2.4Unit 2: Cells, Development, Biodiversity and Conservation
SNAB Differentiation & Variation — IAL Edexcel Modular Revision Notes & Topical Questions
5 min read 9 parts Lesson 6 of 8 1 quiz
Lesson summary
2.4 Differentiation and Variation — IAL EDEXCEL MODULAR 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 an…
- Embryonic stem cells
- Adult stem cells
- Regulatory authorities
- Irreversible
- Transcription factor
- lac operon components
Section 1 of 9Stem 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.
| Type | Can become | Examples |
|---|---|---|
| Totipotent | Any cell type in an embryo plus extra-embryonic cells (placenta, umbilical cord) | Zygote and embryonic cells up to the 16-cell stage |
| Pluripotent | Any embryonic cell type but not placenta or umbilical cord cells | Embryonic stem cells on day 5 |
| Multipotent | A limited range of cell types | Adult 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.
Section 2 of 9Stem 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.
Section 3 of 9Differential 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.
Section 4 of 9Operons 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.
Section 5 of 9Epigenetics
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.
Section 6 of 9Phenotype 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.
Section 7 of 9Discontinuous and continuous variation
| Feature | Discontinuous | Continuous |
|---|---|---|
| Type of difference | Qualitative, in distinct categories with no intermediates | Quantitative, a range of values between two extremes |
| Example | ABO blood group | Height and mass |
| Genetic control | Usually one gene | Usually 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.
- alleles at many loci each have a small effect and add together, and with the environment give continuous variation.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Stem cell | A cell that can divide without limit and differentiate into specialised cells. |
| Totipotent | Able to become any cell type including extra-embryonic cells. |
| Pluripotent | Able to become any embryonic cell type but not extra-embryonic cells. |
| Transcription factor | A protein that controls transcription by binding to DNA. |
| Operon | A cluster of structural genes with a promoter and operator controlled together. |
| Epigenome | All the epigenetic tags on an organism's DNA and histones. |
| Polygenic | Controlled by several genes with additive effects. |
Section 9 of 9Exam technique
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.
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