Biology • Year 12 • Module 5 • Lesson 9
DNA in Prokaryotes and Eukaryotes
Apply the prokaryote-vs-eukaryote contrast to real genome data, a structured compare-and-contrast, and a predict-and-justify scenario.
1. Interpret genome-organisation data across species
The table below shows the form of the main DNA, total genome size, and presence of plasmids for four representative organisms (values rounded; based on data from Ensembl Bacteria and Ensembl, 2023). 8 marks
| Organism | Cell type | Main chromosome form | Genome size (Mb) | Plasmids present? |
|---|---|---|---|---|
| Escherichia coli K-12 | Prokaryote | 1 circular | 4.6 | Yes (e.g. F plasmid, ~0.1 Mb) |
| Mycobacterium tuberculosis | Prokaryote | 1 circular | 4.4 | Rarely |
| Saccharomyces cerevisiae (yeast) | Eukaryote | 16 linear | 12 | 2µ plasmid (small) |
| Homo sapiens | Eukaryote | 46 linear (23 pairs) | 3 200 | No |
1.1 Identify two patterns shown by the table that match the prokaryote–eukaryote distinction made in the lesson. 2 marks
1.2 Approximately how many times larger is the human genome than the E. coli genome? Show your working. 2 marks
1.3 Yeast is a eukaryote but its genome is only ~3× the size of E. coli's. Explain why yeast still organises its DNA the way other eukaryotes do (linear chromosomes inside a nucleus) rather than the way E. coli does. 2 marks
1.4 Using the table, explain why bacterial plasmids — and not human chromosomes — are useful as routine biotechnology vectors. 2 marks
2. Interpret graph — genome size across cell types
The figure below plots total genome size (megabases, log scale) for a sample of prokaryotic and eukaryotic species. Use the figure to answer the sub-questions. 7 marks
Adapted from Ensembl & Ensembl Bacteria genome size data (2023). Note the log-scale y-axis.
2.1 Describe the difference in genome-size range between prokaryotes and eukaryotes shown in the figure. Quote at least one value from each group. 2 marks
2.2 Estimate the genome size of the largest eukaryote shown, and state how many times larger it is than E. coli's 4.6 Mb genome. 2 marks
2.3 Explain why eukaryotes can carry such larger genomes, using lesson terms (chromatin, linear chromosomes, nucleus). 3 marks
3. Structured compare and contrast
Complete the table below to compare DNA organisation in prokaryotes and eukaryotes. Use single phrases or short sentences — no full paragraphs needed. 10 marks (1 per cell)
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Main chromosome form | ||
| Location of main DNA | ||
| Membrane around DNA? | ||
| Number of main DNA molecules | ||
| Additional DNA molecules (and example use) |
4. Predict and justify — antibiotic resistance spreading via plasmids
A hospital lab finds that a strain of E. coli in one ward has become resistant to a previously effective antibiotic. Genome sequencing shows that the resistance gene is carried on a plasmid, not on the main bacterial chromosome. A clinician asks whether the resistance could spread to other bacterial species in the ward. 5 marks
4.1 Predict whether the resistance gene is more likely to spread to nearby bacteria than if it had been on the main chromosome. Justify your prediction using the lesson's definition of a plasmid. 3 marks
4.2 Explain why the same kind of cross-cell spread would not occur if the resistance gene were instead on a human chromosome inside a patient's cells. 2 marks
Q1 — Genome-organisation data (8 marks)
1.1 (2 marks). Acceptable patterns include: (i) both prokaryotes have a single circular main chromosome while both eukaryotes have multiple linear chromosomes; (ii) plasmids are listed as present in the prokaryote rows and (where present in yeast) are very small, whereas the human row has no plasmids; (iii) eukaryote genomes are much larger than prokaryote genomes in this sample. 1 mark per valid pattern, max 2.
1.2 (2 marks). 3 200 ÷ 4.6 ≈ 696 — the human genome is approximately 700 times larger than the E. coli genome [1 for working, 1 for a value between 650 and 750].
1.3 (2 marks). The defining eukaryotic feature is not genome size — it is that DNA is organised into multiple linear chromosomes inside a membrane-bound nucleus and associated with proteins as chromatin [1]. Yeast cells share this organisation regardless of how big their genome is, which is why they are classified as eukaryotic even though their genome is small [1].
1.4 (2 marks). Plasmids are small, separate, circular DNA molecules that can replicate inside a bacterial cell [1], so a gene of interest can be inserted into them and easily moved into bacteria. Human chromosomes are very large linear molecules locked inside a nucleus, packaged with proteins, and do not replicate independently — they cannot be used as routine vectors [1].
Q2 — Graph interpretation (7 marks)
2.1 (2 marks). Prokaryote genomes cluster between roughly 1 and 10 Mb (e.g. E. coli ≈ 4.6 Mb) [1], while eukaryote genomes span roughly 10 Mb up to several thousand Mb (e.g. yeast ≈ 12 Mb, Homo sapiens ≈ 3 200 Mb) [1]. The two groups barely overlap on the log axis.
2.2 (2 marks). Largest eukaryote shown ≈ 3 000–5 000 Mb (accept any value in this range using Homo sapiens ≈ 3 200 Mb) [1]. 3 200 ÷ 4.6 ≈ ~700× the E. coli genome [1].
2.3 (3 marks). Eukaryotes organise their DNA as multiple linear chromosomes [1], package those chromosomes with proteins to form chromatin so the DNA can be compacted and uncompacted as required [1], and store the whole package inside a membrane-bound nucleus [1]. Together these features let a much larger amount of DNA be held, replicated and accessed inside one cell.
Q3 — Compare-and-contrast table (10 marks)
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Main chromosome form | Usually circular | Linear |
| Location of main DNA | Nucleoid region of the cytoplasm | Inside the nucleus |
| Membrane around DNA? | No (no nuclear envelope) | Yes (nuclear envelope / membrane-bound nucleus) |
| Number of main DNA molecules | Usually one main chromosome | Multiple chromosomes (e.g. 46 in humans) |
| Additional DNA molecules | Plasmids — small circular DNA, used as vectors in biotechnology | No equivalent plasmid role in typical HSC comparison (mitochondrial / chloroplast DNA exists but is not the same use) |
Marking notes. 1 mark per correctly filled cell. Accept biologically equivalent phrasing.
Q4 — Predict and justify (5 marks)
4.1 (3 marks). Yes — it is more likely to spread [1]. A plasmid is a small, separate, circular DNA molecule that can replicate independently of the main bacterial chromosome [1], and plasmids can be transferred between bacterial cells (including between species) by mechanisms such as conjugation. If the resistance gene had been on the main chromosome it would only be passed to direct descendants of that one bacterial cell, which is much slower across the ward population [1].
4.2 (2 marks). Human chromosomes are linear DNA molecules locked inside a membrane-bound nucleus within each somatic cell [1]. They are not designed for cross-cell exchange and have no equivalent of bacterial plasmid transfer, so a gene on a human chromosome would not spread laterally between cells the way a plasmid-borne gene can spread between bacteria [1].