Gene expression connects DNA, transcription regulation, epigenetic control, transcript stability, proteomes and environmental signals to phenotype in living cells and tissues.
Gene expression is the mechanism by which information in a gene affects phenotype, most commonly through production and function of a protein.
The DNA base sequence is transcribed into mRNA, the mRNA sequence is translated into a polypeptide, and the folded protein performs a function such as catalysing a reaction. That function contributes to the cell's traits.
Gene information → transcription → mRNA → translation → protein → cellular function → phenotype.
Expression of a gene for a digestive enzyme produces mRNA, then enzyme protein; the enzyme's catalytic activity contributes to the digestive phenotype of that cell.
Possessing a gene does not mean it is expressed in every cell. Regulation can change the amount of mRNA and protein without changing the DNA sequence.
This objective is assessed through structured response, commonly using Explain.
Explain
Build the answer around this relationship: DNA information affects phenotype through gene products.
Representative question
One important chemical in the mobilization of stem cells is a protein, CXCL12, which maintains the stem cells inside the bone marrow. The breakdown of CXCL12 causes the mobilization of stem cells to the blood vessels.
The graph below shows the mobilization of stem cells and the production of mRNA for CXCL12 when the bone marrow is treated with two different chemicals (isoprenaline and clenbuterol).
Explain how the amount of mRNA for CXCL12 gives an indication of the amount of protein CXCL12 produced.
mRNA is translated to protein / involved in protein synthesis
Transcription factors regulate transcription by binding specific DNA sequences such as promoters and enhancers.
| Regulatory element | Role |
|---|---|
| Promoter | Region near the gene where RNA polymerase and transcription factors assemble to begin transcription |
| Enhancer | Regulatory sequence that increases transcription when an activator transcription factor binds |
| Repressor-bound sequence | Binding can reduce polymerase recruitment or activity and lower transcription |
Different cell types contain different combinations or activities of transcription factors, so the same genome can produce different amounts of a gene's mRNA.
An activator bound to an enhancer can contact the promoter complex and increase how often RNA polymerase initiates transcription.
A transcription factor recognizes a particular DNA sequence; it does not bind every promoter or enhancer indiscriminately.
This objective is assessed through structured response, multiple choice, commonly using Suggest / Explain.
Suggest / Explain
Promoters mark where RNA polymerase can bind to begin transcription, while transcription factors bind specific DNA sequences to activate or repress transcription.
Treating promoters as translation start sites instead of RNA polymerase binding regions.
Representative question
Explain how gene expression can be regulated during transcription to determine an organism's phenotype.
a. transcription factors bind to DNA and can be activators or repressors OR
transcription factors promote or block transcription;
b. protein/transcription factor/TF binds to promoter to activate transcription OR
promoter indicates where RNA polymerase binds to begin transcription;
c. transcription factor/activator/protein binds to enhancer to increase the rate of transcription
OR
transcription factor/repressor/protein binds to silencer to decrease the rate of transcription;
d. epigenic tags can alter gene expression / methylation can alter gene expression;
e. methylation of cytosine in the DNA of a promoter represses transcription;
f. methylation of amino acids/protein in histones can cause transcription to be repressed or activated;
g. thereby regulating the production of proteins from the gene
OR
proteins from gene expression display/create phenotypic traits;
h. gene expression may be altered by (steroid) hormones/biochemicals that act as chemical signals to activate receptors that promote transcription;
i. alteration of methyl tags on DNA in response to air pollution or in response to the environment;
j. valid example of gene regulation;
7
Marking guidance:
max
Regulating mRNA degradation controls how long an mRNA remains available for translation and therefore how much protein can be produced.
Human mRNAs can persist from minutes to days. Shortening of the protective 3′ poly-A tail helps initiate degradation, after which nucleases break down the transcript.
Longer mRNA lifetime → more opportunities for ribosomes to translate it. Faster degradation → fewer translation rounds and a shorter-lived protein-production response.
If two cells transcribe equal numbers of mRNA molecules but one transcript is degraded within minutes while the other persists for hours, the longer-lived transcript can usually support more translation.
mRNA abundance reflects both its rate of transcription and its rate of degradation; high transcription alone does not guarantee high protein output.
Epigenesis is the development of patterns of cell differentiation in a multicellular organism from an initially undifferentiated zygote.
Cells descended from the zygote usually contain the same DNA sequence, but different genes become active or repressed. Epigenetic changes such as DNA methylation or histone modification alter how genes are read without altering their base sequences.
One zygote → repeated cell division → different epigenetic states and gene-expression patterns → different proteins → specialized cell phenotypes.
A developing muscle cell activates genes for contractile proteins while maintaining repression of genes needed only in neurons, producing differentiation without changing genotype.
Epigenetic change can alter phenotype but not genotype because the nucleotide sequence remains unchanged. It is therefore not a gene mutation.
A cell's genome is its complete genetic information, its transcriptome is the set of mRNA transcripts present at a particular time, and its proteome is the set of proteins present at that time.
| Layer | What it contains | How it varies |
|---|---|---|
| Genome | Complete DNA information | Usually shared by an individual's somatic cells and relatively stable |
| Transcriptome | mRNAs produced in a cell or tissue at that time | Changes with cell type, development and environment |
| Proteome | Proteins present in a cell, tissue or organism at that time | Dynamic because translation, processing and degradation change protein abundance |
A liver cell and muscle cell usually share a genome but express different genes, so their transcriptomes and proteomes differ and support different functions.
No cell expresses all its genes. Detecting an mRNA also does not guarantee that its protein is abundant or functional.
This objective is assessed through multiple choice, commonly using State / Calculate / Explain.
State / Calculate / Explain
Build the answer around this relationship: The genome is the complete genetic information.
Assuming the genome and proteome are identical in all tissues.
Representative question
Which statement correctly describes genome and proteome?
Only the genome but not the proteome can be analysed using gel electrophoresis.
The genome and the proteome are the same in all tissues in an organism.
In cells of different tissues, the genome is the same while the proteome varies.
Only mutations in the proteome but not in the genome cause any variability.
C
Methyl groups added to promoter DNA or to histones act as epigenetic tags that alter transcription without changing DNA base sequence.
| Tag location | Typical expression effect |
|---|---|
| Cytosine in a DNA promoter | Methylation represses transcription of the downstream gene |
| Amino acids in histone proteins | Methylation can repress or activate transcription, depending on the site and context |
The tags change which regulatory proteins can associate with chromatin and how accessible the promoter is to transcription machinery.
Extensive methylation of promoter cytosines can make a gene less accessible, reducing its mRNA and protein production.
Do not apply one universal rule to all methylation: promoter DNA methylation is generally repressive, while histone methylation may activate or repress.
This objective is assessed through structured response, commonly using Outline / Suggest / Explain.
Outline / Suggest / Explain
Build the answer around this relationship: Promoter DNA methylation usually represses transcription.
Describing histone methylation when the question specifically asks about DNA methylation.
Representative question
Explain how methylation of nucleosomes affects DNA transcription.
a. methylation of amino acids/histones;
b. (can cause) transcription to be inhibited/repressed/activated / OWTTE;
c. methylation of cytosine causes transcription to be repressed/inhibited;
d. (repressed as histones) restricts access to the DNA;
e. (activated as) allows transcription factors to bind to DNA;
f. methylation of promoter (DNA) represses transcription
g. results in decreased gene (downstream) expression/gene silencing / OWTTE;
b. Accept "transcription more difficult".
d. Accept other wording such as "less accessible".
2
Marking guidance:
max
Epigenetic inheritance passes a change in gene expression to daughter cells or offspring without changing the nucleotide sequence of DNA.
If DNA methylation or histone modifications remain in place or are re-established during chromosome replication and division, the inherited chromatin state can keep a gene active or repressed.
During mitosis, persistent tags can transmit a cell-lineage expression pattern to daughter cells. If tags remain through gamete formation and meiosis, an expression state may influence offspring.
A repressive mark maintained through repeated mitoses can help daughter cells retain the same differentiated identity even though every cell division copies the same DNA sequence.
Epigenetic inheritance is inheritance of expression state, not a change in allele sequence, and most tags are not guaranteed to persist through gamete formation.
Environmental exposure can alter gene expression by changing epigenetic tags; air pollution can modify DNA and histone methylation patterns.
Pollutants reaching lung tissue can trigger cellular responses associated with altered methylation. Changed access to regulated genes can affect inflammatory pathways and may contribute to lung disease without changing the DNA sequence.
Air-pollution exposure → altered DNA or histone methylation → altered transcription pattern → changed cell response → possible contribution to inflammation or respiratory disease.
Comparing exposed and less-exposed lung-cell samples may reveal different methylation and expression patterns associated with inflammatory responses.
An association between pollution, methylation and disease does not by itself prove that one methylation change caused the disease; exposure, dose and alternative causes must be evaluated.
This objective is assessed through essay response, multiple choice, commonly using Compare / Suggest / Discuss.
Compare / Suggest / Discuss
Build the answer around this relationship: Environmental conditions can alter gene expression.
Treating altered expression as a necessary DNA base-sequence mutation.
Representative question
Using the data in the bar chart, discuss the evidence for Arabidopsis plants adapting to different daylight regimes by changing the pattern of gene expression.
a. «mRNA» transcripts differ in plants grown in long days and short days
b. indicates different genes are being expressed
c. plants adapt to different daylight regimes by altering gene expression
d. short day length causes struggle to get enough light to photosynthesize and more «mRNA» transcripts related to photosynthesis
OR
plants produce large leaves rapidly when grown in long days which may result in more transcripts for biotic stress
During human egg and sperm development, most epigenetic tags are removed, but retained imprints can make only the maternal or paternal copy of a gene active in offspring.
Resetting prevents most acquired expression states being passed between generations. For an imprinted gene, a retained parent-specific tag silences one allele, so phenotype depends on whether the active copy came from the mother or father.
| Hybrid cross | Epigenetic growth outcome |
|---|---|
| Male tiger × female lion → tigon | Tiger paternal genes lack the lion's strong growth promotion, while the lioness contributes anti-growth imprinting; the hybrid is about parental size or smaller |
| Male lion × female tiger → liger | Lion paternal growth promotion is not opposed by the tigress's imprints in the same way; the hybrid can grow larger than either parent |
The hybrid pattern supports a parent-of-origin epigenetic explanation, but the exact imprinted genes making the largest growth difference are not established in the approved local textbook.
This objective is assessed through multiple choice.
Some marks, such as imprints, may be retained in specific cases and can affect offspring phenotypes.
Representative question
Very soon after fertilization, parental epigenetic methylation is reversed in the DNA. Later, tissue-specific epigenetic modifications are made to the embryonic DNA. The graph follows the degree of methylation from different sources during embryonic development.
According to the graph, what are the changes in DNA methylation during embryonic development?
Only the paternal DNA becomes demethylated.
The maternal DNA becomes demethylated first.
The methylation patterns of the parents' DNA are erased before fertilization.
The methylation patterns of both parents are erased after fertilization.
D
Studies of monozygotic twins compare genetically similar individuals to estimate how environment and epigenetic differences contribute to traits.
If twins differ despite near-identical DNA, differing environments, developmental history or epigenetic states are possible explanations. Concordance and study design determine the strength of inference.
Evaluate a twin result by checking: shared genes; shared environment; age and exposure; trait concordance; alternative causes.
Twins may both inherit risk alleles but develop different symptoms after different exposures, suggesting environment modifies expression or phenotype.
Twin differences do not prove a purely environmental cause; measurement error and non-shared biology also matter.
This objective is assessed through data analysis, multiple choice, commonly using Identify / Compare / Analyse.
Identify / Compare / Analyse
Build the answer around this relationship: Monozygotic twins share essentially the same genome.
Using only numerical values without comparing identical and non-identical twins.
Representative question
Analyse the data to find whether it supports the hypothesis that genetic factors cause some people to have a much higher chance of cocaine dependence than others.
hypothesis supported as identical twins are more likely to behave the same for abuse and dependence than non-identical twins;
identical twins have the same genotype / OWTTE;
hypothesis not supported as environment is the major factor for trying cocaine;
not known if similar results may be due to similar environment;
not enough data for valid statistical analysis/OWTTE;
External chemical signals can change gene expression in both eukaryotes and bacteria, but the regulatory mechanism depends on the cell type.
| External factor | Mechanism and expression outcome |
|---|---|
| Steroid hormone in a eukaryotic cell | The hormone binds an intracellular receptor; the activated receptor acts with transcriptional regulators at DNA and changes transcription |
| Lactose in E. coli | Without lactose, a repressor binds the lac operator and blocks transcription. Lactose binds and inactivates the repressor, allowing transcription of genes needed for lactose metabolism |
Both mechanisms connect an external condition to selective protein production, so the cell makes a response only when the signal or substrate is present.
The lac operon is an inducible bacterial system and is not the mechanism used by steroid hormones in eukaryotic cells. One biochemical example is sufficient for the syllabus scope.