Flashcards · IB Diploma Programme Biology · Topic 4

Continuity and change flashcards

71 cards on Topic 4, subtopics D1.1 to D4.3. Pick a subtopic or study them all.

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D1.1 DNA replication

Semi-conservative replication: Each new DNA molecule consists of one original strand and one new strand. Accuracy comes from complementary base pairing with the template.
Roles of helicase and DNA polymerase: Helicase unwinds the helix by breaking hydrogen bonds between the strands. DNA polymerase builds the new strand by linking nucleotides that pair with the template (covalent backbone bonds).
PCR: the role of primers and Taq polymerase, and the three temperatures: Primers bind the ends of the target and give polymerase a 3' end to extend. Taq polymerase is heat-stable. Denature about 95 °C; anneal about 55–60 °C; extend about 72 °C. Ideal copies after n cycles = 2ⁿ.
Gel electrophoresis of DNA: DNA is negatively charged (phosphate groups), so it moves towards the positive electrode; smaller fragments move further. Sizes are estimated against a ladder. Used in DNA profiling for forensics and paternity.
Leading and lagging strands: DNA is synthesised only 5' to 3'. The leading strand is made continuously from one primer; the lagging strand is made discontinuously as Okazaki fragments, each with its own primer. (HL only)
Roles of primase, DNA polymerase III, DNA polymerase I and DNA ligase (prokaryotes): Primase makes RNA primers; DNA polymerase III extends them with DNA and proofreads; DNA polymerase I replaces RNA primers with DNA; DNA ligase joins fragments into a continuous strand. (HL only)

D1.2 Protein synthesis

Transcription: Synthesis of RNA from a DNA template by RNA polymerase, which unwinds a short region of DNA and links RNA nucleotides by complementary pairing (A–U, T–A, G–C, C–G).
Template strand and coding strand: The mRNA is complementary to the template strand and has the same sequence as the coding strand, with U in place of T.
Elongation in translation: A tRNA with a matching anticodon binds the codon; a peptide bond forms (condensation) between its amino acid and the chain; the ribosome moves one codon along the mRNA; the empty tRNA is released.
Sickle cell disease: the mutation and its effect: In the β chain of haemoglobin, the mRNA codon GAG (glutamic acid) becomes GUG (valine), a missense mutation. The altered haemoglobin aggregates at low oxygen.
Post-transcriptional processing of eukaryotic mRNA: Introns are removed and exons spliced together; a 5' cap and poly-A tail stabilise the mRNA. Alternative splicing joins different exon combinations, so one gene can give different polypeptides. (HL only)
Roles of the A, P and E sites of a ribosome: A site: an incoming tRNA arrives. P site: holds the tRNA carrying the growing chain. E site: the tRNA exits. (HL only)

D1.3 Mutations and gene editing

SNP (as the result of a mutation): A single-nucleotide difference at one position among individuals, arising from a base substitution.
Silent, missense and nonsense mutations: Silent: same amino acid (the code is degenerate). Missense: a different amino acid. Nonsense: a stop codon appears, giving a truncated polypeptide.
Frameshift mutation: An insertion or deletion of bases (not a multiple of three) that changes every codon downstream; the polypeptide is very likely non-functional.
Mutations in germ cells and in somatic cells: Germ cells: the mutation can be inherited and be present in every cell of the offspring. Somatic cells: not inherited, but can cause cancer.
CRISPR-Cas9 gene editing: A guide RNA matches a target sequence; Cas9 cuts the DNA there; repair either disables the gene or uses a supplied template to change it. Example: a CRISPR-based sickle cell treatment authorised in 2023. (HL only)
Conserved and highly conserved sequences: Conserved: similar across a species or group. Highly conserved: similar over long evolutionary periods. Hypotheses: the function of the gene product restricts change, or the mutation rate is slower there. (HL only)

D2.1 Cell and nuclear division

Cytokinesis in animal and plant cells: Animal: a ring of actin and myosin contracts, pinching the membrane inwards (a furrow). Plant: vesicles fuse across the middle and build new membrane and wall outwards (a cell plate).
Mitosis and meiosis compared: Mitosis: one division, two nuclei, same chromosome number, genetically identical; no pairing. Meiosis: two divisions, four nuclei, chromosome number halved, genetically different; homologues pair and cross over in meiosis I.
Non-disjunction and Down syndrome: Chromosomes or chromatids fail to separate in meiosis. A gamete with 24 chromosomes fusing with a normal gamete (23) gives a zygote with 47, three copies of chromosome 21: trisomy 21 (Down syndrome).
How many chromosome combinations can random orientation give when n = 6?: 2ⁿ = 2⁶ = 64, where n is the haploid number. Random orientation of bivalents happens in metaphase I; crossing over in prophase I adds more variation.
Control of the cell cycle by cyclins: Cyclin concentrations rise and fall in a pattern through the cycle; a specific cyclin must reach a threshold for the cell to pass each checkpoint. (HL only)
23 of 310 cells counted are in mitosis. What is the mitotic index?: Cells in mitosis ÷ total cells = 23 ÷ 310 = 0.0742, or 7.42%. (HL only)

D2.2 Gene expression (HL only)

Promoter and enhancer: Promoter: DNA sequence next to a gene where RNA polymerase attaches. Enhancer: DNA sequence where regulatory proteins bind to raise transcription of a gene. (HL only)
Genome, transcriptome and proteome: Genome: all the DNA of a cell (the same in nearly all cells). Transcriptome: all the mRNA in a cell at a given time. Proteome: all the proteins in a cell. Transcriptome and proteome differ between cell types. (HL only)
How does DNA methylation affect gene expression, and is it inherited?: Methylated cytosine in a promoter represses transcription, silencing the gene. Tags stay in place through mitosis; if they survive meiosis, offspring may show the change. The base sequence is unchanged. (HL only)
How does lactose switch on genes in bacteria?: Without lactose the repressor sits on the operator, blocking RNA polymerase. A lactose-derived molecule binds the repressor, which changes shape and leaves the operator, so the genes for lactose use are transcribed. (HL only)
What do studies of monozygotic twins show about epigenetics?: They share a genome, so differences in expression come from the environment. Older pairs, and pairs who spent more years apart, may show larger differences in methylation. (HL only)

D2.3 Water potential

Hypertonic, hypotonic and isotonic: Hypertonic: higher solute concentration than the solution compared with. Hypotonic: lower solute concentration. Isotonic: equal; water still crosses both ways at equal rates (dynamic equilibrium).
Animal and plant cells in hypotonic and hypertonic solutions: Hypotonic: animal cell swells and may burst; plant cell becomes turgid (the wall resists expansion). Hypertonic: animal cell crenates; plant cell plasmolyses (membrane pulls away from the wall).
Plant tissue changes from 1.50 g to 1.38 g in a sucrose solution. Percentage change, and what it shows?: (1.38 − 1.50) ÷ 1.50 × 100 = −8.0%. Mass fell, so the solution was hypertonic to the cells and water left by osmosis.
Why are intravenous fluids and organ-bathing solutions isotonic?: No net water movement into or out of cells, so blood cells and donor organ cells neither swell nor shrink. Distilled water in a vein would make red blood cells swell and burst.
Water potential and the direction of water movement: Potential energy of water per unit volume, relative to pure water at 20 °C and normal air pressure (0 kPa). Solutes lower it. Water moves from higher (less negative) to lower (more negative) water potential. (HL only)
A cell has ψs = −650 kPa and ψp = +400 kPa. What is its water potential?: ψw = ψs + ψp = −650 + 400 = −250 kPa. In a solution of −250 kPa there is no net movement of water. (HL only)

D3.1 Reproduction

Asexual and sexual reproduction compared: Asexual: one parent, mitosis, clones; fast and keeps a successful genotype in a stable environment. Sexual: usually two parents, meiosis and fusion of gametes, varied offspring; new allele combinations may suit a changed environment.
Roles of FSH, LH, oestradiol and progesterone in the menstrual cycle: FSH (pituitary): follicle growth. LH (pituitary): surge triggers ovulation; corpus luteum forms. Oestradiol (follicle): thickens endometrium; feedback on FSH and LH. Progesterone (corpus luteum): maintains endometrium; inhibits FSH and LH.
What causes the LH surge?: Oestradiol at low to moderate levels gives negative feedback on FSH and LH; when oestradiol stays high for long enough it gives positive feedback, causing the LH surge and ovulation about mid-cycle.
Pollination, fertilisation and seed dispersal: Pollination: pollen moves from anther to stigma. Fertilisation: later fusion of gametes in the ovule, after a pollen tube grows down the style. Dispersal: the seed moves away from the parent plant.
Acrosome reaction and cortical reaction: Acrosome reaction (sperm): enzymes digest a route through the zona pellucida. Cortical reaction (egg): cortical granules release contents that change the zona pellucida so no more sperm can pass, preventing polyspermy. (HL only)
Spermatogenesis and oogenesis: gametes from one cell: One primary spermatocyte gives four equal sperm. One primary oocyte gives one large ovum (which keeps the cytoplasm) and small polar bodies. Both involve mitosis, growth, two meiotic divisions and differentiation. (HL only)

D3.2 Inheritance

Codominance and incomplete dominance: Codominance: the heterozygote shows both phenotypes at once (group AB, IᴬIᴮ). Incomplete dominance: the heterozygote has an intermediate phenotype (pink Mirabilis jalapa flowers).
Haemophilia: carrier mother (XᴴXʰ) × unaffected father (XᴴY). Outcomes?: Offspring XᴴXᴴ, XᴴY, XᴴXʰ, XʰY, each 1/4. Half the sons affected, half the daughters carriers, no affected daughters. A son always gets his X from his mother.
Two PKU carriers have three children. Probability that none is affected?: Each child is unaffected with probability 3/4, independently. (3/4)³ = 27/64 ≈ 0.42.
Pedigree: what do these show? (a) two unaffected parents with an affected child (b) two affected parents with an unaffected child: (a) The allele is recessive and both parents are carriers. (b) The allele is dominant and both parents are heterozygous.
RT | rt × rt | rt gives RT 104, rt 96, Rt 52, rT 48. Recombinant frequency?: Recombinants are the smallest classes: 52 + 48 = 100. 100 ÷ 300 × 100 = 33.3%. Below 50%, so the genes are linked. (HL only)
Expected ratios for unlinked genes: AaBb × AaBb and AaBb × aabb: AaBb × AaBb: 9 : 3 : 3 : 1. AaBb × aabb (test cross): 1 : 1 : 1 : 1. Both follow from independent assortment of bivalents in metaphase I. (HL only)

D3.3 Homeostasis

Insulin and glucagon compared: Insulin: beta cells of the pancreas; released when blood glucose is high; liver, muscle and fat cells take up glucose and store it as glycogen; glucose falls. Glucagon: alpha cells; released when glucose is low; mainly liver breaks down glycogen; glucose rises.
Type 1 and type 2 diabetes: Type 1: beta cells destroyed (autoimmune); little or no insulin; treated with insulin. Type 2: target cells less responsive to insulin; risk factors excess body fat, inactivity, diet, age, family history; managed by diet and exercise, medicines, sometimes insulin.
How does brown adipose tissue release heat?: By uncoupled respiration: protons leak back across the inner mitochondrial membrane, so the energy is released as heat rather than used to make ATP.
Thermoregulation responses to cold: Thermoreceptors and the hypothalamus detect the fall. Skin arterioles constrict, muscles shiver, brown adipose tissue releases heat, hairs are raised, and the hypothalamus–pituitary–thyroid route increases thyroxin, raising metabolic rate.
Response of the collecting duct when the blood is too concentrated: Osmoreceptors in the hypothalamus detect it; the pituitary releases more ADH; vesicles carrying aquaporins fuse with collecting duct membranes; more water leaves the duct by osmosis, giving a small volume of concentrated urine. (HL only)
Ultrafiltration in the glomerulus: High pressure pushes plasma through the capillary wall, the basement membrane and the slits between podocytes into Bowman's capsule. Water, glucose, amino acids, urea and ions pass; blood cells and large plasma proteins stay. (HL only)

D4.1 Natural selection

The logic of natural selection: Populations show variation; more offspring are produced than the environment can support, so there is a struggle for resources; better-adapted individuals survive and breed more; if the traits are heritable, their alleles become more common; repeated over generations, the population changes.
Sources of variation: Mutation makes new alleles. Meiosis (crossing over and independent assortment) and random fertilisation give new combinations but no new alleles.
What did Endler's guppy studies show about sexual selection?: Bright colour improves mating success but attracts predators. Where predators are few, males become brighter; where predators are many, males are duller. The trait balances mating benefit against survival cost.
Fitness: The survival value plus reproductive potential of a genotype, judged by the number of offspring that survive to breed.
A recessive trait occurs in 0.49% of a population. What proportion are carriers?: q² = 0.0049, so q = 0.07 and p = 0.93. Carriers = 2pq = 2 × 0.93 × 0.07 = 0.1302, about 13.0%. (HL only)
Is antibiotic resistance in bacteria natural or artificial selection?: Natural selection: the antibiotic is a selection pressure, but nobody chooses which bacteria breed. In artificial selection humans choose the parents (crops, dogs, dairy cattle). (HL only)

D4.2 Stability and change

Four requirements for ecosystem stability: An energy supply, recycling of nutrients, genetic diversity, and climatic variables that stay within tolerance levels.
Keystone species: A species with a disproportionate effect on community structure compared with its abundance. Removing it can cause collapse. Keystone is about impact; dominant is about numbers.
Eutrophication chain, and the role of BOD: Leaching of nitrogen and phosphate; algal bloom; light blocked; plants die; decomposers multiply and respire; BOD (oxygen used by microbes decomposing organic matter) rises; oxygen falls; fish die.
Biomagnification, with two examples: Rising concentration of a persistent toxin at each higher trophic level. DDT (insecticide) caused eggshell thinning in some predatory birds; mercury accumulates in large predatory fish.
When is a harvest sustainable, and how is it checked?: When the harvest rate is lower than the replacement rate. Check indicators over time: catch per unit effort, fish size and age, seedling counts, age structure.
Cyclical and arrested succession: Cyclical: a repeating cycle of communities, such as the phases of heather moorland. Arrested: human activity holds the community at an earlier stage, for example livestock grazing or wetland drainage. (HL only)

D4.3 Climate change

Greenhouse gas: A gas that absorbs and re-emits infrared radiation, reducing heat loss to space. Carbon dioxide (combustion, forest clearance) and methane (cattle, rice fields, waste tips, gas leaks).
Give two positive feedback loops in global warming: Melting ice and snow leaves a darker surface (lower albedo) that absorbs more sunlight, so more melting. Thawing permafrost and peat decompose faster, releasing CO₂ and methane, so more warming.
Ocean acidification and coral bleaching: Acidification: CO₂ dissolves and pH falls, suppressing calcification. Bleaching: higher water temperature makes corals expel their algae, so they starve. Loss of corals collapses the reef habitat and food web.
How are species ranges shifting with climate change?: Poleward and upslope, because the cooler edge of each species' tolerable temperature range moves there. Evidence: montane birds in New Guinea moving upslope; North American trees spreading north. Limits: summits, slow dispersal, fragmentation.
Loss of synchrony: great tits and caterpillars: When two linked populations use different timing cues (one temperature, one photoperiod), warming can separate them. Great tit (Parus major) breeding can miss peak caterpillar biomass, so chicks get less food. (HL only)
Why does warming increase outbreaks of the spruce bark beetle?: Warmth speeds the development of Ips typographus, so there are more generations a year and larger outbreaks. (HL only)