Each new DNA molecule has one original strand and one new strand.
Helicase
Unwinds the double helix and breaks hydrogen bonds between bases.
DNA polymerase
Adds nucleotides complementary to the template strand.
PCR
Polymerase chain reaction: copies a DNA sequence many times.
Okazaki fragments (HL)
Short sections of DNA made on the lagging strand.
Replication makes exact copies for growth, repair and reproduction.
The full notes, worked example and check questions for this subtopic are in the app.
D1.2Protein synthesis
Key terms
Transcription
Making mRNA from a DNA template using RNA polymerase.
Translation
Making a polypeptide from mRNA at a ribosome.
Codon
Three mRNA bases coding for one amino acid.
Anticodon
Three tRNA bases complementary to a codon.
Degenerate code
Most amino acids have more than one codon.
Transcription: RNA polymerase separates DNA strands and builds mRNA complementary to the template strand. DNA isn't used up, so it stays stable as a permanent template.
The full notes, worked example and check questions for this subtopic are in the app.
D1.3Mutations and gene editing
Key terms
Gene mutation
A change in the base sequence of a gene.
Substitution
One base replaced by another; may change one amino acid.
Frameshift
An insertion or deletion that shifts the reading frame.
Mutagen
Agent that increases mutation rate, e.g. UV, X-rays, some chemicals.
CRISPR-Cas9 (HL)
Gene-editing tool that cuts DNA at a chosen sequence.
Types: substitution, insertion, deletion. Insertions and deletions usually cause frameshifts with big effects.
The full notes, worked example and check questions for this subtopic are in the app.
D2.1Cell and nuclear division
Key terms
Mitosis
Nuclear division producing two genetically identical nuclei.
Meiosis
Two divisions producing four haploid, genetically different nuclei.
Cytokinesis
Division of the cytoplasm.
Non-disjunction
Chromosomes fail to separate in meiosis, e.g. causing Down syndrome.
Mitotic index (HL)
Cells in mitosis ÷ total cells observed.
Mitosis phases: prophase (chromosomes condense, spindle forms, nuclear membrane breaks down), metaphase (chromosomes line up on the equator), anaphase (sister chromatids pulled to opposite poles), telophase (nuclear membranes reform).
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
D2.2Gene expression HL
Key terms
Gene expression
Using a gene's information to make a product, linking genotype to phenotype.
Transcription factor
Protein that binds DNA to switch transcription on or off.
Epigenetics
Heritable changes in gene expression without changes to the base sequence.
Methylation
Adding methyl groups to DNA or histones, usually silencing genes.
Transcriptome / proteome
All the mRNA / all the proteins made by a cell at a given time.
Expression is regulated at transcription (promoters, transcription factors), translation and by mRNA degradation.
The full notes, worked example and check questions for this subtopic are in the app.
D2.3Water potential
Key terms
Water potential (ψ)
Potential energy of water per unit volume, in kPa; water moves from higher to lower ψ.
Solute potential (ψs)
Always negative; more solutes = more negative.
Pressure potential (ψp)
Pressure on water, e.g. turgor pressure from a cell wall.
Plasmolysis
Membrane pulls away from the cell wall when a plant cell loses water.
Isotonic
Same water potential as the cell, so no net movement.
Water forms hydration shells around ions and polar molecules (solvation).
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
D3.1Reproduction
Key terms
Gamete
Haploid sex cell (sperm, egg, pollen nucleus, ovule).
FSH and LH
Pituitary hormones controlling follicle development and ovulation.
Oestradiol and progesterone
Ovarian hormones that build and maintain the uterus lining.
IVF
In vitro fertilisation: eggs fertilised outside the body.
Pollination
Transfer of pollen from anther to stigma.
Sexual reproduction needs meiosis and fusion of gametes, producing variation; asexual reproduction gives clones.
The full notes, worked example and check questions for this subtopic are in the app.
D3.2Inheritance
Key terms
Genotype / phenotype
Alleles an organism has / its observable traits.
Homozygous / heterozygous
Two identical alleles / two different alleles.
Codominance
Both alleles are expressed in the heterozygote, e.g. blood group AB.
Sex-linked gene
Gene on the X chromosome, e.g. haemophilia, red-green colour blindness.
Polygenic trait
Controlled by many genes, giving continuous variation, e.g. height.
Gametes are haploid; fertilisation restores diploid zygotes.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
D3.3Homeostasis
Key terms
Homeostasis
Keeping the internal environment within narrow limits.
Negative feedback
A change triggers responses that reverse it.
Insulin / glucagon
Pancreatic hormones that lower / raise blood glucose.
ADH (HL)
Antidiuretic hormone: increases water reabsorption in collecting ducts.
Ultrafiltration (HL)
Filtering blood under pressure in the glomerulus.
Variables kept stable: blood glucose, body temperature, blood pH, water and CO₂ concentration.
The full notes, worked example and check questions for this subtopic are in the app.
D4.1Natural selection
Key terms
Natural selection
Individuals with better-adapted heritable traits survive and reproduce more.
Fitness
Ability to survive and reproduce in an environment.
Sexual selection
Selection for traits that increase mating success.
Gene pool (HL)
All the alleles in a population.
Hardy–Weinberg (HL)
Model of allele frequencies in a population that is not evolving.
Sources of variation: mutation, meiosis and fertilisation.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
D4.2Stability and change
Key terms
Keystone species
A species with a disproportionately large effect on its ecosystem, e.g. sea otters.
Tipping point
A threshold after which an ecosystem shifts to a different state.
Eutrophication
Excess nutrients cause algal blooms, oxygen depletion and death of aquatic life.
Biomagnification
Increase in concentration of a toxin up a food chain, e.g. DDT, mercury.
Succession (HL)
Gradual change in a community over time.
Natural ecosystems can stay stable for long periods if they have an energy supply, nutrient recycling, genetic diversity and climate within tolerance limits.
The full notes, worked example and check questions for this subtopic are in the app.
D4.3Climate change
Key terms
Greenhouse gas
Gas that absorbs and re-emits infrared radiation, e.g. CO₂, methane.
Positive feedback
A change that triggers more of the same change, e.g. melting ice lowering albedo.
Range shift
Species moving towards the poles or uphill as temperatures rise.
Ocean acidification
Lower ocean pH as CO₂ dissolves, harming organisms with calcium carbonate shells.
Phenology (HL)
Timing of seasonal biological events, e.g. flowering, migration.
Human emissions of CO₂ (fossil fuels, deforestation) and methane (livestock, rice paddies, landfill) are the main cause of current warming.
The full notes, worked example and check questions for this subtopic are in the app.