secondary response to the same antigen is faster, larger and longer-lasting — the whole basis of vaccination.

Four kinds of immunity

Active natural (catching the disease), active artificial (vaccination), passive natural (antibodies across the placenta or in breast milk), passive artificial (injected antibodies, e.g. antivenom). Passive immunity acts immediately but is short-lived, because no memory cells are made.

The grown-up words

Check you have got it

The four stages

Glycolysis (cytoplasm): glucose → 2 pyruvate, net 2 ATP and 2 reduced NAD. Link reaction (mitochondrial matrix): pyruvate → acetyl CoA, releasing CO₂ and reducing NAD. Krebs cycle (matrix): acetyl CoA is oxidised, producing CO₂, ATP, reduced NAD and reduced FAD. Oxidative phosphorylation (inner membrane): the reduced coenzymes are oxidised and most ATP is made.

Chemiosmosis makes the ATP

Electrons pass along carriers in the inner membrane, releasing energy used to pump protons into the intermembrane space. The resulting gradient drives protons back through ATP synthase, and that flow makes ATP. Oxygen is the final electron acceptor, combining with electrons and protons to form water.

Mitochondrial structure fits the job

The inner membrane is folded into cristae, giving a large area for electron carriers and ATP synthase. The matrix holds the enzymes of the link reaction and Krebs cycle. The narrow intermembrane space lets a proton gradient build quickly.

Anaerobic respiration buys time

Without oxygen, the electron transport chain stops and NAD is not regenerated — so glycolysis would halt. In mammals, pyruvate is reduced to lactate; in yeast, to ethanol and CO₂. Both regenerate NAD so that glycolysis continues, but the yield is only 2 ATP per glucose.

Respiratory quotient

RQ = CO₂ produced ÷ O₂ consumed. Carbohydrate gives about 1.0, protein about 0.9, lipid about 0.7. A respirometer measures oxygen uptake, with soda lime absorbing the CO₂ produced.

The grown-up words

Check you have got it

The light-dependent stage

In the thylakoid membranes, chlorophyll absorbs light and excites electrons. Photolysis splits water, giving electrons, protons and oxygen as a by-product. Electrons pass along carriers, pumping protons and driving ATP synthesis by chemiosmosis, and finally reduce NADP.

The Calvin cycle

In the stroma, rubisco combines CO₂ with the 5-carbon RuBP to give two molecules of the 3-carbon GP. ATP and reduced NADP convert GP to TP, most of which regenerates RuBP; the rest becomes glucose, starch, lipids and amino acids.

Chloroplast structure fits the job

Stacked thylakoids (grana) give a large surface for pigments and electron carriers. The stroma holds Calvin cycle enzymes. The chloroplast also contains its own DNA and 70S ribosomes, plus starch grains and lipid droplets.

Pigments and chromatography

Chlorophyll a and b, carotene and xanthophyll absorb different wavelengths, broadening the range of light usable. An absorption spectrum shows what a pigment absorbs; an action spectrum shows the rate of photosynthesis at each wavelength. Their close match is the evidence that those pigments drive the process.

Limiting factors

Light intensity, CO₂ concentration and temperature each limit the rate. On a graph, the plateau shows something else has become limiting — and identifying which factor, with a reason, is what the question is really asking.

The grown-up words

Check you have got it

Negative feedback is the mechanism

A receptor detects a change from the set point, a coordinator processes it, and an effector brings about a response that reverses the change. Positive feedback amplifies instead — useful in childbirth and nerve impulses, but destabilising if it happened in temperature control.

The kidney filters, then reclaims

Ultrafiltration at the glomerulus is driven by high hydrostatic pressure through the basement membrane, which holds back blood cells and large proteins. The proximal convoluted tubule then selectively reabsorbs all the glucose and amino acids and most of the water and salts, using microvilli and many mitochondria.

The loop of Henle concentrates urine

A counter-current multiplier: the ascending limb actively pumps out sodium and chloride ions, making the medulla increasingly salty; water then leaves the descending limb and the collecting duct by osmosis. Longer loops allow more concentrated urine — which is why desert mammals have them.

ADH controls water balance

Osmoreceptors in the hypothalamus detect a fall in water potential. The posterior pituitary releases ADH, which makes the collecting duct walls more permeable by inserting aquaporins, so more water is reabsorbed and urine is more concentrated.

Blood glucose control

The islets of Langerhans in the pancreas: β-cells release insulin, which increases glucose uptake and glycogenesis, lowering blood glucose. α-cells release glucagon, which promotes glycogenolysis and gluconeogenesis, raising it. Type 1 diabetes is failure to produce insulin; type 2 is loss of receptor sensitivity.

Plants use homeostasis too

Guard cells control stomatal aperture. Abscisic acid released in water stress triggers loss of potassium ions from guard cells, so water leaves by osmosis, the cells become flaccid and the stoma closes — reducing water loss.

The grown-up words

Check you have got it

The resting potential and the action potential

At rest, the sodium–potassium pump keeps the inside at about −70 mV. If a stimulus reaches threshold, voltage-gated sodium channels open and the membrane depolarises to about +40 mV. Potassium channels then open and it repolarises, briefly overshooting into hyperpolarisation before returning to rest.

All-or-nothing, and the refractory period

An impulse either fires fully or not at all — stimulus strength is coded by frequency, not size. The refractory period prevents a new impulse immediately, which ensures one-way travel and sets the maximum firing rate.

Speed depends on the axon

Myelination forces the impulse to jump between nodes of Ranvier — saltatory conduction, much faster. A wider axon diameter and higher temperature also increase speed.

The synapse

The impulse opens calcium channels; vesicles of acetylcholine fuse with the membrane and release it into the cleft; it binds receptors on the postsynaptic membrane, opening sodium channels. Acetylcholinesterase then breaks it down so the signal stops. Synapses ensure one-way transmission and allow summation.

Muscle contraction: the sliding filament model

Calcium ions released from the sarcoplasmic reticulum move tropomyosin, exposing binding sites on actin. Myosin heads bind, pull the actin inwards, then ATP detaches them and resets them. The sarcomere shortens while the filaments themselves stay the same length.

Plant responses

Auxin accumulates on the shaded side of a shoot, causing greater cell elongation there and bending towards the light. Gibberellin triggers amylase production in germinating seeds. Plants respond to stimuli with growth and chemistry, not nerves.

The grown-up words

Check you have got it

Meiosis halves the number and creates variety

Two divisions produce four genetically different haploid cells. Crossing over in prophase I exchanges sections between homologous chromosomes. Independent assortment in metaphase I means each pair lines up randomly relative to the others. Random fertilisation adds a third layer.

The vocabulary must be exact

Gene — a length of DNA coding for a polypeptide. Allele — a version of a gene. Genotype — the alleles present. Phenotype — the observed characteristics. Homozygous/heterozygous, dominant/recessive/codominant. Marks are routinely lost by writing 'gene' where 'allele' is meant.

Monohybrid and dihybrid crosses

A monohybrid cross of two heterozygotes gives 3:1. A dihybrid cross of two double heterozygotes gives 9:3:3:1 — but only if the genes are on different chromosomes. Always draw parental genotypes, gametes, the Punnett square and the ratio; the working carries the marks.

Linkage and sex linkage

Genes on the same chromosome are linked and tend to be inherited together, distorting expected ratios; crossing over produces a minority of recombinants. Sex-linked genes are on the X chromosome, so males, having only one X, show recessive conditions such as haemophilia and red–green colour blindness far more often.

The chi-squared test

χ² = Σ (O − E)² ÷ E. Compare with the critical value at p = 0.05 for the correct degrees of freedom (categories − 1). If χ² is greater than the critical value, the difference is significant and you reject the null hypothesis.

Control of gene expression

The lac operon: without lactose, a repressor blocks the operator and the genes are off. Lactose binds the repressor, changing its shape so it releases, and transcription proceeds. Transcription factors and gibberellin acting on DELLA proteins are the eukaryotic examples in the syllabus.

The grown-up words

Check you have got it

Variation has two sources

Genetic variation arises from mutation, meiosis and random fertilisation, and is heritable. Environmental variation is not. Continuous variation (height, mass) is polygenic and gives a normal distribution; discontinuous variation (blood group) is controlled by one or few genes.

Three types of selection

Stabilising favours the mean and reduces variation — human birth mass is the standard example. Directional favours one extreme and shifts the mean, as in antibiotic resistance. Disruptive favours both extremes and can begin to split a population.

Genetic drift and the founder effect

In small populations, allele frequencies change by chance alone. A founder effect occurs when a few individuals start a new population, carrying an unrepresentative sample of alleles. A genetic bottleneck does the same after a crash in numbers.

Hardy–Weinberg lets you calculate allele frequencies

p + q = 1 and p² + 2pq + q² = 1, where p² is homozygous dominant, 2pq heterozygous and q² homozygous recessive. It assumes a large population, random mating, no migration, no mutation and no selection — assumptions worth stating when the question asks.

Speciation needs isolation

Allopatric speciation follows geographical separation; sympatric speciation happens without it, through behavioural, temporal or polyploid isolation. Once gene flow stops, the two populations accumulate different changes until they can no longer interbreed.

Artificial selection

Humans choose the parents — as in modern bread wheat, dairy cattle and inbred maize. It is much faster than natural selection and can produce extremes, but it narrows the gene pool and can fix harmful recessive alleles.

The grown-up words

Check you have got it

The three domains and the hierarchy

Domains: Bacteria, Archaea, Eukarya — based on ribosomal RNA differences. The hierarchy runs domain, kingdom, phylum, class, order, family, genus, species. The binomial name is genus plus species, italicised, with the genus capitalised.

Defining a species is harder than it looks

The usual definition is a group that can interbreed to produce fertile offspring. It fails for asexual organisms, extinct species and organisms that hybridise — which is why molecular evidence from DNA and protein sequences is now used alongside it.

Viruses sit outside the system

Viruses are not placed in any domain because they are acellular, have no metabolism of their own and cannot reproduce without a host. They are classified separately by their nucleic acid type and structure.

Measuring biodiversity

Species richness counts species; species evenness considers how numbers are distributed. Simpson's index of diversity combines both: D = 1 − Σ(n/N)². A value closer to 1 means higher diversity. Sampling uses random quadrats, transects for gradients, and mark–release–recapture for mobile animals.

Statistics you must be able to use

The t-test compares two means. Pearson's correlation is for linear relationships between normally distributed data; Spearman's rank is for ranked or non-normal data. Standard deviation and standard error give the spread; if error bars overlap, a difference is unlikely to be significant.

Conservation, in situ and ex situ

In situ — national parks, protected areas, wildlife corridors — conserves species in their habitat with the ecosystem intact. Ex situ — zoos, botanic gardens, seed banks, captive breeding — is a safety net when the habitat cannot be secured, but the population is small and can lose genetic diversity.

The grown-up words

Check you have got it

The toolkit

Restriction endonucleases cut DNA at specific recognition sequences, often leaving sticky ends. DNA ligase joins fragments. Reverse transcriptase makes DNA from mRNA. Plasmids and viruses act as vectors. Marker genes — fluorescence or antibiotic resistance — identify which cells took up the gene.

PCR amplifies tiny samples

Repeated cycles of denaturing at about 95 °C, annealing primers at 50–65 °C and extending with heat-stable Taq polymerase at about 72 °C. Each cycle doubles the DNA, so a trace sample becomes enough to analyse — the basis of forensic and diagnostic work.

Gel electrophoresis separates by size

DNA is negatively charged, so it moves towards the anode. Shorter fragments travel further through the gel. Compared against a ladder of known sizes, this gives DNA profiles used in forensics, paternity testing and detecting genetic disease.

Applications

Bacteria engineered to make human insulin. Golden Rice engineered to make β-carotene, addressing vitamin A deficiency. Herbicide-resistant crops. Gene therapy for conditions such as cystic fibrosis and SCID. Genetic screening for inherited disease.

The concerns are real, not decorative

Ecological: gene flow to wild relatives, effects on non-target species. Health: allergenicity, antibiotic resistance markers. Social and economic: patents, seed costs, dependence of small farmers. Ethical: consent, insurance discrimination, genetic privacy. A good answer names a specific concern, not just 'people worry about it'.

The grown-up words

Check you have got it