A-Level Biology flashcards

About this deck

230 free AQA A-Level Biology flashcards across 8 topics. Every card is a key term and its definition, taken word for word from a published Revise Right lesson, and links back to that lesson so you can read the full notes behind it.

Biological molecules

33 cards from 21 lessons. Each term links to the lesson it comes from.

Monomers, Polymers, and Life's Biochemical Basis

• Monomer: A monomer is a small molecule that acts as the repeating subunit from which larger molecules are constructed. Monomers join together through chemical reactions to form polymers.
• Polymer: A polymer is a large molecule made from a large number of monomers joined together by covalent bonds. Polymers are formed by condensation reactions and broken down by hydrolysis reactions.

Condensation and Hydrolysis Reactions

• Condensation reaction: A reaction that joins two molecules together with the formation of a new chemical bond and involves the elimination of a molecule of water. The water is produced from atoms contributed by both reacting molecules.
• Hydrolysis reaction: A reaction that breaks a chemical bond between two molecules by using a molecule of water. The water molecule is split, with one part (-OH) going to one product and the other part (-H) going to the other product.

Monosaccharides and Disaccharides

• Monosaccharide: A single-unit sugar that acts as a monomer for larger carbohydrates. Glucose, galactose, and fructose are all monosaccharides with the formula C6H12O6.
• Glycosidic bond: A covalent bond formed between two monosaccharides during a condensation reaction. One molecule of water (H2O) is released for every glycosidic bond formed.

Testing for Sugars and Starch

• Reducing sugar: A carbohydrate that can reduce copper(II) ions in Benedict's solution to copper(I) oxide, producing a colour change from blue to brick-red. All monosaccharides and the disaccharides maltose and lactose are reducing sugars.
• Iodine/potassium iodide test for starch: A biochemical test in which iodine/potassium iodide solution is added to a sample. A positive result is a blue-black colour, caused by iodine molecules becoming trapped within the helical structure of amylose. A negative result leaves the solution orange-brown.

Introduction to Lipids: Triglycerides

• Fatty acid: An organic molecule with the general formula RCOOH, consisting of a carboxyl group (-COOH) attached to a hydrocarbon R-group chain. The R-group varies in length and degree of saturation between different fatty acids.

Phospholipids and the Emulsion Test

• Phospholipid: A lipid in which one of the three fatty acid chains of a triglyceride is substituted by a phosphate-containing group. The result is a molecule with a hydrophilic phosphate 'head' and two hydrophobic fatty acid 'tails'.

Amino Acids, Dipeptides, and Polypeptides

• R group (side chain): The variable portion of an amino acid that distinguishes one amino acid from another. The twenty amino acids common to all organisms share an identical backbone (amine group, carboxyl group, and central carbon) but differ only in their R group. R groups vary in size, charge, polarity, and chemical reactivity.

Protein Structure and Function

• Primary structure: The sequence of amino acids in a polypeptide chain, held together by peptide bonds. This sequence is determined by the base sequence of the gene (via mRNA codons). Every other level of structure follows directly from the primary structure.
• Tertiary structure: The overall three-dimensional conformation of a single polypeptide chain, produced by further folding of the secondary structure. Maintained by interactions between R groups: hydrogen bonds, ionic bonds, and disulfide bridges (as listed in the AQA specification). Hydrophobic interactions also play a role in folding but are not considered a named bond type for AQA.

The Biuret Test for Proteins

• Peptide bond: A covalent bond formed between the carboxyl group (–COOH) of one amino acid and the amino group (–NH2) of another, with the loss of a water molecule (condensation reaction). The –CO–NH– linkage is what the biuret reagent detects.

Enzyme Action: Specificity and Mechanisms

• Enzyme specificity: The property of an enzyme that restricts it to catalysing one particular reaction (or a narrow class of reactions), arising because only a substrate with a complementary shape and chemical character can bind to the active site and form an enzyme-substrate complex.
• Induced-fit model: A model of enzyme action in which the active site is flexible; when the substrate binds, the active site changes conformation to produce a precise fit. This conformational change strains bonds in the substrate and lowers the activation energy of the reaction.

Factors Affecting Enzyme Activity

• Active site: The specific region of an enzyme with a shape complementary to a particular substrate. When the substrate binds, an enzyme-substrate complex forms, lowering the activation energy of the reaction. The active site's shape depends on the precise tertiary structure of the enzyme protein.
• Competitive inhibitor: A molecule with a shape complementary to the enzyme's active site. It competes with the substrate for the active site. When bound, it blocks the substrate from entering. The effect can be reduced by increasing substrate concentration, because substrate molecules can outcompete the inhibitor for active sites.
• Non-competitive inhibitor: A molecule that binds to a site other than the active site — the allosteric site. This binding changes the tertiary structure of the enzyme, altering the shape of the active site so that the substrate can no longer bind effectively. Increasing substrate concentration does NOT overcome non-competitive inhibition.

Nucleotide Structure: DNA vs RNA

• Nucleotide: A monomer consisting of a pentose sugar, a phosphate group, and a nitrogen-containing organic base. Nucleotides are the repeating units from which polynucleotides (DNA and RNA) are built.

Unwinding and Preparing DNA for Replication

• Complementary base pairing: The specific pairing of nitrogenous bases in DNA: adenine pairs with thymine (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen bonds), holding the two polynucleotide strands together.
• DNA helicase: An enzyme that moves along the DNA double helix, breaking the hydrogen bonds between complementary base pairs and unwinding the two polynucleotide strands to form a replication fork.

Building New DNA Strands

• Template strand: The existing, unwound strand of DNA whose exposed bases direct the sequence of incoming free nucleotides during replication, acting as a molecular blueprint for the new complementary strand.

Semi-Conservative Replication and its Validation

• Helicase: An enzyme that breaks the hydrogen bonds between complementary base pairs, unwinding the double helix and separating the two parental strands to form a replication fork. It does not break the covalent bonds of the sugar-phosphate backbone.
• DNA polymerase: An enzyme that catalyses the formation of phosphodiester bonds between adjacent nucleotides, joining them into a new complementary strand. It can only add nucleotides in the 5' to 3' direction and requires a pre-existing strand as a template.

ATP: Structure and Hydrolysis

• ATP — adenosine triphosphate: A nucleotide derivative formed from three components: the pentose sugar ribose, the nitrogenous base adenine, and three phosphate groups arranged in a chain. The adenine and ribose together form the unit called adenosine.

ATP: Energy Coupling, Phosphorylation and Resynthesis

• Hydrolysis of ATP: The reaction in which a water molecule is used to break the bond between the second (beta) and third (gamma/terminal) phosphate groups of ATP, yielding ADP (adenosine diphosphate), inorganic phosphate (Pi), and a release of energy. This reaction is catalysed by the enzyme ATPase.
• Condensation reaction (ATP resynthesis): The reaction in which ADP and inorganic phosphate (Pi) are joined by the formation of a phosphoanhydride bond, producing ATP and water. This reaction requires an input of energy and is catalysed by the enzyme ATP synthase.

Water: Solvent & Metabolite

• Hydrogen bond: A weak electrostatic attraction between the slightly positive hydrogen atom (δ+) of one water molecule and the slightly negative oxygen atom (δ-) of an adjacent molecule. Individual hydrogen bonds are weak, but the large number formed collectively gives water its distinctive properties.

Water: Thermal Properties

• Heat capacity (specific heat capacity of water): Water has a specific heat capacity of approximately 4,200 J kg⁻¹ °C⁻¹ — one of the highest of any known liquid. This means 4,200 joules of energy must be transferred to raise the temperature of 1 kg of water by just 1°C.
• Latent heat of vaporisation: The energy required to convert 1 kg of a liquid into vapour at constant temperature. For water at body temperature (~37°C), this is approximately 2,430 kJ kg⁻¹ — far higher than most other liquids. Crucially, this energy comes from the surface the water is evaporating from, cooling it down.

Introduction to Inorganic Ions

• Inorganic ion: A charged atom or small group of atoms, not derived from carbon-based (organic) molecules, that is dissolved in the cytoplasm or body fluids of an organism and performs a specific physiological role determined by its chemical properties.

Key Inorganic Ions and Their Roles

• pH: A logarithmic scale that measures the concentration of hydrogen ions (H^+) in a solution. pH = -10[H^+]. A pH of 7 is neutral; below 7 is acidic (higher H^+ concentration); above 7 is alkaline (lower H^+ concentration).

Cells

35 cards from 17 lessons. Each term links to the lesson it comes from.

Prokaryotic Cell Structure: Inside and Out

• Nucleoid region: The area within the prokaryotic cytoplasm where the single circular DNA molecule is located. It is not enclosed by a nuclear envelope and the DNA is not associated with histone proteins.
• Murein (peptidoglycan): A glycoprotein that forms the structural component of the prokaryotic cell wall. It consists of polysaccharide chains cross-linked by short peptides, forming a rigid mesh that maintains cell shape and resists osmotic lysis.

Prokaryotic Cell Features: Plasmids, Capsule, and Flagella

• Plasmid: A small, circular DNA molecule found in prokaryotic cells, separate from the main chromosomal DNA. Plasmids replicate independently and carry genes that may provide survival advantages, such as antibiotic resistance.
• Capsule: A thick, gel-like layer composed mainly of polysaccharides that surrounds the cell wall of some prokaryotic cells. The capsule provides protection against desiccation, phagocytosis by white blood cells, and attack by bacteriophages.

Viruses: Acellular Structure

• Non-living (in the context of viruses): Viruses are described as non-living because they cannot carry out any of the seven characteristics of life independently. They cannot respire, grow, respond to stimuli, or reproduce without using a host cell's machinery. Outside a host, a virus particle is essentially an inert chemical structure.

Microscopy Techniques

• Resolution (optical microscope): The minimum distance between two points at which they can still be distinguished as separate structures. For an optical microscope, the maximum resolution is approximately 200 nm (0.2 micrometres), because resolution is limited by the wavelength of visible light.
• Transmission Electron Microscope (TEM): A microscope that fires a beam of electrons through an extremely thin specimen (typically 100 nm or less). Denser regions of the specimen absorb or scatter more electrons, producing contrast in the final image. TEMs produce 2D, high-contrast images of internal cell structures.
• Scanning Electron Microscope (SEM): A microscope that scans a focused beam of electrons across the surface of a specimen coated in a thin layer of metal (usually gold). Electrons scattered from the surface are detected to build up a detailed 3D image of the specimen's external surface.

Cell Fractionation and Ultracentrifugation

• Differential centrifugation: A technique in which a cell homogenate is centrifuged at a series of increasing speeds, with the pellet collected and the supernatant re-spun at each stage, so that organelles are separated in order of decreasing size and density.

Exploring Eukaryotic Cells: Nucleus, Membranes, and Energy

• Cell-surface membrane: A partially permeable membrane, approximately 7–10 nm thick, composed of a phospholipid bilayer with embedded proteins. It forms the outer boundary of the cell and controls the exchange of substances between the cell and its environment.
• Nucleus: The largest organelle in most eukaryotic cells, bounded by a double membrane (the nuclear envelope). It contains the cell's genetic material in the form of linear DNA bound to proteins, organised into chromosomes, and one or more nucleoli responsible for ribosome production.
• Mitochondrion: A double-membrane-bound organelle found in eukaryotic cells that is the primary site of aerobic respiration. It produces ATP through oxidative phosphorylation. Mitochondria contain their own circular DNA and ribosomes, and can replicate independently within the cell.
• Chloroplast: A double-membrane-bound organelle found in plant cells and algae that is the site of photosynthesis. It contains a system of flattened membrane sacs called thylakoids (stacked into grana) and a fluid-filled stroma, and possesses its own circular DNA and ribosomes.

Eukaryotic Organelles: Golgi, Lysosomes, and Endoplasmic Reticulum

• Ribosome: A small organelle (approximately 20–30 nm in diameter) composed of ribosomal RNA (rRNA) and protein, arranged into a large and a small subunit. Ribosomes are the site of translation — the assembly of amino acids into polypeptide chains using mRNA as a template.
• Golgi apparatus: A stack of flattened, membrane-bound cisternae that receives proteins and lipids from the endoplasmic reticulum, modifies them (e.g. by further glycosylation or proteolytic cleavage), and packages them into Golgi vesicles for transport to the cell surface membrane, lysosomes, or for secretion.
• Lysosome: A membrane-bound organelle, approximately 0.1–1.2 µm in diameter, that contains hydrolytic enzymes (including proteases, lipases, nucleases, and glycosidases) maintained at an acidic pH of around 4.5–5.0. Lysosomes digest engulfed pathogens, worn-out organelles (autophagy), and macromolecules, recycling their components for reuse.

Specialisation and Organisation in Eukaryotic Cells

• Cell wall: A layer external to the cell surface membrane that provides structural support and rigidity. In plants it is composed of cellulose microfibrils; in fungi it is composed of chitin; in algae it is typically cellulose (though composition varies by species). The cell wall is fully permeable to water and solutes.
• Cell vacuole (plant): A large, membrane-bound organelle in plant cells, enclosed by the tonoplast (vacuolar membrane). It is filled with cell sap — a solution of water, sugars, mineral ions, and pigments. The permanent central vacuole pushes the cytoplasm against the cell wall, generating turgor pressure that keeps the cell firm.

The Cell Cycle and Mitosis

• Interphase: The longest phase of the cell cycle, divided into G1 (first growth phase), S phase (DNA synthesis/replication), and G2 (second growth phase). During S phase, every chromosome is replicated so that each consists of two identical sister chromatids joined at the centromere.

Binary Fission and Viruses

• Binary fission: The asexual reproduction of prokaryotic cells in which the circular chromosomal DNA and plasmids are replicated, and the cytoplasm divides to produce two daughter cells, each with a single copy of the circular DNA and a variable number of plasmid copies.
• Virus: A non-living infectious agent consisting of nucleic acid (DNA or RNA) enclosed in a protein coat (capsid). Viruses lack the cellular machinery required for independent metabolism or reproduction, and so are not classified as living organisms.

Passive Transport: Diffusion and Osmosis

• Cholesterol: A lipid molecule interspersed among the phospholipids in the bilayer. Cholesterol restricts the movement of other molecules that make up the membrane (such as phospholipids), reducing fluidity and making the membrane less permeable to most water-soluble substances. It also prevents the membrane from becoming too rigid at low temperatures.
• Channel proteins: Intrinsic membrane proteins that form a hydrophilic pore or channel through the bilayer. They are specific — each type allows only particular ions or molecules (often water) to pass through. They do not change shape during transport. Aquaporins are a well-known example, forming water-specific channels.
• Carrier proteins: Intrinsic membrane proteins that bind to a specific molecule on one side of the membrane, undergo a conformational change (a change in shape), and release the molecule on the other side. They are highly specific and transport larger polar molecules such as glucose and amino acids.

Active Transport and Co-transport

• Active transport: The movement of molecules or ions across a cell membrane against their concentration gradient, using energy released from the hydrolysis of ATP and specific carrier proteins.
• Co-transport: A transport mechanism in which a single carrier protein simultaneously moves two different substances across a membrane. The movement of one substance down its electrochemical gradient provides the energy to drive the other substance against its gradient.

Cell Adaptations and Membrane Permeability

• Phospholipid bilayer: A double layer of phospholipid molecules in which the hydrophilic phosphate heads face the aqueous environments on either side of the membrane and the hydrophobic fatty acid tails face inward, forming a barrier to most water-soluble substances.
• Colorimeter: An instrument that measures the absorbance or transmission of light through a coloured solution at a specific wavelength. In membrane permeability investigations, a colorimeter measures the absorbance of the betalain solution — higher absorbance indicates more pigment has leaked out, meaning greater membrane permeability.

Cell Recognition and Immune Response: Antigens and Phagocytosis

• Antigen: A molecule, usually a protein or glycoprotein, on the surface of a cell, pathogen, or toxin that is recognised as foreign by the immune system and stimulates an immune response.

T and B Lymphocytes: Cellular and Humoral Responses

• Antigen-presenting cell (APC): A cell that has engulfed a pathogen and displays antigen fragments on its surface, bound to MHC proteins, thereby activating T lymphocytes. Macrophages are the primary example.
• Clonal selection: The process by which a B lymphocyte with a receptor complementary to a specific antigen is selected by that antigen and stimulated (with helper T cell signals) to divide by mitosis, producing a clone of genetically identical cells.

Immunity, HIV, Monoclonal Antibodies, and Ethical Considerations (Part 1/2)

• Primary immune response: The immune response that occurs on first exposure to a specific antigen. It is relatively slow (days to weeks) and produces a modest antibody titre, because naive B and T lymphocytes must first be activated and clonally expanded before plasma cells can secrete antibodies in large quantities.
• Secondary immune response: The immune response on second (or subsequent) exposure to the same antigen. It is faster, larger in magnitude, and longer-lasting than the primary response, because memory B and T cells are already present and can rapidly differentiate into plasma cells and effector T cells without the need for a prolonged activation phase.
• Active immunity: Immunity resulting from the production of antibodies by an individual's own immune system, either following natural infection or vaccination. It involves clonal expansion of lymphocytes and the formation of memory cells, so protection is long-lasting.

Immunity, HIV, Monoclonal Antibodies, and Ethical Considerations (Part 2/2)

• NICE cost-effectiveness threshold: NICE generally approves treatments that cost less than £20,000–£30,000 per quality-adjusted life year (QALY) gained. Treatments above this threshold require exceptional justification. This threshold is used to allocate NHS resources fairly across all patient groups.

Organisms exchange substances with their environment

22 cards from 13 lessons. Each term links to the lesson it comes from.

Surface Area to Volume Ratio: Size Matters

• Fick's Law of Diffusion (applied context): Rate of diffusion ∝ (Surface area × Concentration difference) ÷ Thickness of exchange surface. A lower SA:V ratio means less surface area is available per unit of metabolic demand, reducing the rate of exchange relative to the organism's needs.

Gas Exchange in Organisms

• Fick's Law (qualitative statement): Rate of diffusion is directly proportional to (surface area × concentration difference) and inversely proportional to the thickness of the exchange surface. At A Level, you apply this principle to evaluate and compare gas exchange adaptations.
• Stomata (singular: stoma): Pores in the leaf epidermis, each formed by two guard cells, through which gas exchange and transpiration occur. Guard cells change shape in response to water potential changes, opening the pore in light and closing it in darkness or water stress.

The Human Gas Exchange System

• Alveolar epithelium: The single layer of squamous (flattened) epithelial cells lining each alveolus. It forms the gas exchange surface across which oxygen diffuses into the blood and carbon dioxide diffuses out.
• Ventilation: The process of moving air into (inhalation) and out of (exhalation) the lungs by creating pressure differences between the thoracic cavity and the atmosphere, driven by the contraction and relaxation of the diaphragm and intercostal muscles.

Lung Disease and Risk Factors

• Pulmonary fibrosis: A condition in which scar tissue (fibrous connective tissue) accumulates in the walls of the alveoli and surrounding lung tissue. The thickened walls increase the diffusion distance for oxygen and carbon dioxide, reducing the rate of gas exchange. Lung compliance also falls, making ventilation harder and tidal volume smaller.
• Emphysema: A condition in which the walls between adjacent alveoli break down, merging smaller alveoli into larger, irregular air spaces. This dramatically reduces the total surface area available for gas exchange and destroys the elastic recoil of the lung tissue, impairing expiration.
• Asthma: A condition characterised by reversible narrowing of the airways (bronchoconstriction), inflammation of the bronchiole lining, and increased mucus secretion. Airflow resistance rises sharply during an attack, reducing ventilation rate and disrupting the concentration gradient across alveolar membranes.
• Chronic obstructive pulmonary disease (COPD): An umbrella term covering chronic bronchitis and emphysema together. Characterised by persistent airflow limitation that is not fully reversible. Chronic bronchitis involves long-term inflammation and excess mucus production in the bronchi, further obstructing airflow.

Digestive Hydrolysis of Macromolecules

• Hydrolysis: The breaking of a chemical bond by the addition of a water molecule. In digestion, hydrolysis converts polysaccharides to monosaccharides, lipids to fatty acids and glycerol, and proteins to amino acids — all small enough to be absorbed across cell membranes.
• Emulsification by bile salts: Bile salts (produced in the liver, stored in the gall bladder, and released into the duodenum) are amphipathic molecules — they have a hydrophilic region and a hydrophobic region. They surround large lipid globules and break them into many smaller droplets, dramatically increasing the total surface area available for lipase to act upon. Emulsification is a physical process, not a chemical one: no bonds in the lipid are broken.

Absorption in the Ileum

• Co-transport (sodium-glucose co-transport): A secondary active transport mechanism in which a carrier protein (e.g. SGLT1 for glucose) simultaneously binds a sodium ion and a monosaccharide or amino acid, then changes shape to move both into the epithelial cell. The sodium ion moves down its electrochemical gradient, releasing enough energy to pull the glucose or amino acid in against its own concentration gradient. No ATP is consumed directly at this step.
• Micelle: A spherical aggregate formed in the small intestinal lumen when bile salts surround fatty acids and monoglycerides. The hydrophilic surfaces of the bile salts face the aqueous environment, making the micelle water-soluble and allowing it to transport lipid digestion products to the apical surface of epithelial cells.

Investigating Digestion Experimentally

• Rate of enzyme-catalysed reaction: The change in substrate concentration (or product concentration) per unit time. In digestion investigations, this is often expressed as the time taken for a visible endpoint to be reached — for example, the time for starch to be completely hydrolysed, detected by iodine solution no longer turning blue-black.

Xylem Transport: Cohesion-Tension Theory

• Xylem vessel: A dead, hollow tube formed from cells whose end walls have broken down. The walls are thickened with lignin, which provides structural support and prevents collapse under the negative pressures generated during water transport.
• Cohesion: The attraction between molecules of the same substance. In water, cohesion arises from hydrogen bonding between adjacent water molecules, allowing the water column in xylem to be pulled upwards as a continuous unit without breaking.

Phloem Transport: Mass Flow Hypothesis

• Hydrostatic pressure: The physical pressure exerted by a fluid within a confined space. In phloem sieve tubes, hydrostatic pressure builds at the source end as water enters by osmosis, driving the bulk flow of solution toward the sink.

Evidence for Mass Flow: Interpreting Tracer & Ringing Experiments

• Correlation: A statistical association between two variables: as one changes, the other tends to change in a predictable direction. A correlation does not, by itself, establish that one variable causes the other.

The Cardiac Cycle & Cardiovascular Disease

• Atrioventricular (AV) valves: Valves located between each atrium and its ventricle (tricuspid on the right, bicuspid/mitral on the left). They open when atrial pressure exceeds ventricular pressure and close when ventricular pressure rises above atrial pressure, preventing backflow.
• Correlation vs causation: A correlation is a statistical association between two variables — as one changes, the other tends to change. Causation means one variable directly produces a change in the other. A correlation does not automatically imply causation; a third (confounding) variable may explain the relationship.

Haemoglobin and Oxygen Transport

• Quaternary structure: The level of protein structure in which two or more polypeptide chains are held together by bonds (including hydrogen bonds, ionic bonds, and disulfide bridges) to form a single functional protein. Haemoglobin is a classic example, with four polypeptide subunits.

Capillaries and Tissue Fluid Formation

• Endothelium: A single layer of flattened epithelial cells that lines the inner surface of all blood vessels. In capillaries, this single layer forms the entire wall, minimising the diffusion distance for exchanged substances.

Genetic information, variation and relationships between organisms

36 cards from 19 lessons. Each term links to the lesson it comes from.

DNA, Genes, and the Genetic Code

• Gene: A base sequence of DNA that codes for the amino acid sequence of a polypeptide. Each gene occupies a fixed position, called a locus, on a particular DNA molecule.
• Triplet: A sequence of three consecutive DNA bases that codes for a specific amino acid. The triplet is the fundamental unit of the genetic code.

DNA Organisation: Prokaryotes vs. Eukaryotes

• Prokaryotic DNA: A short, circular DNA molecule found free in the cytoplasm of prokaryotic cells. It is not associated with histone proteins.
• Chromosome: A structure consisting of a single, very long, linear DNA molecule associated with histone proteins. Chromosomes are found in the nucleus of eukaryotic cells.
• Exon: A coding sequence within a eukaryotic gene that specifies part of the amino acid sequence of a polypeptide. Multiple exons within a gene are separated by introns.
• Intron: A non-coding sequence within a eukaryotic gene, located between exons. Introns are transcribed into the primary mRNA transcript but are removed by splicing before translation.

Functional RNA

• Ribosomal RNA (rRNA): An RNA molecule that forms an integral part of ribosome structure. rRNA provides the catalytic site (peptidyl transferase centre) where peptide bonds are formed between amino acids during translation. It is transcribed from DNA but never translated into protein.
• Anticodon: A sequence of three bases on the anticodon loop of a tRNA molecule. The anticodon is complementary and antiparallel to a specific mRNA codon, ensuring that the correct amino acid is delivered to the ribosome during translation.

Transcription: From DNA to mRNA

• Introns and exons: Introns are non-coding intervening sequences within a eukaryotic gene that are transcribed into pre-mRNA but removed during splicing. Exons are the coding sequences that are retained and joined together to form the mature mRNA.

Translation: mRNA to Polypeptides and the Genetic Code

• Codon: A sequence of three consecutive nucleotide bases on mRNA that specifies a particular amino acid or a stop signal during translation. For example, AUG is the start codon and also codes for methionine.
• Transfer RNA (tRNA): A small, clover-leaf-shaped RNA molecule that acts as an adaptor between the mRNA codon and the amino acid sequence. One end carries a specific amino acid; the other end carries a three-base anticodon complementary to the corresponding mRNA codon.

Genome, Proteome, mRNA, and tRNA Structures

• Genome: The complete set of genes in a cell. In humans, this means all the DNA found in the nucleus (across the chromosomes—46 in diploid somatic cells, 23 in gametes) and a small amount in mitochondria. Chromosome number varies between species and between cell types.
• Proteome: The full range of proteins that a cell is able to produce. Unlike the genome, the proteome is not fixed — it varies between different cell types and changes over time depending on the cell's conditions and developmental stage.

Meiosis & Genetic Variation

• Haploid (n) and Diploid (2n): A diploid cell contains two complete sets of chromosomes (2n). A haploid cell contains one set (n). In humans, diploid cells have 46 chromosomes (2n = 46); haploid gametes have 23 (n = 23).

Random Fertilisation & Chromosome Combinations

• Independent assortment (2^n rule): During meiosis I, each homologous pair aligns independently at the metaphase plate. With n pairs, there are 2^n possible combinations of maternal and paternal chromosomes in the resulting gametes. This calculation assumes no crossing over has occurred.

Gene & Chromosome Mutations

• Base substitution: A mutation in which one base (and its complementary base on the opposite strand) is replaced by a different base pair. For example, the sequence...CAT... might become...CGT... if adenine is replaced by guanine. Only the codon containing the substituted base is directly affected.
• Base deletion: A mutation in which a single base pair is removed from the DNA sequence. Because the genetic code is read in non-overlapping triplets, removing one base shifts the reading frame for every codon downstream of the deletion. This is called a frameshift mutation.

Genetic Diversity and Natural Selection

• Mutation (in this context): A random, heritable change in the base sequence of DNA that can produce a new allele of a gene. Mutations arise spontaneously during DNA replication or as a result of mutagens such as ionising radiation or certain chemicals.

Selection, Adaptation, and Evolution

• Natural selection: The process by which heritable variation in a population leads to differential reproductive success. Individuals whose phenotype is better matched to environmental conditions survive longer, reproduce more, and pass their alleles to a greater proportion of the next generation. Over time, allele frequencies in the population change.
• Stabilising selection: A mode of natural selection in which intermediate phenotypes have the highest fitness. Selection pressures act against both extremes of the phenotypic range, reducing variation and maintaining the population mean over time.
• Anatomical adaptation: A structural feature of an organism's body that increases its fitness. Anatomical adaptations are physical, observable characteristics — for example, the streamlined body shape of a dolphin reducing drag in water, or the large surface area of a leaf maximising light absorption.

Defining Species & Courtship Behaviour

• Biological Species Concept: Two organisms belong to the same species if they are able to interbreed and produce fertile offspring. Organisms that cannot produce fertile offspring together are classified as separate species.
• Courtship Behaviour: A set of innate behavioural patterns performed by one or both members of a potential mating pair before mating occurs. Courtship is a necessary precursor to successful mating and plays a critical role in species recognition.

Phylogenetic Classification: Organising Life

• Taxon: A named group at any level within a classification hierarchy. Each taxon is contained within a larger taxon and may contain smaller taxa within it. The term applies equally to a species, a genus, a family, or any other level of the hierarchy.
• Binomial nomenclature: The system by which each species is given a two-part name consisting of its genus name (first, with a capital letter) followed by its species name (second, lower case). The full name is written in italics when typed, or underlined when handwritten. For example, Homo sapiens identifies modern humans.

Modern Advances in Taxonomy

• Immunological comparison (precipitation test): A technique in which antibodies raised against the proteins of one species are mixed with proteins from a second species. The degree of precipitation produced indicates how similar the proteins are, and therefore how closely related the two species are evolutionarily. A stronger reaction indicates greater similarity and closer relatedness.
• Molecular clock: The principle that neutral mutations accumulate in DNA at an approximately constant rate over evolutionary time. By comparing the number of sequence differences between two species and applying a known mutation rate, scientists can estimate the time since those species shared a common ancestor.

Understanding Biodiversity and its Calculation

• Biodiversity: The variety of living organisms in a given area. Biodiversity can relate to a range of habitats, from a small local habitat to the entire Earth, and encompasses the number of different species as well as the relative abundance of individuals within those species.
• Species richness: A measure of the number of different species in a community. It gives no information about how many individuals of each species are present.

Farming's Impact on Biodiversity

• Monoculture: The cultivation of a single crop species across a large area of land. Because only one plant species is present, the range of insects, birds, and mammals that can feed, shelter, or breed in that habitat collapses dramatically.
• Eutrophication: The enrichment of a water body with excess nutrients, leading to rapid algal growth (algal bloom), reduced light penetration, death of submerged plants, bacterial decomposition of dead algae consuming dissolved oxygen, and the subsequent death of aerobic aquatic organisms.

Measuring Diversity: Observable Traits & DNA

• Frequency of measurable or observable characteristics: A quantitative method of investigating diversity in which the number of individuals displaying each value of a trait is recorded across a sample. The resulting distribution reflects the phenotypic variation within a population.
• Amino acid sequence comparison: A method of investigating evolutionary relationships in which the order of amino acids in a shared protein (such as cytochrome c or haemoglobin) is compared between species. A greater number of amino acid differences indicates greater evolutionary divergence.

Quantitative Sampling Techniques

• Random sampling: A method of data collection in which sample positions are determined by chance, so that every point in the study area has an equal probability of being selected. This eliminates sampling bias and allows valid statistical analysis.
• Mean: The arithmetic average of a data set, calculated by summing all values and dividing by the number of values. Represented by the symbol x.

Random Sampling Methods

• Random number generator: A tool — calculator function, app, or random number table — used to produce unpredictable coordinate pairs that determine sampling locations. Using one ensures the investigator has no influence over which areas are sampled.

Energy transfers in and between organisms (A-level only)

18 cards from 12 lessons. Each term links to the lesson it comes from.

Photosynthesis: Light-Dependent Reactions

• Photoionisation of chlorophyll: When chlorophyll absorbs light energy, electrons within the molecule gain sufficient energy to be ejected from the chlorophyll. This loss of electrons is photoionisation. The chlorophyll molecule is left in an oxidised, positively charged state.
• Reduced NADP (NADPH): Formed when NADP accepts two electrons and one proton at Photosystem I. It acts as a hydrogen carrier, transferring reducing power from the light-dependent reactions to the Calvin cycle.

Photosynthesis: Light-Independent Reactions (Calvin Cycle)

• Carbon fixation: The process by which inorganic carbon dioxide is incorporated into an organic molecule. In the Calvin cycle, rubisco fixes one molecule of CO₂ onto one molecule of RuBP, producing two molecules of GP.

Limiting Factors and Agricultural Practices

• Carbon dioxide compensation point: The CO2 concentration at which the rate of CO2 fixation in photosynthesis exactly equals the rate of CO2 release in respiration, resulting in no net change in CO2 concentration around the plant.

Glycolysis and the Link Reaction

• Glycolysis: The metabolic pathway in the cytoplasm in which one glucose molecule (6 carbons) is converted into two pyruvate molecules (3 carbons each), with a net yield of 2 ATP and 2 reduced NAD. It is the first stage of both aerobic and anaerobic respiration.
• Active transport of pyruvate: The ATP-dependent movement of pyruvate molecules from the cytoplasm across the inner mitochondrial membrane and into the mitochondrial matrix, against its concentration gradient, via specific carrier proteins.

The Krebs Cycle and Oxidative Phosphorylation

• Acetylcoenzyme A (acetyl-CoA): A molecule consisting of a two-carbon acetyl group attached to coenzyme A. It delivers the acetyl group to the Krebs cycle by condensing with the four-carbon molecule oxaloacetate, forming the six-carbon molecule citrate. Coenzyme A is released and can be reused.
• Chemiosmosis: The movement of protons (H⁺ ions) down their electrochemical gradient through the enzyme ATP synthase, which is embedded in the inner mitochondrial membrane. The flow of protons drives the rotation of ATP synthase, catalysing the synthesis of ATP from ADP and inorganic phosphate (Pi). This mechanism is the basis of the chemiosmotic theory proposed by Peter Mitchell.

Anaerobic Respiration and Required Practical

• Lactate fermentation (animals and some bacteria): Pyruvate accepts hydrogen from reduced NAD, catalysed by lactate dehydrogenase, converting pyruvate directly to lactate. This reoxidises NADH to NAD, which re-enters glycolysis. No carbon dioxide is released in this step.
• Alcoholic fermentation (yeast and some plant cells): Pyruvate is first decarboxylated to ethanal (acetaldehyde) by pyruvate decarboxylase, releasing carbon dioxide. Ethanal then accepts hydrogen from reduced NAD, catalysed by alcohol dehydrogenase, producing ethanol. This reoxidises NADH to NAD.

Energy Flow and Productivity in Ecosystems

• Biomass: The mass of biological material in organisms, expressed as mass of carbon or dry mass of tissue per given area. The chemical energy stored in dry biomass can be estimated using calorimetry, where a known mass of dried tissue is combusted and the heat released is measured.
• Primary Productivity: The rate at which plants (producers) synthesise and store chemical energy as biomass per unit area per unit time. Measured in units such as kJ ha⁻¹ year⁻¹ or g dry mass m⁻² year⁻¹.

Calculating Net Production and Efficiency

• Net production (N): The chemical energy remaining in an organism's biomass after faecal, urinary, and respiratory losses have been accounted for. It represents the energy available for growth, reproduction, and storage — and therefore the energy potentially available to the next trophic level. Measured in kJ m⁻² yr⁻¹ (or equivalent units).

Improving Energy Transfer in Agriculture

• Net productivity: The energy stored in biomass per unit area per unit time after respiratory losses have been subtracted. Net productivity = Gross productivity − Respiratory losses. Measured in kJ m⁻² yr⁻¹.

Decomposition and the Nitrogen Cycle

• Saprobiotic nutrition: A mode of heterotrophic nutrition in which an organism secretes enzymes onto dead or decaying organic matter, digests it externally, and absorbs the soluble products. Carried out by saprobionts such as decomposer bacteria and fungi.

Mycorrhizae, Phosphorus, and Recycling

• Mycorrhizae: Mutualistic symbioses between fungi and plant roots. Fungal hyphae extend far beyond the root surface into surrounding soil, massively increasing the volume of soil from which the plant can absorb water and inorganic ions such as phosphate, nitrate, and potassium.
• Mineralisation: The process by which decomposer microorganisms break down organic compounds in dead organisms and waste, converting organically bound elements such as phosphorus and nitrogen into inorganic ions (e.g. phosphate, ammonium) that can be absorbed by producers.

Fertilisers and Eutrophication

• Leaching: The washing of soluble minerals — particularly nitrates and phosphates — out of the soil by percolating rainwater, transporting them into groundwater or surface waterways.

Organisms respond to changes in their internal and external environments (A-level only)

37 cards from 21 lessons. Each term links to the lesson it comes from.

Survival and Plant Growth Responses

• Indoleacetic acid (IAA): IAA is the primary growth factor (auxin) in flowering plants. It is synthesised mainly in the shoot tip (the apical meristem) and moves by diffusion and active transport to cells below, where it promotes cell elongation by loosening cell walls.

Simple Responses and Reflexes

• Taxis: A directional movement response of a whole organism in which the direction of movement is determined by the direction of the stimulus. Movement towards the stimulus is described as positive; movement away is negative.
• Kinesis: A non-directional movement response in which the rate of movement (orthokinesis) or the rate of turning (klinokinesis) changes in proportion to the intensity of a stimulus. The direction of movement is random, not oriented towards or away from the stimulus.

Myogenic Heart Stimulation

• Myogenic stimulation: The ability of cardiac muscle to initiate its own electrical excitation without requiring a nerve impulse. The impulse originates within the muscle tissue itself, making the heartbeat intrinsic and self-sustaining.

Chemo- and Pressure Receptors in Heart Rate Control

• Chemoreceptor: A receptor cell or organ that detects changes in the chemical environment — specifically CO₂ concentration, O₂ levels, and pH in the context of heart rate control. When stimulated, it sends nerve impulses to the cardiovascular centre in the medulla oblongata.
• Baroreceptor (pressure receptor): A mechanoreceptor located in the walls of the carotid sinus and aortic arch that detects the degree of stretch caused by blood pressure. Higher pressure stretches the arterial wall more, increasing the rate of nerve impulse firing to the cardiovascular centre.

Receptor Specificity and Potentials

• Adequate stimulus: The specific type of energy or change in the environment to which a given receptor is structurally adapted to respond. Applying any other form of stimulus to that receptor will not produce a normal response.
• Habituation (of touch receptors): The progressive reduction in action potential frequency fired by a touch receptor in response to a constant, sustained stimulus. It allows the nervous system to filter out unchanging background stimuli and respond preferentially to new or changing inputs.

Pacinian Corpuscle Structure and Function

• Lamellae: The concentric layers of connective tissue and fluid that form the outer capsule of a Pacinian corpuscle. They transmit and focus mechanical pressure onto the central sensory neurone ending.
• Generator potential: A graded, local depolarisation of the sensory receptor membrane caused by the opening of ion channels in response to a stimulus. It is not an action potential itself, but if it reaches the threshold level it will trigger an action potential in the sensory neurone.

Sensitivity to light, sensitivity to colour and visual acuity

• Rhodopsin: The light-sensitive pigment in rod cells. It consists of a protein (opsin) bound to a light-absorbing molecule called retinal. When even a small amount of light strikes rhodopsin, it breaks down (bleaches), triggering a generator potential and ultimately an action potential in the optic nerve.
• Convergence (retinal): The arrangement by which multiple photoreceptor cells synapse onto a single bipolar cell. High convergence (many rods to one bipolar cell) increases sensitivity through summation but reduces visual acuity. Low convergence (one cone to one bipolar cell) preserves spatial resolution, giving high acuity.

Neurone Structure & Resting Potential

• Myelin sheath: A lipid-rich insulating layer wrapped around the axon, formed by Schwann cells. Each Schwann cell wraps around a section of the axon, leaving small gaps called nodes of Ranvier between adjacent Schwann cells. The sheath prevents ion movement across the membrane except at the nodes, forcing the impulse to jump between nodes — a process called saltatory conduction — which dramatically increases conduction speed.
• Differential membrane permeability: The property of the plasma membrane whereby it allows different ions to cross at different rates, due to the presence of specific ion channels. At rest, the membrane is significantly more permeable to potassium ions than to sodium ions, because potassium leak channels are open while most sodium channels are closed.

Action Potential Generation & Propagation

• Threshold potential: The critical membrane potential (approximately −55 mV) that must be reached for voltage-gated Na⁺ channels to open and trigger a full action potential. Stimuli that do not reach this value produce no action potential.
• Saltatory conduction: The propagation of an action potential along a myelinated axon by jumping between successive nodes of Ranvier, rather than depolarising every point along the membrane. From the Latin 'saltare', meaning to jump.

Refractory Period & Conduction Speed

• Absolute refractory period: The phase immediately following an action potential during which no stimulus, however large, can trigger a new action potential. Voltage-gated sodium channels are inactivated and cannot open.
• Relative refractory period: The phase following the absolute refractory period during which a new action potential can be generated, but only if the stimulus is larger than the normal threshold. Some Na⁺ channels have recovered, but K⁺ channels are still open, keeping the membrane hyperpolarised.
• Nodes of Ranvier: Small, unmyelinated gaps in the myelin sheath of a myelinated axon, spaced approximately 1–2 mm apart, where voltage-gated ion channels are concentrated and action potentials are generated.

Synaptic Structure and Function

• Synaptic cleft: The narrow fluid-filled gap (~20 nm) between the presynaptic and postsynaptic membranes at a synapse. Neurotransmitter molecules diffuse across it to reach postsynaptic receptors.

Synaptic Properties and Drug Effects

• Unidirectionality: The property ensuring nerve impulses cross a synapse in one direction only — from presynaptic to postsynaptic neurone — because neurotransmitter vesicles are located exclusively on the presynaptic membrane and receptor proteins are located exclusively on the postsynaptic membrane.
• Temporal summation: The addition of excitatory postsynaptic potentials (EPSPs) generated at the same synapse in rapid succession. If action potentials arrive at the presynaptic terminal at a high frequency, each EPSP begins before the previous one has fully decayed, so the depolarisations accumulate and may reach threshold.
• Spatial summation: The addition of EPSPs generated simultaneously at different synapses on the same postsynaptic neurone. Multiple presynaptic neurones each release neurotransmitter at the same moment; the resulting depolarisations from different locations on the postsynaptic membrane combine to reach threshold.

Muscle Structure and Antagonistic Action

• Antagonistic pair: Two muscles arranged so that contraction of one (the agonist) produces a movement, while the other (the antagonist) relaxes to allow it. The roles of agonist and antagonist reverse for the opposite movement. They act against an incompressible skeleton to generate controlled joint movement.
• Sarcomere: The repeating contractile unit of a myofibril, running from one Z-line to the next. It contains overlapping actin and myosin filaments arranged to produce the characteristic banding pattern of striated muscle.

Energy Sources and Muscle Fibre Types

• Phosphocreatine (PCr): A phosphorylated compound stored in the sarcoplasm of muscle fibres. It acts as an immediate phosphate donor, regenerating ATP from ADP via the enzyme creatine kinase. The reaction is: PCr + ADP → creatine + ATP. This system requires no oxygen and produces no lactate.

Insulin, Glucagon and Blood Glucose Regulation

• Negative feedback: A homeostatic mechanism in which a change away from the set point triggers a response that opposes the change and returns the system towards the set point. In blood glucose regulation, high glucose triggers insulin release; low glucose triggers glucagon release.
• Glycogenesis: The synthesis of glycogen from glucose, catalysed by glycogen synthase. Occurs in hepatocytes and muscle cells. Activated by insulin in response to high blood glucose concentration.
• Gluconeogenesis: The synthesis of glucose from non-carbohydrate precursors, including glycerol and amino acids. Occurs in hepatocytes. Activated by glucagon when blood glucose is low and glycogen stores are insufficient.

Adrenaline, Second Messengers, and Factors Influencing Blood Glucose

• Glycogenolysis: The hydrolysis of glycogen into glucose-1-phosphate (and subsequently glucose), catalysed by phosphorylase enzymes in the liver and muscle. Adrenaline and glucagon both stimulate this process, raising blood glucose concentration.

Diabetes: Types, Control, and Glucose Concentration Measurement

• Beta cells: Cells located in the islets of Langerhans within the pancreas that synthesise and secrete insulin in response to rising blood glucose concentration. In type I diabetes, these cells are destroyed by the immune system.

Homeostasis: Maintaining a Stable Internal Environment

• Homeostasis: The maintenance of the internal environment of an organism within restricted limits, using physiological control systems. In mammals, this includes regulating core body temperature, blood pH, and blood glucose concentration.
• Enzyme denaturation (pH-induced): A permanent change in the tertiary structure of an enzyme caused by altered hydrogen ion concentration, which disrupts ionic bonds and hydrogen bonds in the active site, preventing substrate binding.

Negative Feedback: Restoring Balance

• Positive feedback: A control mechanism in which a deviation from the set point triggers a response that amplifies the original change, moving the variable further away from the set point rather than restoring it.

Filtration & Reabsorption in the Nephron

• Ultrafiltration: The pressure-driven filtration of blood in the glomerulus. High hydrostatic pressure forces water, glucose, urea, ions, and amino acids through the capillary walls, basement membrane, and podocyte foot processes into the Bowman's capsule. Large proteins and blood cells are too large to pass through and remain in the blood.

Fine-Tuning Water Balance: Distal Tubule & ADH

• Osmoregulation: The homeostatic control of the water potential of the blood. It ensures that cells neither shrink (in a solution of low water potential) nor burst (in a solution of high water potential), keeping the internal environment stable.
• Antidiuretic hormone (ADH): A peptide hormone produced by neurosecretory cells in the hypothalamus and released from the posterior pituitary gland. It acts on the distal convoluted tubule and collecting duct to increase their permeability to water, promoting water reabsorption by osmosis.

Genetics, populations, evolution and ecosystems (A-level only)

21 cards from 9 lessons. Each term links to the lesson it comes from.

Genotypes, Phenotypes, and Alleles

• Genotype: The genetic constitution of an organism — specifically, the combination of alleles present at one or more loci. For a single gene, this is written as two allele symbols (e.g. Bb), reflecting the diploid nature of most organisms.
• Phenotype: The observable characteristics of an organism, produced by the expression of its genotype in interaction with environmental factors. Phenotype includes morphological, physiological, and behavioural traits.
• Homozygous: A diploid organism is homozygous at a locus when both alleles at that locus are identical — for example, BB (homozygous dominant) or bb (homozygous recessive).

Monohybrid and Dihybrid Crosses

• Homozygous and heterozygous: Homozygous describes an organism carrying two identical alleles for a gene (e.g. AA or aa). Heterozygous describes an organism carrying two different alleles (e.g. Aa).

Advanced Inheritance Patterns

• Sex-linked gene: A gene located on a sex chromosome. In AQA Biology, sex-linked genes are almost always X-linked. Males (XY) carry only one copy of any X-linked gene and are therefore hemizygous — they express whichever allele is present, regardless of dominance.
• Autosomal linkage: Two genes located on the same autosome are said to be linked. Linked alleles that entered the organism together on the same chromosome are in coupling (cis configuration). During meiosis, crossing over between homologous chromosomes can separate linked alleles, producing recombinant gametes at a frequency that reflects the distance between the loci.

Understanding Populations and Gene Pools

• Population: A group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed. All four criteria — same species, shared space, shared time, and potential to interbreed — must be met.
• Gene pool: The complete set of alleles present in all individuals of a population at a given time. It represents the total genetic variation available within that population.
• Allele frequency: The proportion of all alleles at a given gene locus in a population that are a particular allele. It is expressed as a decimal between 0 and 1, or as a percentage.

Variation and Natural Selection

• Phenotypic variation: Differences in the observable characteristics of individuals within a population, resulting from genetic differences, environmental influences, or an interaction of both.

Genetic Drift and Speciation

• Genetic drift: A random change in allele frequency within a population caused by chance events in survival and reproduction, independent of natural selection. Its effect is greatest in small populations.
• Species (biological species concept): A group of organisms that share a common gene pool and are capable of interbreeding to produce fertile offspring. Two populations are considered separate species when reproductive isolation prevents the production of fertile offspring between them.

Ecosystems, Niches and Population Dynamics

• Ecosystem: A community of organisms (the biotic component) together with the non-living (abiotic) components of the environment in which they live. Ecosystems can range in size from the very small (a single rotting log) to the very large (the Amazon basin).
• Carrying capacity: The maximum population size of a given species that an ecosystem can sustainably support over time, determined by the availability of resources and other limiting factors.

Abiotic Factors, Interspecific Interactions and Quadrat Sampling

• Abiotic factor: A non-living physical or chemical variable in an environment that affects the survival, distribution, and abundance of organisms. Examples include light intensity, temperature, soil pH, water availability, and mineral ion concentration.
• Intraspecific competition: Competition for the same limited resources between individuals of the same species. As population density increases, intraspecific competition intensifies, acting as a density-dependent factor that limits further population growth.
• Interspecific competition: Competition for limited resources between individuals of different species. The stronger competitor gains more resources, grows faster, and reproduces more successfully, often reducing the population size of the weaker competitor or excluding it from the habitat entirely.
• Frame quadrat: A square frame of known area (commonly 0.25 m² or 1 m²) placed in a habitat to define a sampling unit. The organisms within the frame are counted or their cover estimated. Used for slow-moving or non-motile organisms.
• Belt transect: A continuous strip of known width (typically 0.5 m or 1 m) laid across a habitat along a line transect. Quadrats are placed at regular intervals along the strip, allowing changes in species distribution across an environmental gradient to be recorded systematically.

Mark-Release-Recapture, Succession and Conservation

• Lincoln-Petersen Index: The formula used to estimate population size from mark-release-recapture data: N = (n1 × n2) ÷ m2, where n1 is the number marked and released, n2 is the total number in the second sample, and m2 is the number of marked individuals in the second sample.
• Pioneer species: The first organisms to colonise a bare, inhospitable substrate during primary succession. They tolerate extreme abiotic conditions and begin the process of soil formation, making the environment progressively less hostile for subsequent species.

The control of gene expression (A-level only)

28 cards from 16 lessons. Each term links to the lesson it comes from.

Types and Effects of Gene Mutations

• Degenerate genetic code: The genetic code is described as degenerate because most amino acids are specified by more than one codon. For example, leucine is encoded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This redundancy means that some base changes do not alter the amino acid inserted into the polypeptide chain.
• Frameshift mutation: A mutation caused by the insertion or deletion of a number of bases that is not a multiple of three. The reading frame of all codons downstream from the mutation site is shifted, so a completely different sequence of amino acids is encoded from that point onwards. The resulting polypeptide is almost always non-functional.

Causes and Consequences of Mutations

• Substitution: One base is replaced by a different base. For example, ATG-CGT-AAC becomes ATG-CAT-AAC (G replaced by A in the second codon). Only the codon containing the substituted base is directly affected.
• Addition (insertion): One or more extra bases are inserted into the DNA sequence. This shifts the reading frame for every codon downstream of the insertion point, altering all subsequent amino acids.
• Deletion: One or more bases are removed from the DNA sequence. Like addition, this causes a frameshift affecting all codons downstream of the deletion point.
• Inversion: A sequence of bases is removed from its original position, flipped so that the order of the bases within the segment is reversed, and reinserted at the same location. The bases in the inverted segment now run in the opposite orientation (for example, 5'→3' becomes 3'→5'), changing which codons are read from this region.
• Duplication: A sequence of one or more bases is copied and the copy is inserted adjacent to the original sequence. This increases the total number of bases in the gene.
• Translocation: A sequence of bases is moved from one position in the genome to a different position, either within the same chromosome or to a different chromosome. This can disrupt the reading frame at both the original and new locations.

Stem Cell Potency: Totipotent and Pluripotent Cells

• Totipotent cell: A cell that can divide by mitosis and differentiate into any type of body cell, including extraembryonic tissues such as the placenta. In mammals, totipotency is restricted to the zygote and the cells of the very early embryo (up to approximately the 16-cell morula stage).
• Pluripotent cell: A cell that can divide in unlimited numbers by mitosis and can differentiate into almost any body cell type, but cannot form extraembryonic tissues. Embryonic stem cells derived from the inner cell mass of the blastocyst are pluripotent.

Multipotent and Unipotent Cells: Specialization and iPS Cells

• Cardiomyocyte: A specialised cardiac muscle cell produced by the differentiation of unipotent cardiac progenitor cells. Cardiomyocytes express a specific set of contractile proteins and are adapted for continuous, rhythmic contraction.

Oncogenes and Tumour Suppressor Genes

• Proto-oncogene: A normal gene encoding proteins that promote cell growth and division. It operates under tight regulatory control and is expressed only when cell proliferation is required.
• Tumour suppressor gene: A gene encoding proteins that inhibit cell division, promote apoptosis, or facilitate DNA repair. Loss of function of both alleles removes the normal restraint on cell proliferation.

Oestrogen and Tumour Types

• Transcription factor: A protein that binds to a specific DNA sequence in a gene's promoter region and either stimulates or inhibits transcription of that gene by RNA polymerase.

Genetic/Environmental Factors & Cancer Treatment

• Targeted therapy: A treatment that exploits a specific molecular alteration in cancer cells (such as an oncogene product) to selectively inhibit tumour growth while minimising damage to normal cells.

Epigenetic Control: Methylation, Acetylation & Heritability

• Epigenetics: Heritable changes in gene function that do not involve any alteration to the base sequence of DNA. These changes are caused by chemical modifications to DNA or to the histone proteins around which DNA is wound, and they can be passed on when cells divide.

RNA Interference and Epigenetics in Disease

• RNA-induced silencing complex (RISC): A multi-protein complex that incorporates a single-stranded guide RNA (siRNA or miRNA) and uses it to locate complementary mRNA sequences, leading to mRNA degradation or translational repression.

Creating DNA Fragments

• Reverse transcriptase: An enzyme that catalyses the synthesis of a single-stranded DNA strand from an mRNA template, using free DNA nucleotides and following complementary base pairing rules.
• Palindromic recognition sequence: A short DNA sequence (typically 4–8 base pairs) that reads the same on both strands in the 5' to 3' direction. Restriction enzymes bind and cut at these sequences.

Assembling and Amplifying DNA

• Promoter region: A DNA sequence upstream (5') of the coding sequence to which RNA polymerase binds, initiating transcription. Without a promoter, the gene of interest will not be transcribed in the host cell.
• Sticky ends: Short, single-stranded overhangs produced when a restriction endonuclease cuts DNA at a staggered position within its recognition sequence. Complementary sticky ends on the fragment and vector allow them to anneal by hydrogen bonding before ligation.

Applications and Implications

• Recombinant DNA technology: The transfer of DNA fragments from one organism or species to another. Because the genetic code, transcription, and translation mechanisms are universal, the transferred DNA can be expressed within cells of the recipient (transgenic) organism.
• Gene therapy: The use of recombinant DNA technology to introduce functional copies of a gene into a patient's cells, aiming to treat or prevent disease caused by a non-functional or absent gene.

Understanding Genetic Fingerprinting (Part 1/3)

• Variable Number Tandem Repeats (VNTRs): Short, non-coding DNA sequences (typically 10–100 base pairs long) that are repeated consecutively at specific loci on chromosomes. The number of repeat units at each locus varies between individuals, making the overall pattern highly individual-specific.
• Polymerase Chain Reaction (PCR): An in vitro technique that uses repeated cycles of heating and cooling to denature DNA, anneal specific primers, and extend new strands using a heat-stable DNA polymerase (typically Taq polymerase). Each cycle doubles the number of target DNA copies, producing millions of copies from a minimal starting sample.

Understanding Genetic Fingerprinting (Part 2/3)

• Locus (plural: loci): The specific, fixed position of a gene or DNA sequence on a chromosome. In genetic fingerprinting, each STR locus is analysed separately; the combined profile across multiple loci gives the fingerprint its discriminating power.

Interpreting Gel Electrophoresis Results (Part 1/3)

• Gel electrophoresis: A technique that separates charged molecules — including DNA fragments — by size. DNA is loaded into wells at the negative end of an agarose gel; an electric current drives fragments towards the positive electrode. Smaller fragments migrate further in a given time, producing a size-dependent banding pattern.

Interpreting Gel Electrophoresis Results (Part 2/3)

• DNA ladder (marker): A reference lane in gel electrophoresis containing DNA fragments of known sizes. It allows scientists to estimate the size of unknown fragments by comparing band positions. Always loaded into the first or last lane of the gel.