4.5.3Additional lessons
Environment and Gene Expression — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 49 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.
Part 1 of 5Epigenetics
Differences between cells arise from the epigenome and transcription factors, not from different DNA.
Epigenome
- Epigenome The collection of chemical tags on DNA and histones that influence gene activity without changing the base sequence.
Genome
- Genome All the DNA of an organism, which is almost identical in each of its cells.
Transcriptome
- Transcriptome The complete set of mRNA molecules made in a cell at a given time, which varies between cell types.
Proteome
- Proteome The complete set of proteins in a cell, which depends on which mRNAs are transcribed and translated.
| Level | What it contains | Same in all body cells? |
|---|---|---|
| Genome | All genes | Yes, nearly |
| Epigenome | Chemical tags | No |
| Transcriptome | mRNAs | No |
| Proteome | Proteins | No |
Part 2 of 5Epigenetic Tags
DNA methylation
- DNA methylation A methyl group (-CH₃) added to cytosine bases, often near promoters, blocks transcription factors and silences the gene.
Histone methylation
- Histone methylation Methyl groups added to histone tails can make chromatin tighter or looser depending on the position modified.
Histone acetylation
- Histone acetylation Acetyl groups on histones loosen the DNA packaging, making genes more accessible to transcription factors and RNA polymerase.
Reversible
- Reversible Tags can be added and removed by enzymes, so patterns of expression can change during a life.
Part 3 of 5Inheritance of Tags
Cell division
- Cell division Methylation patterns are copied when DNA replicates, so daughter cells keep the same expression pattern as the parent cell.
Differentiation
- Differentiation This is how a liver cell divides to give only liver cells, because its specialised pattern is passed on.
Between generations
- Between generations Most tags are erased in gametes and early embryos, although some can pass to offspring in certain cases.
Part 4 of 5Environment and Phenotype
Genotype and phenotype
- Genotype and phenotype Identical genotypes can give different phenotypes if the environment changes which genes are switched on.
Environmental influences
- Environmental influences Chemicals, diet, temperature and stress can alter the activity of enzymes that add or remove epigenetic tags.
Importance
- Importance Epigenetics explains how genes and environment interact to affect health, development and disease risk.
Part 5 of 5Exam Checklist & Tips
4.5.3 – Key Points for the Exam
- Epigenetic tag A chemical group attached to DNA or histones that alters gene expression.
- Methylation Addition of a methyl group, often silencing a gene when added to DNA.
- Chromatin DNA combined with histone proteins in eukaryotic nuclei.
- Differentiation Development of a cell into a specialised type by expressing particular genes.
Common Mistakes
- Saying that epigenetic tags change the DNA base sequence.
- Believing methylation always increases expression; on promoters it generally reduces it.
- Mixing up genome (same in all cells) and proteome (varies between cells).
- Assuming all epigenetic changes are passed to offspring.
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