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3.4.1
DNA, Genes and Chromosomes
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
6
Total marks
15
Marks / Q
2.5
Accessibility
63.0%
Mastery
35.8%
Student strength
49.5%
01
At a glance
3.4.1 · DNA, Genes and Chromosomes
8YRSYNTHESIS
3.4.1 (DNA, Genes and Chromosomes) appeared in 6 of the 8 years between 2017 and 2024, contributing 6 questions and 15 marks across Papers 1, 2 and 3. KNOWLEDGE dominates the mark distribution at 80.0% of total marks. The accessibility–mastery gap sits at 27.2 percentage points (63.0% vs 35.8%) — 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 2023 (25.0%) and highest in 2022 (41.0%).
Access–mastery gap
+27 pp
Lowest mastery
2023 · 25.0%
Highest mastery
2022 · 41.0%
02
Question type split
By marks · compound to dominant
15MARKS
15
marks
Knowledge80.0%12 marks
Application20.0%3 marks
Calculation0.0%0 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
11TERMS
Tier 1 · Always credit
Tier 2 · Sometimes credit
genome
Reject · Never credit
same allelesone from each parentidentical chromosomeshydrolyses hydrogen bonds; A (for adenine — abbreviation not in spec); DNA polymerase catalyses complementary base pairingpositioned in intronsbetween exonsdoes not code for an amino acid (singular)DNA/genes within species/populationnumber of proteins (unqualified)genetic information
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 | 0 | 0 | — COVID | — COVID |
| 2018 | 1 | 1 | 37.0% | 37.0% |
| 2019 | 1 | 5 | 75.0% | 40.0% |
| 2020 | 1 | 2 | — COVID | — COVID |
| 2021 | 1 | 3 | — COVID | — COVID |
| 2022 | 1 | 2 | 70.0% | 41.0% |
| 2023 | 1 | 2 | 70.0% | 25.0% |
| 2024 | 0 | 0 | — COVID | — COVID |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
15TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| genome | 2 | 2022, 2023 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| same alleles | 1 | 2018 | |
| one from each parent | 1 | 2018 | |
| identical chromosomes | 1 | 2018 | |
| hydrolyses hydrogen bonds; A (for adenine — abbreviation not in spec); DNA polymerase catalyses complementary base pairing | 1 | 2019 | |
| positioned in introns | 1 | 2020 | |
| between exons | 1 | 2020 | |
| does not code for an amino acid (singular) | 1 | 2020 | |
| DNA/genes within species/population | 1 | 2022 | |
| number of proteins (unqualified) | 1 | 2022 | |
| genetic information | 1 | 2023 | |
| genetic code | 1 | 2023 | |
| genetic constitution | 1 | 2023 | |
| all DNA in a species/population | 1 | 2023 | |
| all proteins in a cell | 1 | 2023 |
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
1CASE STUDIES
Conceptual errors
- Homologous chromosomes defined as having the same alleles rather than the same genes — "same genes at the same loci" is the specification requirement; "same alleles" implies identical base sequences at every locus, which would mean every organism in a species was genetically identical; only 36.9% scored the 2018 mark (2018 P1 Q01.5)
- Homologous chromosomes confused with sister chromatids — sister chromatids are the two identical strands of a single replicated chromosome joined at the centromere; homologous chromosomes are the two versions of the same chromosome, one inherited from each parent; this confusion appeared in both 2018 and 2019 examiner notes (2018 P1 Q01.5, 2019 P1 Q06.2)
- DNA polymerase role conflated with complementary base pairing — in 2019, students described DNA polymerase as joining bases together rather than catalysing the formation of phosphodiester bonds between adjacent nucleotides; complementary base pairing positions nucleotides but DNA polymerase performs the covalent bond formation (2019 P1 Q06.2)
- Genome or proteome defined for a species rather than for an individual organism or cell — this error appeared in both 2022 and 2023; the genome is the complete DNA of one organism; the proteome is all the proteins one cell can produce; extending either definition to a species or population is rejected (2022 P3 Q04.1, 2023 P1 Q06.1)
Vocabulary errors
- "Genetic information" or "genetic code" or "genetic constitution" used to define genome — none of these phrases earns the mark; the genome must be defined as all the DNA of an organism; the AQA examiner explicitly listed all three rejected phrasings in 2023 (2023 P1 Q06.1)
- "All proteins in a cell" rather than "all proteins a cell can produce" — the "can produce" qualifier is essential; it distinguishes the proteome from the subset of proteins currently expressed; missing this qualifier was the most common precision failure across both 2022 and 2023 (2022 P3 Q04.1, 2023 P1 Q06.1)
- "A" used instead of "adenine" in sequence answers — base-name abbreviations are not credited in AQA mark schemes; single letters must be written as full names; this was penalised in the DNA replication question (2019 P1 Q06.2)
Application errors
- Bases described as joining during DNA replication rather than nucleotides — the phosphodiester bonds form between the phosphate group of one nucleotide and the sugar of the next; describing "bases joining" omits the sugar-phosphate backbone and does not earn the mark (2019 P1 Q06.2)
- Paternal/maternal origin given as the definition of homologous chromosomes — describing where the chromosomes came from explains inheritance but does not define homology; the structural definition (same genes at the same loci) is what AQA requires (2018 P1 Q01.5)
High-impact failures · examiner narrative
2023 P1 Q06.12 marks
Tested definitions of genome and proteome. 70% scored at least one mark, but precision failures were widespread on both definitions. For genome, "the genetic information/code of a cell" earned zero; AQA requires "all the DNA" with reference to a specific organism. For proteome, "all the proteins in a cell" was rejected because it describes currently expressed proteins, not the full set a cell is capable of producing — the "can produce" qualifier was the mark. The examiner also noted that some students used "expression" incorrectly ("the proteome is the expression of all proteins in a cell"), demonstrating confusion between the proteome concept and gene expression.
08
Difficulty signals
Performance metric synthesis
27PP GAP
Mean accessibility
63.0%
Mean mastery
35.8%
Mean student strength
49.5%
The accessibility–mastery gap of 27.2 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), 3.4.1 ranks 4 of 6 sub-sections by mean mastery (1 = hardest). Mastery trajectory is falling across the cohort window: 37.0% in 2018 → 25.0% in 2023 (-12.0 percentage points). Mean mastery was lowest in 2023 (25.0%) and highest in 2022 (41.0%).