4.5.4Additional lessons
Environment and Gene Expression- Examples — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 50 of 101
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.
Section 1 of 5Gene Control in Bacteria: the lac Operon
Bacteria make lactose-digesting enzymes only when lactose is available, saving resources.
Structure
- Structure The genes lacZ, lacY and lacA are controlled by one promoter and one operator, so they are transcribed together.
Repressor
- Repressor A regulatory gene makes a repressor that binds the operator when lactose is absent, blocking RNA polymerase.
Lactose present
- Lactose present Lactose, or its derivative allolactose, binds the repressor and changes its shape so it leaves the operator.
Result
- Result RNA polymerase can transcribe the structural genes, including lacZ for the enzyme that breaks down lactose.
| Condition | Repressor | Transcription of lacZYA |
|---|---|---|
| No lactose | Bound to operator | Blocked |
| Lactose present | Released from operator | Occurs |
Section 2 of 5Hormones and Imprinting
Steroid hormones
- Steroid hormones Hormones like oestradiol and testosterone enter cells and bind receptors that act as transcription factors for target genes.
Genomic imprinting
- Genomic imprinting Some genes are marked by methylation during gamete formation, so only the copy from one parent is expressed.
Ligers and tigons
- Ligers and tigons Offspring of male lion and female tiger are larger than either parent, which is proposed to result from imprinted growth genes.
Resetting
- Resetting Imprints are erased and re-established in each generation according to the sex of the parent.
Section 3 of 5Twins and Pollution
Monozygotic twins
- Monozygotic twins They share the same genotype, so differences in phenotype, such as body mass, point to environmental effects.
Epigenetic drift
- Epigenetic drift Twins have similar methylation patterns early on, but patterns diverge with age and different life experiences.
Dizygotic twins
- Dizygotic twins These develop from two eggs and are genetically different, so they provide a comparison against identical twins.
Air pollution
- Air pollution Exposure to polluted air is linked to altered DNA methylation in blood cells and to higher disease risk.
Section 4 of 5Reasoning from Twin Data
1
Compare a trait between pairs of monozygotic twins; a high match suggests a strong genetic effect.
2
Compare the same trait between pairs of dizygotic twins.
3
If monozygotic pairs match much more closely than dizygotic pairs, genes are likely influential.
4
Differences within monozygotic pairs are attributed to environment, including epigenetic change.
Section 5 of 5Exam Checklist & Tips
4.5.4 – Key Points for the Exam
- Operon A cluster of genes controlled together by one promoter and operator.
- Repressor A protein that blocks transcription by binding to the operator.
- Genomic imprinting Silencing of a gene according to which parent it was inherited from.
- Monozygotic twins Twins from one fertilised egg, with identical genotypes.
Common Mistakes
- Saying that lactose is made by the lac operon; the operon codes for enzymes that break it down.
- Thinking the repressor binds the promoter rather than the operator.
- Claiming that identical twins are identical in phenotype.
- Stating that imprinting changes the DNA sequence of the gene.
Last updated:
\n