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3.8.2
Gene Expression and Control
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
50
Total marks
123
Marks / Q
2.5
Accessibility
54.8%
Mastery
21.2%
Student strength
30.8%
01
At a glance
3.8.2 · Gene Expression and Control
8YRSYNTHESIS
3.8.2 (Gene Expression and Control) appeared in 7 of the 8 years between 2017 and 2024, contributing 50 questions and 123 marks across Papers 1, 2 and 3. APPLICATION dominates the mark distribution at 77.2% of total marks. The accessibility–mastery gap sits at 33.6 percentage points (54.8% vs 21.2%) — most students reach partial credit, but full marks remain harder to secure. The largest single question observed is worth 5 marks, signalling that AQA expects complete hierarchical accounts in this sub-section. Mastery varied year-to-year, lowest in 2022 (6.0%) and highest in 2017 (25.0%). Calculation marks are a small share (13.0%) but typically sit at the lower end of the mastery distribution.
Access–mastery gap
+34 pp
Lowest mastery
2022 · 6.0%
Highest mastery
2017 · 25.0%
02
Question type split
By marks · compound to dominant
123MARKS
123
marks
Application77.2%95 marks
Calculation13.0%16 marks
Knowledge9.8%12 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
28TERMS
Tier 1 · Always credit
transcription factortranscriptionRNA polymerasemethylationcomplementarytumour suppressor genepromoter
Tier 2 · Sometimes credit
acetylationhistonesssRNAmRNAtranslationrejection/immune responseprevents translationlogarithmic scalesmall samplesignificant differencephospholipid bilayer
Reject · Never credit
active siteenzymesubstrateinduced fitgenes being expressed/switched on alonetranslationgenderlifestyle (too vague)less made (insufficient)transcription instead of translation
04
Question patterns
Recurring formats & tariff structure
0PARAGRAPHS
05
Year-over-year presence
P1 + P3 · 2017–2024
8YEARS
| Year | Questions | Total marks | Mean accessibility | Mean mastery |
|---|---|---|---|---|
| 2017 | 5 | 11 | 52.4% | 25.0% |
| 2018 | 12 | 28 | 53.2% | 20.9% |
| 2019 | 11 | 22 | 56.6% | 24.4% |
| 2020 | 9 | 23 | — COVID | — COVID |
| 2021 | 8 | 22 | — COVID | — COVID |
| 2022 | 2 | 6 | 37.5% | 6.0% |
| 2023 | 3 | 11 | 70.3% | 14.7% |
| 2024 | 0 | 0 | — COVID | — COVID |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
33TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| transcription factor | 5 | 2017, 2018, 2020, 2022 | |
| transcription | 3 | 2017, 2018, 2022 | |
| RNA polymerase | 3 | 2017, 2020, 2022 | |
| methylation | 3 | 2017, 2018, 2022 | |
| complementary | 3 | 2017, 2020, 2021 | |
| tumour suppressor gene | 3 | 2018, 2021, 2022 | |
| promoter | 3 | 2018, 2020, 2022 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| acetylation | 2 | 2017, 2022 | |
| histones | 2 | 2017, 2022 | |
| ssRNA | 2 | 2017 | |
| mRNA | 2 | 2017 | |
| translation | 2 | 2017, 2021 | |
| rejection/immune response | 2 | 2018, 2020 | |
| prevents translation | 2 | 2018, 2019 | |
| logarithmic scale | 2 | 2018, 2019 | |
| small sample | 2 | 2019 | |
| significant difference | 2 | 2019, 2023 | |
| phospholipid bilayer | 2 | 2020, 2021 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| active site | 4 | 2018, 2020, 2022 | |
| enzyme | 2 | 2020 | |
| substrate | 2 | 2020 | |
| induced fit | 2 | 2020 | |
| genes being expressed/switched on alone | 1 | 2017 | |
| translation | 1 | 2017 | |
| gender | 1 | 2017 | |
| lifestyle (too vague) | 1 | 2017 | |
| less made (insufficient) | 1 | 2017 | |
| transcription instead of translation | 1 | 2017 | |
| decreased acetylation | 1 | 2018 | |
| protein not formed (too vague) | 1 | 2018 | |
| binds to transcription factor | 1 | 2018 | |
| green cones (without 'sensitive'); grow (without differentiation); green-sensitive pigment (not cones) | 1 | 2018 | |
| non-functional TF unqualified (given in stem) | 1 | 2018 |
Marks in this sub-section are typically awarded for precise terminology and correct application of biological principles. Sequential mark schemes — where each mark requires building on the previous one — are common in multi-mark questions; stating the first step without progression rarely earns more than one mark. Calculation marks are typically split between method (correct setup and value extraction) and answer (accurate numerical result), allowing partial credit when arithmetic errors occur.
07
Where students lose marks
Examiner-anchored error patterns
4CASE STUDIES
Conceptual errors
- Describing RNA interference as siRNA binding to DNA or preventing transcription — siRNA binds complementary mRNA sequences and either blocks translation or triggers mRNA degradation; transcription is unaffected
- Attributing lactase persistence to a mutation in the lactase gene itself, rather than a mutation in the gene encoding the transcription factor that normally suppresses lactase gene transcription in adults
- Confusing the direction of epigenetic modifications: increased methylation of DNA inhibits transcription; increased acetylation of histones opens chromatin and promotes transcription — students frequently applied each modification to the wrong molecule or omitted the direction entirely
- Treating iPS cell derivation as a single-step reprogramming event; the resulting stem cells must then undergo cell division before they can differentiate into the target cell type
- Equating "genes switched on/off" or "gene expression changes" with a full mechanistic answer — without specifying transcription factor binding, methylation state, or chromatin remodelling, this language earns no marks in multi-step schemes
Vocabulary errors
- "Grow" in place of "divide" or "cell division" for iPS cells — the mark scheme explicitly requires the cell division step; "grow" was rejected across multiple years
- "Less protein made" without the intermediate chain — methylation → transcription factor cannot bind promoter → less mRNA produced → less protein; omitting any link in the chain loses marks in sequential schemes
- Naming acetylcholine or mutagenic agents when asked about the molecular mechanism of epigenetic change — these are potential environmental triggers, not the mechanism itself
- "Binds to the gene/DNA" when describing siRNA action — siRNA binds mRNA, not DNA; this substitution was explicitly rejected in 2017, 2018, and 2019 mark schemes
- "Green cones" without specifying "sensitive to [green] light" — the full descriptor is required; the abbreviated form was rejected in 2018
Application errors
- On lactase persistence: attributing the molecular change to a mutation in the transcription factor protein rather than in the gene encoding it — the mark scheme requires mutation in the TF gene or in the promoter region, not a change to the TF protein directly
- On siRNA mechanism questions in context: giving a rote account of how RNAi works generally without applying it to the specific gene or protein named in the stem — responses not connected to CENP-W or PIP1 scored zero
- On evaluation questions about iPS cells or gene therapy: describing the mechanism of the therapy rather than comparing it to the alternative treatment — the evaluative framing requires both advantages and limitations, and students who described only the mechanism failed to score
- On mRNA quantification questions involving bacterial populations: treating the measured mRNA level as a direct indicator of population size or cell number — the bacteria were never counted; mRNA concentration reflects gene expression, not population growth, and the link between the two must be explicitly questioned
High-impact failures · examiner narrative
2018 P3 Q06.54 marks
The lowest mastery recorded in this subtopic at approximately 2%, with 11% of students not attempting the question. Students needed to identify four distinct points: bacteria were never directly counted at any stage; the mRNA copies measured reflect amoA gene expression, not cell number; the amount of mRNA produced per cell may vary across bacteria; and it is not established that amoA expression is linked to cell division or population growth. Almost every student either conflated mRNA quantity with population count or gave a generic statement about gene expression. The four-mark structure with a single command word ("explain") required four independently reasoned points, but students concentrated on one or two at length and omitted the rest.
2018 P2 Q10.52 marks
Approximately 3% mastery, with 48% scoring zero. The question asked how iPS cells could correct red-green colour blindness. The consistently missing step was that reprogrammed iPS cells must divide before differentiating — students knew the cells would differentiate but omitted division entirely. "Grow" was not credited and "green cones" without "sensitive" was also penalised. The combination of two vocabulary-level traps (grow/divide and green cones/green-sensitive cones) in a two-mark question meant that students who understood the biology in principle still scored zero on both points.
2018 P2 Q09.32 marks
Only 18% of students scored at least one mark — the second-lowest accessibility figure in this subtopic. The question required identifying that a mutation occurred in the gene encoding the transcription factor (not the lactase gene itself), changing the TF's tertiary structure so it could no longer bind the lactase gene promoter, thereby allowing continued lactase gene transcription. The dominant error was misidentifying the target: students named the lactase gene, cited a frameshift in an adjacent gene, or described methylation or acetylation as the mechanism. Lactase/lactose confusion also appeared. Students who understood that a TF was involved often located the mutation in the TF protein rather than in the gene coding for it, which was not credited.
2017 P3 Q06.53 marks
Approximately 15% mastery, with 63% of students scoring zero. The question required explaining how foreign DNA produced ssRNA that specifically inhibited PIP1 expression via RNA interference. The near-universal errors were: describing the ssRNA as binding to DNA (rejected), stating that transcription is prevented rather than translation (wrong process), and concluding "less protein is made" without specifying the binding mechanism that causes the reduction. Students who described RNAi at the DNA level scored nothing; the critical distinction between transcriptional inhibition and translational silencing via mRNA binding was the conceptual hurdle that most students could not clear.
08
Difficulty signals
Performance metric synthesis
34PP GAP
Mean accessibility
54.8%
Mean mastery
21.2%
Mean student strength
30.8%
The accessibility–mastery gap of 33.6 percentage points characterises this sub-section's difficulty profile. Most students reach partial credit; full marks remain harder to achieve. Within 3.8 (The control of gene expression), 3.8.2 ranks 1 of 4 sub-sections by mean mastery (1 = hardest). Mastery trajectory is falling across the cohort window: 25.0% in 2017 → 14.7% in 2023 (-10.3 percentage points). Mean mastery was lowest in 2022 (6.0%) and highest in 2017 (25.0%).