3.2.10Unit 3: Practical Skills in Biology I
Cell Specialisation — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 12 of 27 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.
Section 1 of 4Differential Gene Expression
Specialised cells come from stem cells by switching different genes on or off.
Same genome
- Same genome All the cells in an organism come from one zygote, so every nucleus contains the same genes.
Selective expression
- Selective expression Only certain genes are switched on in each cell type, so cells make different proteins despite having the same DNA.
Differentiation
- Differentiation Specialisation occurs as particular genes are expressed and the proteins they code for modify the structure and activity of the cell.
Active genes
- Active genes mRNA is transcribed only from the active genes and is then translated to make proteins.
Progressive
- Progressive As these proteins accumulate the cell becomes more specialised and, once differentiated, the process is generally irreversible.
Control of development
- Control of development Controlling gene expression is the key to development, because it decides which cell types arise.
Section 2 of 4Transcription Factors
1
A transcription factor is a protein that controls transcription by binding to a specific region of DNA.
2
Many bind near the start of a gene in the promoter region, where they interact with RNA polymerase.
3
Activators help RNA polymerase to bind and start transcription, and so increase the rate of gene expression.
4
Repressors prevent RNA polymerase from binding, so they decrease or stop transcription of that gene.
5
Some hormones act by affecting transcription factors, which allows organisms to respond to their environment.
Section 3 of 4Examples and Implications
Examples and Implications
Correct cells and times
- Correct cells and times Transcription factors make sure that genes are expressed in the right cells, at the right time and at the right level.
Many genes
- Many genes Around one tenth of human genes are estimated to code for transcription factors, showing their importance.
Red blood cells
- Red blood cells A differentiated red blood cell expresses genes for haemoglobin, but does not express genes for digestive enzymes.
Stem cells
- Stem cells Stem cells have the potential to differentiate because many of their genes have not yet been switched off.
| Transcription factor | Effect on RNA polymerase | Effect on transcription |
|---|---|---|
| Activator | Helps it bind to the promoter | Increases |
| Repressor | Blocks its binding | Decreases |
Section 4 of 4Exam Checklist & Tips
3.2.10 – Key Points for the Exam
- Differentiation The process by which an unspecialised cell becomes specialised.
- Differential gene expression Different genes being switched on in different cell types.
- Transcription factor A protein that controls whether a gene is transcribed.
- Promoter The region of DNA at the start of a gene where RNA polymerase binds.
- Genome The complete set of genes of an organism.
Common Mistakes
- Saying specialised cells have lost the genes they do not use, when they still contain the same genome.
- Writing that transcription factors are made of DNA, when they are proteins.
- Saying that all genes are expressed in all cells.
- Mixing up activators, which increase transcription, with repressors, which decrease it.
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.