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3.1.5
Nucleic Acids
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
14
Total marks
39
Marks / Q
2.8
Accessibility
68.4%
Mastery
34.4%
Student strength
58.2%
01
At a glance
3.1.5 · Nucleic Acids
8YRSYNTHESIS
3.1.5 (Nucleic Acids) appeared in 7 of the 8 years between 2017 and 2024, contributing 14 questions and 39 marks across Papers 1, 2 and 3. KNOWLEDGE dominates the mark distribution at 79.5% of total marks. The accessibility–mastery gap sits at 34.0 percentage points (68.4% vs 34.4%) — 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 2023 (15.0%) and highest in 2017 (92.0%). Calculation marks are a small share (5.1%) but typically sit at the lower end of the mastery distribution.
Access–mastery gap
+34 pp
Lowest mastery
2024 · 15.0%
Highest mastery
2017 · 92.0%
02
Question type split
By marks · compound to dominant
39MARKS
39
marks
Knowledge79.5%31 marks
Application15.4%6 marks
Calculation5.1%2 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
23TERMS
Tier 1 · Always credit
histoneshydrogen bondsphosphodiester bondsdeoxyribosedouble helix
Tier 2 · Sometimes credit
DNA polymerasecircularplasmidsnucleotidesphosphatenitrogenous baseadenine-thyminecytosine-guanine
Reject · Never credit
RNA polymerasedifferent allelesstain propertiesdifferent amounts of DNAsingle-strandedsmallerin cytoplasm (without further detail)helicase hydrolyses H bondsDNA polymerase joins complementary nucleotides (forming H bonds)description of data only
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 | 1 | 2 | 92.0% | 92.0% |
| 2018 | 4 | 9 | 58.8% | 28.2% |
| 2019 | 0 | 0 | — COVID | — COVID |
| 2020 | 4 | 9 | — COVID | — COVID |
| 2021 | 2 | 7 | — COVID | — COVID |
| 2022 | 1 | 3 | 90.0% | 40.0% |
| 2023 | 1 | 6 | 75.0% | 15.0% |
| 2024 | 1 | 3 | 55.0% | 15.0% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
28TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| histones | 3 | 2018, 2020, 2023 | |
| hydrogen bonds | 3 | 2018, 2021, 2023 | |
| phosphodiester bonds | 3 | 2018, 2020, 2023 | |
| deoxyribose | 3 | 2021, 2022, 2023 | |
| double helix | 3 | 2021, 2022, 2023 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| DNA polymerase | 2 | 2017, 2018 | |
| circular | 2 | 2018, 2020 | |
| plasmids | 2 | 2018, 2020 | |
| nucleotides | 2 | 2018, 2023 | |
| phosphate | 2 | 2021, 2023 | |
| nitrogenous base | 2 | 2021, 2023 | |
| adenine-thymine | 2 | 2021, 2023 | |
| cytosine-guanine | 2 | 2021, 2023 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| RNA polymerase | 1 | 2017 | |
| different alleles | 1 | 2018 | |
| stain properties | 1 | 2018 | |
| different amounts of DNA | 1 | 2018 | |
| single-stranded | 1 | 2018 | |
| smaller | 1 | 2018 | |
| in cytoplasm (without further detail) | 1 | 2018 | |
| helicase hydrolyses H bonds | 1 | 2018 | |
| DNA polymerase joins complementary nucleotides (forming H bonds) | 1 | 2018 | |
| description of data only | 1 | 2018 | |
| cyclin A is an enzyme | 1 | 2018 | |
| forms hydrogen bonds | 1 | 2020 | |
| joins complementary bases | 1 | 2020 | |
| nucleotide bases | 1 | 2020 | |
| prokaryotic DNA only exists as plasmids | 1 | 2020 |
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
- Prokaryotic DNA is assumed to be single-stranded — the most common conceptual error in 2018, capping many responses at one mark; prokaryotic DNA is a double helix exactly as eukaryotic DNA is, the structural differences are circularity and histone absence, not strandedness (2018 P1 Q01.6)
- Helicase is described as "hydrolysing" hydrogen bonds — AQA explicitly rejects this phrasing; helicase breaks hydrogen bonds but does not catalyse a hydrolysis reaction, and the distinction was enforced at mark-scheme level in 2018 (2018 P1 Q09.1)
- DNA polymerase is described as forming hydrogen bonds or joining complementary bases — DNA polymerase catalyses phosphodiester bond formation between adjacent nucleotides; including hydrogen bonding as part of its role negated mark point 2 and was the primary reason only 24.4% achieved full marks (2018 P1 Q09.1)
- tRNA is described as double-stranded or possessing a double helix — tRNA is single-stranded and folds into a clover-leaf shape via intrastrand hydrogen bonding; the helix misconception conflates structural folding with double-strandedness (2024 P1 Q06.1)
- mRNA is assigned thymine and tRNA is assigned uracil — both are RNA molecules and both contain uracil; only DNA contains thymine; this inverted recall error appeared explicitly in examiner notes for 2024 and represents a confident wrong answer, not a gap (2024 P1 Q06.1)
Vocabulary errors
- Base initials (A, T, G, C) written instead of full names — not credited in 2022 or 2023; adenine, thymine, guanine, cytosine must be spelled out; the examiner note "ignore: bases identified with letters" has appeared in consecutive years, confirming this is a persistent loss (2022 P1 Q07.1, 2023 P1 Q10.2)
- "Helix" written without qualifying it as "double helix" — the mark scheme requires the full descriptor; "helix" alone is insufficient and was penalised explicitly in 2022 Q07.1 (2022 P1 Q07.1)
- "Polymer of nucleotides" omitted — this was the mark point observed least frequently in the 2023 Q10.2 extended response; students described structural components in detail but missed the initial classification statement that is itself a direct mark point (2023 P1 Q10.2)
- "tRNA has an amino acid" written instead of "amino acid binding site" — the binding site is the credited concept, not the molecule it carries; this level of precision was flagged explicitly in the 2024 examiner report (2024 P1 Q06.1)
Application errors
- Data described rather than explained — in the cyclin A question (2018 P1 Q09.2), restating which treatments showed more or less replication earned zero; the question word "suggest" required a mechanistic explanation; 15.4% made no attempt, the highest non-attempt rate in the 3.1.5 dataset
- A structural feature named without explaining how it differs along the chromosome's length — in 2018 P1 Q01.4, "histones" or "bases" alone was not credited; the question asked how that feature varies along the chromosome to produce differential staining, not merely what the feature is
- Comparative statements made for only one molecule — in the tRNA/mRNA comparison questions (2022 P1 Q07.1, 2024 P1 Q06.1), students listed properties of one molecule without providing the paired comparison; "mRNA has codons but tRNA does not" was explicitly noted as an invalid A-level comparison
- Prokaryotic description replaced with eukaryotic description — stating what eukaryotic DNA does rather than describing prokaryotic arrangement earned no credit even when factually accurate; converse statements were not accepted (2018 P1 Q01.6)
High-impact failures · examiner narrative
2018 P1 Q09.23 marks12%full marks
Tested application of knowledge about DNA replication regulation using a cyclin A experimental dataset with four treatment groups. Only 12% scored full marks, and 15.4% made no attempt. The examiner reported that most students described the data rather than explaining the mechanism: the common answer for treatment D was that replication was reduced, where the mark scheme required "antibody binds cyclin A so it cannot bind to DNA/enzyme/initiate replication." Treatment F, designed as a rescue control to confirm cyclin A's function, was rarely identified as such. The question also exposed a systematic error: many students described cyclin A as an enzyme, which was explicitly rejected — it is a protein that initiates replication, not a catalyst.
2018 P1 Q09.13 marks
Tested recall of helicase and DNA polymerase roles in semi-conservative replication. Only 24.4% achieved all three marks despite the question being flagged as AO1 recall from a named specification section. Two phrasing errors concentrated the failure: "helicase hydrolyses hydrogen bonds" was explicitly rejected at mark point 1 (the correct verb is "breaks"), and describing DNA polymerase as forming hydrogen bonds between complementary bases negated mark point 2. The second error is particularly costly because it both fails to earn the mark and actively cancels a correct preceding statement — students lose more than one mark from a single well-intentioned sentence.
2023 P1 Q10.26 marks15%full marks
Tested the structure of DNA and chromosomes in a 6-mark extended response. Only 15% achieved full marks; more than half scored 4. The examiner identified two consistent loss points: "polymer of nucleotides" was the least frequently observed mark point despite being a direct specification term — students opened their answers mid-structure and never stated the classification — and base names were given as single letters, which has been rejected across multiple years and recurred here. The chromosome portion (two chromatids joined at centromere) was well handled; marks 5–6 on the DNA structural account were disproportionately missed.
2024 P1 Q06.13 marks15%full marks
Tested structural differences between mRNA and tRNA. Only 15% scored full marks, and the examiner listed multiple explicit misconceptions: mRNA has thymine and tRNA has uracil (inverted — both have uracil), tRNA is double-stranded or forms a double helix (it is single-stranded), and tRNA contains exons. A blank table was provided to direct students toward comparative statements, but some still listed individual properties on single rows without pairing them. The "amino acid binding site" versus "amino acid" precision distinction was flagged: naming what tRNA carries is insufficient, the binding site must be specified.
08
Difficulty signals
Performance metric synthesis
34PP GAP
Mean accessibility
68.4%
Mean mastery
34.4%
Mean student strength
58.2%
The accessibility–mastery gap of 34.0 percentage points characterises this sub-section's difficulty profile. Most students reach partial credit; full marks remain harder to achieve. Within 3.1 (Biological molecules), 3.1.5 ranks 3 of 7 sub-sections by mean mastery (1 = hardest). Mastery trajectory is falling across the cohort window: 92.0% in 2017 → 15.0% in 2024 (-77.0 percentage points). Mean mastery was lowest in 2023 (15.0%) and highest in 2017 (92.0%).