3.2.12Unit 3: Practical Skills in Biology I
Gene Interaction and Epigenetics — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 14 of 27 Unit quiz
Curriculum Equation Key Mendelian Genetics & Punnett Square LawsFast Exam Solution
Monohybrid: Aa × Aa ➔ 1 AA : 2 Aa : 1 aa (Phenotype 3:1) | Dihybrid: 9:3:3:1
Part 1 of 4Genotype and Environment
The phenotype of an organism depends on its genes and on how they interact with the environment.
Phenotype
- The observable characteristics of an organism, which come from its genotype and the effect of the environment.
Genetic variation
- Small differences in the DNA base sequence between individuals of a species cause phenotypic variation that can be inherited.
Environmental variation
- Differences caused by environment, such as diet and light, are not inherited because they do not change gamete DNA.
Chlorosis
- Plants lacking magnesium or light become yellow even though their genes are for green leaves.
Etiolation
- Plants grown in the dark develop long, thin stems and small leaves, although their genes would normally give healthy growth.
Combined effect
- Phenotype = genotype + environment, and many characteristics are the result of both together.
Part 2 of 4Epigenetic Control of Gene Expression
Epigenetic Control of Gene Expression
Epigenetics
- The control of gene expression by factors other than changes in the DNA base sequence.
Chromatin
- Nuclear DNA is wrapped around histone proteins to form chromatin, which can be chemically modified.
DNA methylation
- Methyl groups are added to cytosine bases,
- which prevents transcription factors and RNA polymerase from binding and switches the gene off.
Histone acetylation
- Adding acetyl groups to lysine in histone tails reduces their positive charge,
- so the DNA is wound less tightly and genes are more likely to be expressed.
Part 3 of 4Epigenetic Changes in Context
1
Epigenetic tags are chemical groups added to DNA or histones, and all the tags in an organism are called the epigenome.
2
Tightly wound DNA hides the promoter from transcription factors and RNA polymerase, so genes in that region are switched off.
3
Loosely wound DNA exposes the promoter, so transcription factors can bind and transcription can occur.
4
Environmental factors such as smoking, stress, diet and exercise can alter the epigenome.
5
Histone modifications are reversible, so they can differ between cell types and change with age.
| Modification | Effect on DNA coiling | Effect on transcription |
|---|---|---|
| DNA methylation | Inhibits binding of factors | Decreased |
| Histone acetylation | DNA wound less tightly | Increased |
Part 4 of 4Exam Checklist & Tips
3.2.12 – Key Points for the Exam
- Phenotype The observable characteristics of an organism.
- Epigenetics Control of gene expression without changing the base sequence of the DNA.
- Histone A protein around which DNA is wound to form chromatin.
- Methylation The addition of a methyl group (–CH₃) to DNA, which usually represses a gene.
- Acetylation The addition of acetyl groups to histones, which usually increases gene expression.
Common Mistakes
- Saying that epigenetic changes alter the base sequence of DNA; they do not.
- Writing that acetylation switches genes off, when it generally switches them on.
- Believing that every environmental effect on phenotype is inherited.
- Forgetting that methylation of DNA prevents the binding of transcription factors.
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