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3.4.2
DNA and Protein Synthesis
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
14
Total marks
38
Marks / Q
2.7
Accessibility
79.2%
Mastery
54.4%
Student strength
66.6%
01
At a glance
3.4.2 · DNA and Protein Synthesis
8YRSYNTHESIS
3.4.2 (DNA and Protein Synthesis) appeared in 8 of the 8 years between 2017 and 2024, contributing 14 questions and 38 marks across Papers 1, 2 and 3. KNOWLEDGE dominates the mark distribution at 94.7% of total marks. The accessibility–mastery gap sits at 24.8 percentage points (79.2% vs 54.4%) — most students reach partial credit, but full marks remain harder to secure. The largest single question observed is worth 6 marks, signalling that AQA expects complete hierarchical accounts in this sub-section. Mastery varied year-to-year, lowest in 2022 (13.0%) and highest in 2024 (80.0%). Calculation marks are a small share (2.6%) but typically sit at the lower end of the mastery distribution.
Access–mastery gap
+25 pp
Lowest mastery
2022 · 13.0%
Highest mastery
2024 · 80.0%
02
Question type split
By marks · compound to dominant
38MARKS
38
marks
Knowledge94.7%36 marks
Application2.6%1 marks
Calculation2.6%1 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
22TERMS
Tier 1 · Always credit
tRNAcodonanticodonpeptide bondsplicingATP
Tier 2 · Sometimes credit
mRNAintronsexonsrough endoplasmic reticulumcodonsspecific amino acid
Reject · Never credit
any sequence with errorsintrons present in mRNA (disqualifies mp1)introns made of DNAproduction/producesbases alone (not codons/triplets)codes for multiple amino acids4 bases per codonincluding intronslocust (for locus); smooth endoplasmic reticulum; quaternary (single polypeptide context); RER (abbreviation not in spec)hydrolysing hydrogen bonds
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 | 3 | 6 | 84.7% | 75.7% |
| 2018 | 2 | 3 | 73.5% | 67.5% |
| 2019 | 1 | 3 | 90.0% | 33.0% |
| 2020 | 2 | 11 | — COVID | — COVID |
| 2021 | 1 | 3 | — COVID | — COVID |
| 2022 | 1 | 2 | 45.0% | 13.0% |
| 2023 | 0 | 0 | — COVID | — COVID |
| 2024 | 1 | 1 | 85.0% | 80.0% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
27TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| tRNA | 4 | 2017, 2020, 2021, 2025 | |
| codon | 3 | 2017, 2018, 2021 | |
| anticodon | 3 | 2017, 2021, 2025 | |
| peptide bond | 3 | 2017, 2021, 2025 | |
| splicing | 3 | 2017, 2020, 2025 | |
| ATP | 3 | 2020, 2021, 2025 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| mRNA | 2 | 2017, 2025 | |
| introns | 2 | 2017, 2025 | |
| exons | 2 | 2017, 2025 | |
| rough endoplasmic reticulum | 2 | 2019, 2020 | |
| codons | 2 | 2020, 2022 | |
| specific amino acid | 2 | 2020, 2025 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| any sequence with errors | 1 | 2017 | |
| introns present in mRNA (disqualifies mp1) | 1 | 2017 | |
| introns made of DNA | 1 | 2017 | |
| production/produces | 1 | 2018 | |
| bases alone (not codons/triplets) | 1 | 2018 | |
| codes for multiple amino acids | 1 | 2018 | |
| 4 bases per codon | 1 | 2018 | |
| including introns | 1 | 2018 | |
| locust (for locus); smooth endoplasmic reticulum; quaternary (single polypeptide context); RER (abbreviation not in spec) | 1 | 2019 | |
| hydrolysing hydrogen bonds | 1 | 2020 | |
| RNA polymerase forms hydrogen bonds | 1 | 2020 | |
| RNA polymerase joins complementary bases | 1 | 2020 | |
| DNA is universal (unqualified) | 1 | 2022 | |
| genetic code is degenerate | 1 | 2022 | |
| Lys Ala Arg (complementary bases used) | 1 | 2024 |
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
- Introns described as being made of DNA rather than as non-coding RNA sequences removed from pre-mRNA — splicing occurs at the RNA level after transcription; introns are sequences within the pre-mRNA, not DNA fragments; stating they are removed DNA shows confusion about where in the pathway splicing occurs (2017 P1 Q01.4)
- Genetic code described as universal because it is degenerate — these are independent properties; universality means all organisms use the same codons to specify the same amino acids; degeneracy means multiple codons can specify the same amino acid; they are not causally linked and conflating them was the primary error in 2022 (2022 P3 Q04.2)
- ATP's role in amino acid attachment to tRNA omitted — ATP is required to activate each amino acid before it can bind to its specific tRNA; this mark was rarely scored in the 2025 translation question and was cited by the examiner as a systematic gap (2025 P1 Q10.1)
Vocabulary errors
- "Bases" used instead of "codons" or "base triplets" for the degenerate code definition — the definition must reference triplets; "bases" implies single nucleotides coding for amino acids, which is mechanistically wrong (2018 P1 Q05.2)
- "Produces" used instead of "codes for" in genetic code definitions — codons code for amino acids; "produces" implies direct synthesis by the codon itself, which is not the mechanism (2018 P1 Q05.2)
- "RER" used as abbreviation for rough endoplasmic reticulum — RER is not an abbreviation recognised by AQA; using it in place of the full term in 2019 lost the mark for that point (2019 P1 Q04.2)
- Phosphodiester bonds confused with hydrogen bonds during RNA synthesis — RNA polymerase forms phosphodiester bonds between adjacent ribonucleotides; hydrogen bonds form between the template DNA strand and incoming nucleotides; these are distinct bonds at distinct stages and cannot be swapped (2025 P1 Q02.2)
Application errors
- Transcription steps included in a translation-only question — when the question specified translation, including RNA polymerase, mRNA production, and intron splicing earned no marks and sometimes demonstrated that students could not distinguish where transcription ends and translation begins (2017 P1 Q01.2)
- Complementary DNA strand used to read codons instead of the mRNA sequence — in 2024 Q06.2, some students translated the complementary strand AAA GCC CGC rather than the mRNA UUU CGG GCG, producing a completely wrong amino acid sequence; the codon table must be applied to the mRNA, not to the antisense DNA (2024 P1 Q06.2)
- Quaternary structure given for a singular polypeptide — quaternary structure requires more than one polypeptide chain; when a passage referred to "the polypeptide" (singular), answering quaternary was explicitly rejected in 2019; the examiner noted this appeared among students who otherwise scored well (2019 P1 Q04.2)
High-impact failures · examiner narrative
2022 P3 Q04.22 marks
Tested why the genetic code and protein synthesis machinery being universal is useful for genetic engineering. 55% scored zero; only 13.5% scored both marks. The most common errors: giving irrelevant answers about sticky ends and restriction enzymes (which concern DNA cutting, not universality of the code); stating "DNA is universal" without qualification; and claiming the code is universal because it is degenerate. The correct answer required understanding that universality means a gene from one organism will produce the same protein in any other organism because all ribosomes use the same codon-amino acid assignments.
2025 P1 Q10.16 marks
Tested the roles of ribosomes, tRNA, and ATP in translation. Fewer than 15% achieved all six marks. Three marks were disproportionately missed: ATP's role in amino acid activation before tRNA attachment (rarely mentioned); the ribosome covering two codons simultaneously (almost never mentioned); and the codon-anticodon interaction described without both "complementary" and "binding" (partial credit only). Students who earned marks on start codon recognition and polypeptide release often could not access the mechanistic precision required for the middle mark points.
2018 P1 Q05.22 marks
Tested the definition of a degenerate genetic code with an example. 67.5% achieved both marks, leaving a substantial fraction losing marks through vocabulary precision: writing "bases" rather than "base triplets/codons" implies a different coding unit; writing that triplets "produce" amino acids rather than "code for" them misrepresents the relationship. The examiner flagged both as conceptually imprecise rather than genuinely wrong — students understood what degeneracy means but applied vocabulary too loosely to earn the mark.
2019 P1 Q04.23 marks
Tested gene expression terminology in context. The question discriminated well. RER as an abbreviation was not credited, which affected students who used it consistently throughout their answer. Quaternary was rejected when the passage described "the polypeptide" as singular — a context-reading failure that did not reflect a conceptual misunderstanding of quaternary structure but of when to apply it. Transcription and translation were sometimes swapped at positions (3) and (4) in the passage — an ordering error that produced a cascade, since the steps downstream of the swap were also incorrect.
08
Difficulty signals
Performance metric synthesis
25PP GAP
Mean accessibility
79.2%
Mean mastery
54.4%
Mean student strength
66.6%
The accessibility–mastery gap of 24.8 percentage points characterises this sub-section's difficulty profile. Most students reach partial credit; full marks remain harder to achieve. Within 3.4 (Genetic information, variation and relationships), 3.4.2 ranks 6 of 6 sub-sections by mean mastery (1 = hardest). Mastery trajectory is falling across the cohort window: 75.7% in 2017 → 36.7% in 2025 (-39.0 percentage points). Mean mastery was lowest in 2022 (13.0%) and highest in 2024 (80.0%).