Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

2.4Unit 2: Cells, Development, Biodiversity and Conservation

SNAB Differentiation & Variation — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 26 1 quiz

📚 IAL Edexcel Linear Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
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.
0%read
Lesson summary

2.4 Differentiation and Variation — IAL EDEXCEL LINEAR notes: Explain stem cell potency and use, how gene expression controls differentiation, epigenetic control, and the causes of phenotypic variation.Stem cell = a cell that can divide by mitosis an unlimited number of times and…

  • Embryonic stem cells
  • Adult stem cells
  • Regulatory authorities
  • Irreversible
  • Transcription factor
  • lac operon components

Part 1 of 7Stem cells and potency

Stem cell
Stem cell a cell that can divide by mitosis an unlimited number of times and either remain a stem cell or differentiate into a specialised cell.
Potency
Potency the ability of a stem cell to differentiate into different cell types.
TypeCan becomeExamples
TotipotentAny cell type in an embryo plus extra-embryonic cells (placenta, umbilical cord)Zygote and embryonic cells up to the 16-cell stage
PluripotentAny embryonic cell type but not placenta or umbilical cord cellsEmbryonic stem cells on day 5
MultipotentA limited range of cell typesAdult stem cells, for example intestinal stem cells replacing epithelial cells
  • Plants
    • they also contain stem cells in areas of growth such as shoots and roots.

Part 2 of 7Stem cells in medicine

Therapies
  • Therapies stem cells can replace damaged tissue and cells.
  • Bone marrow stem cell transplants treat leukaemia after the patient's own marrow stem cells are killed.
  • Research aims to replace damaged nerve tissue in spinal cord injuries and damaged heart tissue after heart attacks.
Embryonic stem cells
  • Embryonic stem cells they are totipotent in the first 3–4 days after fertilisation and pluripotent on day 5, with huge therapeutic potential.
  • They often come from spare embryos from IVF, which could develop into human beings, so many people object.
  • Use is banned even for research in some countries such as the USA and parts of the EU, but is tightly regulated in the UK.
Adult stem cells
  • Adult stem cells they divide without limit but form only a limited range of cells, and are found in bone marrow (blood cells) and brain.
    • (neural and glial cells)
  • They remain for growth, cell replacement and tissue repair.
  • Use is less controversial because the donor can give permission.
  • Donor cells must closely match the patient's blood type and antigens or the immune system will reject them, and the patient's own cells are ideal.
Regulatory authorities
  • Regulatory authorities they review research proposals, license and monitor research centres, issue guidelines, monitor developments and advise governments.

Part 3 of 7Differential gene expression

  • Differentiation
    • all cells of an organism have the same genome, but only certain genes are expressed in each cell type.
1
Under certain conditions some genes in a stem cell are activated and others inactivated.
2
mRNA is transcribed only from the active genes.
3
The mRNA is translated into proteins.
4
The proteins modify the cell's structure and processes, so it becomes increasingly specialised.
Irreversible
  • Irreversible once differentiated, the cell stays specialised.
Transcription factor
Transcription factor a protein that controls transcription by binding to a specific region of DNA, such as a promoter.
  • About 10% of human genes code for transcription factors.
  • Activators help RNA polymerase bind to the start of a gene and increase transcription.
  • Repressors stop RNA polymerase binding and decrease transcription.
  • Some hormones act through transcription factors, letting organisms respond to the environment.

Part 4 of 7Operons and the lac operon

Operons and the lac operon
Operon
Operon a section of prokaryotic DNA with structural genes transcribed together, a promoter where RNA polymerase binds and an operator where transcription factors bind.
lac operon components
  • lac operon components promoter, operator, lacZ, lacY (permease, letting lactose in) and lacA (transacetylase).
    • (lactase, also called beta-galactosidase)
  • Upstream, regulatory gene lacI with its own promoter codes for the lac repressor.
  • The repressor has one site for the operator and one for lactose, the effector molecule.
Lactose absent
  • Lactose absent the repressor binds the operator, RNA polymerase cannot bind the promoter and the structural genes are not transcribed,
    • so no lactase is made.
Lactose present
  • Lactose present lactose enters the cell and binds the repressor, distorting its shape so it leaves the operator.
  • RNA polymerase binds the promoter and the structural genes are transcribed and translated.
  • Lactase is inducible, so the bacterium does not waste energy and materials making it when lactose is absent.

Part 5 of 7Epigenetics

Epigenetics
Epigenetics control of gene expression by factors other than the DNA sequence, switching genes on or off without changing the genetic code.
  • Light, drugs, hormones, smoking, stress, exercise and diet can alter gene expression, for example UV radiation increases melanin gene expression.
Epigenetic tags
  • Epigenetic tags chemical modifications of chromatin are called epigenetic tags, and all of them make up the epigenome.
    • (DNA wrapped around histones)
  • They control how tightly DNA is wound, so tightly wound genes are hidden from transcription factors and RNA polymerase and switched off.
DNA methylation
  • DNA methylation methyl groups (–CH3) added to cytosine bases suppress transcription by blocking binding of transcription factors and RNA polymerase, locking genes off.
Histone acetylation
  • Histone acetylation acetyl groups added to lysine on histones remove its positive charge,
    • which weakens the ionic bond with the DNA phosphate backbone.
  • DNA is less tightly wrapped, so RNA polymerase and transcription factors bind and the gene is activated.
  • Deacetylation restores the positive charge and the attraction, so the gene is switched off.
Heritability
  • Heritability epigenetic changes can pass to daughter cells and sometimes to the next generation because methyl groups are not always removed during gamete production.
Compared with mutation
  • Compared with mutation mutation changes the nucleotide sequence, whereas epigenetic change affects only how the code is read.

Part 6 of 7Phenotype and sources of variation

Phenotype and sources of variation
Phenotype
Phenotype genotype + environment, and phenotypic variation is the difference between organisms of the same species.
  • Genetic causethe ABO blood groups arise from combinations of three alleles of one gene.
  • Environmental causegenetically identical plant clones grow to different heights in different conditions.
  • Combined causethe sickle cell allele is frequent where malaria is common because heterozygotes resist malaria.
Genetic variation
  • Genetic variation small differences in DNA base sequences between individuals of a species are inherited and generate phenotypic variation.
  • Monogenic characteristics are controlled by one gene and usually show discontinuous variation.
  • Polygenic characteristics are controlled by several genes and usually show continuous variation.
Environmental factors
  • Environmental factors sunlight hours, nutrients, water, temperature and oxygen affect growth and development.
  • Variation caused only by the environment is not inherited, because only changes to the DNA of gametes are passed on.
  • Silver salts in the diet turn the larvae of normal grey Drosophila yellow.
  • Plants short of light or magnesium show chlorosis, and plants grown in the dark show etiolation with long stems and small curled leaves.

Part 7 of 7Discontinuous and continuous variation

FeatureDiscontinuousContinuous
Type of differenceQualitative, in distinct categories with no intermediatesQuantitative, a range of values between two extremes
ExampleABO blood groupHeight and mass
Genetic controlUsually one geneUsually many genes plus the environment
  • Polygenic inheritance
    • alleles at many loci each have a small effect and add together, and with the environment give continuous variation.
      • (additive effect)
    • For two genes where each dominant allele adds 2x cm and each recessive allele adds x cm, hhtt is 4x, HhTt is 6x and HHTT is 8x.
    • The effect of each gene is not always equal, so the information given must be checked.

Key terms

TermMeaning
Stem cellA cell that can divide without limit and differentiate into specialised cells.
TotipotentAble to become any cell type including extra-embryonic cells.
PluripotentAble to become any embryonic cell type but not extra-embryonic cells.
Transcription factorA protein that controls transcription by binding to DNA.
OperonA cluster of structural genes with a promoter and operator controlled together.
EpigenomeAll the epigenetic tags on an organism's DNA and histones.
PolygenicControlled by several genes with additive effects.

Avoid these mistakes

1
Saying differentiated cells lose genes — all cells keep the same genome and differ in which genes are expressed.
2
Saying epigenetic changes alter the DNA sequence — they alter only how the code is read.
3
Saying acetylation tightens DNA around histones — acetylation loosens it and allows transcription.
4
Saying all variation is inherited — variation caused only by the environment is not inherited.

Last updated:

\n