Consolidating Cell Differentiation, Genes & Environment, and Controlling Gene Expression.
Test what you remember from across Topic 3C. Try each question before checking.
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Use the word bank to complete the passage about gene expression and regulation.
All cells in a multicellular organism contain the same DNA. However, through cell , different cells express different genes. All cells produce proteins, but specialised cells also produce unique proteins. The complete set of proteins in a cell is called the .
Gene expression is controlled at the transcriptional level by , which bind to promoters or enhancers. After transcription, alternative RNA allows different to be joined together, producing different proteins from the same gene.
Epigenetic mechanisms also regulate genes. DNA adds -CH₃ groups to cytosine, and the gene is typically . Changes to proteins can loosen or tighten DNA around nucleosomes.
Phenotype is determined by genotype and the . Traits controlled by many genes show inheritance and often display variation.
Click each cell to reveal the answer. Test yourself first!
| Level | Mechanism | Effect |
|---|---|---|
| Transcriptional | Click to reveal | Click to reveal |
| Epigenetic | Click to reveal | Click to reveal |
| Post-transcriptional | Click to reveal | Click to reveal |
| Non-coding RNA | Click to reveal | Click to reveal |
Identical twins raised in different countries have different heights as adults. One twin had access to a protein-rich diet; the other did not. Explain this observation using the concepts from Topic 3C.
Identical twins have the same genotype (100% shared DNA). However, phenotype is determined by the interaction between genotype and environment. Height is a polygenic trait — controlled by many genes — and is strongly influenced by environmental factors such as nutrition.
The twin with the protein-rich diet had the environmental conditions to maximise expression of growth-related genes, reaching a greater height. The other twin, despite having the same alleles, could not express this potential fully due to nutritional limitation. This demonstrates that genotype sets the potential range, but environment determines where within that range the phenotype falls.
A study found that the grandchildren of individuals who experienced famine had higher rates of metabolic disease, even though the grandchildren had plenty of food. Suggest an explanation involving epigenetic mechanisms.
During famine, epigenetic changes such as DNA methylation patterns may have been altered in the grandparents’ cells, including their gametes. Genes involved in metabolism may have been methylated (silenced) or demethylated (activated) in response to the environmental stress of starvation.
These epigenetic modifications can be heritable — passed through the gametes to the next generation without changing the DNA sequence. The grandchildren therefore inherited altered patterns of gene expression that predisposed them to metabolic disease (e.g. storing fat more efficiently), even though their own environment had adequate nutrition.
This is an example of transgenerational epigenetic inheritance.
The human genome contains approximately 20,000 protein-coding genes, yet human cells produce over 100,000 different proteins. Explain how this is possible.
This is possible because of alternative RNA splicing. After transcription, the pre-mRNA contains exons (coding regions) and introns (non-coding regions). The introns are removed, and different combinations of exons can be joined together in different orders.
This means that one gene can produce multiple different mature mRNA molecules, each of which is translated into a different protein. For example, the Drosophila DSCAM gene can produce over 38,000 different mRNA variants through alternative splicing. This is a form of post-transcriptional regulation of gene expression.
Explain how gene expression is regulated at different levels, from transcription to post-translational modification. Include examples of mechanisms at each level. (6 marks)
Structure your answer around these levels:
1. Transcriptional control — transcription factors, promoters, enhancers
2. Epigenetic control — DNA methylation, histone modification
3. Post-transcriptional — alternative RNA splicing
4. Non-coding RNA — X-inactivation, XIST
5. Link to cell differentiation — why does this matter?
Gene expression is regulated at multiple levels. At the transcriptional level, transcription factors are proteins that bind to specific DNA sequences called promoters (near the gene) or enhancers (further away). Activator transcription factors recruit RNA polymerase and increase transcription, while repressors prevent it.
At the epigenetic level, DNA methylation (adding -CH₃ groups to cytosine bases) prevents transcription factors from binding, silencing the gene. Histone modification also plays a role: acetylation of histones loosens the DNA-histone interaction, making DNA accessible for transcription, while deacetylation tightens it, reducing gene expression. These changes are heritable but do not alter the DNA sequence.
At the post-transcriptional level, alternative RNA splicing allows different combinations of exons from the same pre-mRNA to be joined together. This means one gene can produce multiple different proteins, increasing the diversity of the proteome beyond the number of genes.
Non-coding RNA molecules also regulate expression. For example, the XIST gene produces a non-coding RNA that coats one X chromosome in female mammals, causing it to condense into a Barr body and become transcriptionally inactive. This ensures dosage compensation between XX and XY individuals.
These mechanisms collectively allow cell differentiation — cells with identical DNA can express different subsets of genes, producing different proteins and therefore performing different functions.
Evaluate the relative importance of genetic and environmental factors in determining phenotype. Use examples of continuous and discontinuous variation in your answer. (6 marks)
Phenotype is determined by the interaction between genotype and environment, but the balance varies depending on the trait.
Some traits show discontinuous variation — distinct categories with no intermediates. For example, ABO blood group is determined entirely by genotype (the alleles IA, IB, and i). Environment has no effect on blood group. These traits are typically controlled by a single gene with clear dominant/recessive or codominant relationships.
In contrast, traits showing continuous variation — such as height or skin colour — are influenced by both genetics and environment. Height is polygenic (controlled by many genes, each contributing a small additive effect) and is also strongly affected by environmental factors like nutrition. Identical twins with the same genotype may reach different heights if raised in different nutritional environments.
Research using twin studies can estimate the relative contributions. Monozygotic twins share 100% of DNA; differences between them must be environmental. Studies show height has a heritability of ~80% in well-nourished populations, meaning genetics contributes more than environment, but environment still plays a significant role.
Evaluation: Neither factor alone is sufficient. For discontinuous traits, genotype is usually the sole determinant. For continuous traits, both factors interact, and the relative importance depends on the specific trait and population. It is misleading to say one is ‘more important’ without specifying the context.
The table below shows concordance rates for identical (MZ) and non-identical (DZ) twins for various conditions. Concordance rate = the percentage of twin pairs where both twins show the trait.
| Condition | MZ twins | DZ twins |
|---|---|---|
| Schizophrenia | 48% | 17% |
| Type 1 diabetes | 50% | 10% |
| Eye colour | 99% | 28% |
| Height | 95% | 55% |
(a) Which condition is most strongly determined by genetics? Explain your reasoning.
Eye colour has the highest MZ concordance (99%) and the largest difference between MZ and DZ twins. If MZ twins (who share 100% DNA) almost always match, the trait is primarily genetic. The lower DZ rate (28%) shows that sharing ~50% of genes gives much lower concordance.
(b) Schizophrenia has an MZ concordance of only 48%. What does this tell us about the role of environment?
Since MZ twins share 100% of their DNA but the concordance is only 48%, more than half of the variation must be due to environmental factors. If schizophrenia were purely genetic, we would expect ~100% concordance in MZ twins. The 48% rate shows there is a significant genetic predisposition, but environmental triggers (stress, trauma, prenatal factors, epigenetic changes) determine whether the condition actually develops.
(c) Suggest how epigenetic changes could explain why one MZ twin develops type 1 diabetes while the other does not.
Although MZ twins have identical DNA sequences, they may develop different epigenetic modifications over time due to different environmental exposures. For example, DNA methylation patterns may diverge, silencing or activating different genes in each twin. One twin may experience demethylation of immune-related genes, leading to overactive immune responses that attack pancreatic beta cells (an autoimmune response causing type 1 diabetes). The other twin, with different epigenetic marks due to different environmental triggers, may keep these genes silenced and remain unaffected.
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