D Continuity and Change

• Produces exact DNA copies before cell division • Maintains genetic continuity for reproduction, growth, and tissue replacement

• Each new DNA molecule has one original strand and one new strand • Complementary base pairing gives accurate copying; Meselson-Stahl isotope evidence supports the model

• Helicase unwinds DNA and breaks hydrogen bonds between strands • DNA polymerase joins complementary nucleotides to build new strands

• PCR amplifies selected DNA using primers, temperature cycles, and Taq polymerase • Gel electrophoresis separates DNA fragments by size and charge

• PCR and gel electrophoresis support DNA profiling • Applications include forensic identification and paternity testing

• Transcription makes mRNA as a mobile copy of gene information • RNA polymerase synthesizes RNA complementary to the DNA template strand

• Free RNA nucleotides align by complementary base pairing and hydrogen bonding • DNA adenine pairs with RNA uracil, while cytosine pairs with guanine

• DNA template strands are transcribed without altering the base sequence • Sugar-phosphate backbone and base pairing preserve genetic information

• Transcription is the first stage of gene expression • Cells regulate which genes are transcribed according to tissue, stage, and signals

• Translation decodes mRNA at ribosomes to synthesize polypeptides • mRNA codon order determines amino acid sequence

• mRNA provides codons; tRNA carries activated amino acids with anticodons • Ribosomes hold mRNA and tRNAs so peptide bonds can form

• tRNA anticodons pair with complementary mRNA codons by hydrogen bonding • Specific tRNA-amino acid attachment helps ensure correct amino acid addition

• The genetic code is triplet, degenerate, and almost universal • Codons specify amino acids, a start signal, or stop signals

• Genetic code tables use mRNA codons, not DNA triplets • Convert template DNA to mRNA first, then read codons 5' to 3'

• Ribosomes move along mRNA one codon at a time from start to stop • Peptide bonds join amino acids; multiple ribosomes can form a polysome

• Mutations can change codons and therefore amino acid sequence • Changed primary structure may alter folding and function, such as sickle-cell haemoglobin

• Gene mutations are changes in the base sequence of DNA • Main types are substitution, insertion, deletion, and duplication

• Base substitutions can create SNPs and change codons • Degeneracy can make substitutions silent, missense, or nonsense

• Insertions or deletions not in multiples of three cause frameshifts • Frameshifts alter downstream codons and often disrupt protein function

• Mutations can arise from replication errors, repair errors, or chromosome damage • Mutagens include chemicals, ionizing radiation, and ultraviolet radiation

• Mutations occur randomly with respect to organism need or advantage • Mutation rate varies with DNA sequence, gene expression, repair, and mutagen exposure

• Germ-line mutations can be inherited by offspring • Somatic mutations affect only descendant body cells and can contribute to cancer

• Mutation is the original source of new alleles and genetic variation • Many mutations are neutral or harmful, but variation supplies material for natural selection

• All cells arise from pre-existing parent cells by cell division • Division produces daughter cells for growth, replacement, repair, or reproduction

• Cytokinesis splits cytoplasm after nuclear division • Animal cells use a contractile ring; plant cells form a vesicle-derived cell plate

• Equal cytokinesis gives daughter cells similar amounts of cytoplasm • Unequal cytokinesis occurs in oogenesis and yeast budding

• Mitosis maintains chromosome number for growth, repair, and asexual reproduction • Meiosis halves chromosome number for gametes and generates genetic diversity

• DNA replication in interphase produces chromosomes with sister chromatids • Sister chromatids remain joined at centromeres until separation

• Mitosis and meiosis both condense chromatin into movable chromosomes • Histones, nucleosomes, spindle microtubules, and motor proteins organize movement

• Prophase condenses chromosomes; metaphase aligns them at the equator • Anaphase separates chromatids; telophase reforms nuclei, producing identical nuclei

• Mitosis phases are identified in diagrams, micrographs, and root-tip squashes • Chromosome condensation, equator alignment, separation, and nuclear membranes are cues

• Meiosis has two nuclear divisions after one round of DNA replication • Homologous chromosomes separate in meiosis I, producing haploid nuclei

• Non-disjunction is failed separation of homologues or sister chromatids in meiosis • Down syndrome usually results from trisomy 21 after non-disjunction

• Crossing over at chiasmata exchanges DNA between non-sister chromatids • Random orientation of bivalents and fertilization create new allele combinations

D2.3.1Solvation with water

• Water forms hydration shells around ions and polar solutes • Hydrogen bonding and charge attraction reduce free water movement

D2.3.2Water movement

• Water moves by osmosis across partially permeable membranes • It moves from hypotonic/lower solute solutions toward hypertonic/higher solute solutions

D2.3.3Osmosis into/out of cells

• Osmosis direction depends on internal and external solute concentration • Isotonic conditions have dynamic water movement but no net osmosis

D2.3.4Changes in plant tissue

• Plant tissue changes mass or length when placed in sucrose solutions • Percentage change graphs estimate isotonic/osmotic concentration

D2.3.5Effects on cells without wall

• Animal cells can lyse in hypotonic solutions and crenate in hypertonic solutions • Freshwater protists use contractile vacuoles to expel excess water

D2.3.6Effects on cells with wall

• Plant cells become turgid in hypotonic solutions as vacuoles swell • Hypertonic solutions cause flaccidity and plasmolysis from water loss

D2.3.7Medical applications

• Isotonic saline prevents harmful water gain or loss in body cells • IV fluids and transplant organ baths must match tissue osmotic concentration

• Asexual reproduction uses mitosis without gametes or fertilization, producing clones • Sexual reproduction uses meiosis and fertilization to generate genetic variation

• Meiosis produces haploid gametes and prevents chromosome doubling each generation • Random fertilization fuses gametes to form unique diploid zygotes

• Male gametes are small, numerous, and usually motile • Female gametes are larger, fewer, and contain resources for early development

• Male structures include testes, epididymis, sperm duct, glands, urethra, and penis • Female structures include ovaries, oviducts, uterus, endometrium, cervix, vagina, and vulva

• FSH, LH, oestradiol, and progesterone coordinate ovarian and uterine cycles • Follicle growth, ovulation, corpus luteum, endometrium build-up, and menstruation are linked

• Fertilization occurs in the oviduct after sperm reaches the egg • Sperm and egg nuclei fuse so paternal and maternal chromosomes form the zygote genome

• IVF uses hormones to stimulate superovulation and control egg maturation • Eggs are collected, fertilized outside the body, and embryos transferred to the uterus

• Flowering plants produce male gametes in pollen and female gametes in ovules • Pollination, pollen-tube growth, and fertilization produce embryos inside seeds

• Insect-pollinated flowers often have petals, scent, nectar, sticky pollen, and sticky stigma • Floral structures position pollinators to transfer pollen from anther to stigma

• Cross-pollination increases variation by transferring pollen between different plants • Mechanisms include dioecy, self-incompatibility, and different maturation times

• Self-incompatibility prevents pollen from fertilizing ovules of the same plant • Recognition systems block self-pollen growth and reduce inbreeding

• Seed dispersal reduces competition with parent plants and spreads offspring • Germination uses water uptake, enzyme activation, and food reserve mobilization

• Haploid gametes fuse during fertilization to form a diploid zygote • Diploid organisms usually carry two alleles for each autosomal gene

• Genetic crosses track parental, F1, and F2 generations using Punnett grids • Flowering plant crosses control pollen transfer to study inheritance ratios

• Genotype is the allele combination inherited for a gene or genes • Homozygous genotypes have matching alleles; heterozygous genotypes have different alleles

• Phenotype is the observable characteristic or trait • Phenotype can be determined by genotype, environment, and their interaction

• Dominant alleles are expressed in heterozygotes • Recessive alleles are expressed when no dominant allele masks them

• Phenotypic plasticity is environment-driven phenotype change without genotype change • It depends on altered gene expression and can be adaptive

• PKU is an autosomal recessive disorder affecting phenylalanine metabolism • Low-phenylalanine diet and newborn screening reduce harmful effects

• SNPs are single-base differences that can create different alleles • Populations can have multiple alleles, but diploid individuals carry at most two

• ABO blood group is controlled by IA, IB, and i alleles • IA and IB are codominant; i is recessive, producing four blood phenotypes

• Codominance expresses both heterozygous alleles, as in AB blood type • Incomplete dominance gives an intermediate heterozygote phenotype

• Human chromosomal sex is usually determined by XX or XY chromosome combination • The SRY/TDF region on the Y chromosome directs testis development

• Haemophilia is an X-linked recessive blood-clotting disorder • Carrier females and affected males are represented with X-linked allele notation

• Pedigree charts show family inheritance across generations • Patterns help infer autosomal dominant, autosomal recessive, or sex-linked inheritance

• Continuous variation often results from polygenic inheritance plus environment • Human skin colour, height, and body mass show many intermediate phenotypes

• Box-and-whisker plots summarize non-normal continuous data • They show median, quartiles, interquartile range, maximum/minimum, and outliers

• Homeostasis maintains a stable internal environment within narrow limits • Variables include body temperature, blood pH, glucose, gases, ions, and osmotic concentration

• Negative feedback detects deviation from a set point and reverses it • Receptors, coordinators, effectors, and feedback loops restore normal conditions

• Beta cells secrete insulin when blood glucose rises • Alpha cells secrete glucagon when blood glucose falls, affecting liver and muscle stores

• Type 1 diabetes results from autoimmune destruction of pancreatic beta cells • Type 2 diabetes involves insulin-receptor/response failure and is linked to lifestyle risk factors

• Thermoregulation uses negative feedback to maintain core temperature near 37 °C • Thermoreceptors signal the hypothalamus, which coordinates skin, muscles, liver, and hormones

• Cooling uses vasodilation, sweating, and hairs lying flat • Warming uses vasoconstriction, reduced sweating, shivering, metabolic heat, and brown fat

• Natural selection is the mechanism driving evolutionary change • It acts on heritable variation and can produce adaptation, speciation, and biodiversity

• Mutation creates new alleles, especially when germ-line mutations are inherited • Meiosis and random fertilization create new combinations of existing alleles

• Overproduction of offspring leads to high mortality in limited environments • Competition for food, space, mates, and other resources promotes selection

• Abiotic factors can act as density-independent selection pressures • Temperature, drought, light, salinity, and pH can favour different variants

• Individuals vary in adaptation, survival, and reproductive success • Fitness means passing alleles to offspring in a particular environment

• Natural selection causes evolution only if traits are heritable • Acquired characteristics are not inherited through DNA base sequences

• Sexual selection favours traits that increase mate choice or mating competition success • Displays, ornaments, and behaviours such as birds-of-paradise plumage can be selected

• Selection can be modelled by experimentally controlling selection pressures • Endler's guppy experiments test predation pressure, colour pattern, and mating success

• Some ecosystems remain stable over long timescales despite local fluctuations • Ancient forests, deserts, and long-lived communities provide evidence of stability

• Stability requires energy input, nutrient cycling, biodiversity, and genetic diversity • Abiotic conditions must remain within tolerance ranges for key species

• Amazon deforestation reduces transpiration, rainfall recycling, and regional cooling • Loss of forest can push rainforest toward savanna-like tipping points

• Mesocosms model ecosystem stability under controlled experimental conditions • Closed bottle systems restrict matter exchange but allow light energy input

• Keystone species have disproportionate effects on community structure • Removal can trigger trophic cascades and ecosystem instability

• Sustainable harvesting removes biomass at or below replacement rate • Managed plant harvests and marine fish stocks require monitoring population recovery

• Agricultural sustainability depends on soil conservation, nutrient balance, and biodiversity • Erosion, leaching, fertilizers, agrochemicals, irrigation, and carbon footprint are key factors

• Eutrophication follows nitrate and phosphate enrichment of water • Algal blooms, decomposition, high biochemical oxygen demand, and hypoxia harm aquatic life

• Biomagnification increases toxin concentration at higher trophic levels • DDT and mercury show how top predators receive the highest doses

• Macroplastics and microplastics persist, fragment, and move through marine ecosystems • Effects include entanglement, ingestion, toxin transport, and food-web impacts

• Rewilding restores natural processes, trophic interactions, and habitat connectivity • Keystone species reintroductions can restart trophic cascades and reduce intensive management

• Human activities increase atmospheric CO₂, methane, and other greenhouse gases • Fossil fuels, cement, agriculture, deforestation, and land-use change are major sources

• Positive feedback amplifies warming after an initial climate change • Examples include ice-albedo loss, permafrost methane, ocean CO₂ release, and wildfire feedback

• Boreal forests can shift from carbon sinks to carbon sources • Warming, drought, reduced snowfall, browning, insects, and fire increase tipping-point risk

• Melting landfast ice and sea ice alters breeding, feeding, and resting habitat • Emperor penguins and walruses illustrate species dependent on ice timing and extent

• Ocean warming strengthens stratification and can reduce nutrient upwelling • Reduced upwelling lowers phytoplankton productivity and food supply

• Species ranges can shift poleward, upslope, or contract as climate zones move • Montane birds and North American tree species show changing distribution limits

• Warming causes coral bleaching by disrupting coral-zooxanthellae mutualism • Ocean acidification suppresses calcification, threatening reef biodiversity and collapse

• Carbon sequestration captures and stores atmospheric carbon dioxide • Afforestation, agroforestry, forest regeneration, and peatland rewetting increase carbon stores