Theme D — Continuity and change
Theme D — Continuity and change — IB Biology HL Revision Notes & Topical Questions
5 min read 8 parts Lesson 1 of 18 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
Theme D — Continuity and change — IB BIOLOGY HL notes: Link DNA replication, mutation, cell division, osmosis and stability in Theme D.Purpose — replication gives each daughter cell a complete copy of the genome.Semi-conservative — each new double helix has one old strand and one…
- Transcription pairing
- Base substitution
- Mutation rate
- Gene editing
- Diploid (2n) nucleus
- Replication in S phase
Part 1 of 6DNA replication
Purpose
- Purpose replication gives each daughter cell a complete copy of the genome.
Semi-conservative
- Semi-conservative each new double helix has one old strand and one new strand.
HL: the leading strand is synthesised continuously in the 5' to 3' direction towards the replication fork.
Enzymes of replication
- Helicase unwinds the helix and breaks hydrogen bonds.
- DNA polymerase III adds nucleotides at the 3' end.
- Primase makes RNA primers.
- DNA polymerase I replaces the primers.
- Ligase joins fragments on the lagging strand.
Transcription pairing
- Transcription pairing in RNA, A pairs with U, so the mRNA is complementary to the template strand.
Part 2 of 6Mutation and gene editing
Base substitution
Base substitution replacement of one base by another.
- Silentthe same amino acid, because the code is degenerate.
- Missensea different amino acid.
- Nonsensea stop codon.
Mutation rate
- Mutation rate it is not the same everywhere.
- Hotspots mutate more often.
- Mutagens raise the overall rate.
Gene editing
Gene editing changing an organism's own DNA at a chosen site, instead of adding DNA from another species.
- Earlier gene therapy used viral vectors, which sometimes inserted DNA into the wrong gene.
- Liposome nasal sprays worked briefly because the target cells were short-lived.
Cancer genes
- A mutated proto-oncogene (oncogene) makes too much protein or protein that is always active, so division is constantly stimulated.
- Tumour suppressor proteins halt division or trigger apoptosis when DNA is damaged, and mutations that inactivate them remove this brake.
Part 3 of 6Chromosomes and cell division
1Diploid (2n) nucleus
- Diploid (2n) nucleus it contains two sets of chromosomes (humans 2n = 46).
2Replication in S phase
- Replication in S phase each chromosome has two sister chromatids joined at the centromere by cohesin.
3Condensation
- Condensation supercoiling around histones makes chromosomes short and compact so they move without tangling or breaking.
4
Functions of division
- Mitosis maintains chromosome number and genetic identity, for growth, replacement, repair and asexual reproduction.
- Meiosis halves chromosome number in sexual life cycles so that fertilisation restores it.
5Cytokinesis
- Cytokinesis usually equal but sometimes unequal.
- In oogenesis most cytoplasm goes to the oocyte and the polar bodies are tiny.
- Yeast budding gives a small bud.
- Both daughter cells must receive at least one nucleus and mitochondria.
Part 4 of 6Meiosis and variation
Crossing over
- Crossing over it exchanges alleles between non-sister chromatids in prophase I.
Random orientation
- Random orientation bivalents in metaphase I line up randomly, giving different combinations of maternal and paternal chromosomes in gametes.
- In humans random orientation alone gives 223 (about 8.4 million) combinations.
Non-disjunction
- Non-disjunction chromosomes failing to separate give abnormal numbers, for example trisomy 21.
Part 5 of 6Osmosis and water potential
Isotonic
- Isotonic a solution with a similar concentration to the cytoplasm.
Hypertonic IV solution
- Hypertonic IV solution it causes net movement of water out of red blood cells, which shrivel and become crenated.
Plant cells
- Plant cells as water enters the vacuole, its volume increases.
Estimating osmotic concentration of plant tissue
Osmosis and water potential
Place tissue such as potato in solutions of different osmotic concentration.
Measure the percentage mass change.
Where the percentage change is zero, the solution has the same concentration as the cytoplasm, so there is no net water movement.
Part 6 of 6Natural selection and stability
Natural selection needs variation, overproduction of offspring, a struggle for existence and differential survival.
- Over generations the frequency of advantageous alleles rises.
Stability of ecosystems
- Continuous energy input.
- Nutrient recycling by decomposers.
- Genetic diversity, so populations can survive change.
Tipping points
- Tipping points the system moves to a new state, such as forest to savanna after heavy deforestation.
Key terms
| Term | Meaning |
|---|---|
| Semi-conservative replication | each new DNA molecule has one old and one new strand. |
| Base substitution | replacement of one base by another in DNA. |
| Oncogene | a mutated proto-oncogene that stimulates division continuously. |
| Isotonic | having the same solute concentration as another solution. |
| Diploid | having two sets of chromosomes. |
| Tumour suppressor gene | a gene whose protein stops division or triggers apoptosis. |
Avoid these mistakes
1
The whole DNA molecule is not copied — it is semi-conservative, with each new molecule having one old and one new strand.
2
All mutations are harmful — many are silent or neutral.
3
Hypertonic and hypotonic mean the same — hypertonic has the higher solute concentration and hypotonic the lower.
4
Meiosis only halves the chromosome number — it also creates variation by crossing over and random orientation.
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