These are original Marlbridge practice questions, not past exam questions. The mark points are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. No constants are needed. Organisms named by a letter, compounds and all data sets are fictional. Give percentages to 2 or 3 significant figures.
Question 1 (multiple choice · foundation)
Which observation is evidence for the common ancestry of all eukaryotes in particular, rather than of all living things?
- (A) Fungi, plants and animals all make proteins on ribosomes.
- (B) Fungi, plants and animals all store genetic information in DNA.
- (C) Fungi, plants and animals all remove introns from pre-mRNA using spliceosomes built from related RNAs and proteins.
- (D) Fungi, plants and animals all have cells surrounded by a phospholipid bilayer.
Answer and explanation
Answer: (C). Spliceosomes and spliced genes are found in eukaryotes but not in bacteria or archaea. Because the spliceosome parts are related in all three groups, they point to an ancestor of eukaryotes that already had them.
- (A) Ribosomes are found in every cell, including bacteria and archaea, so they link all life, not eukaryotes in particular.
- (B) DNA is the genetic material of all cellular life.
- (D) Every cell, prokaryote or eukaryote, has a phospholipid plasma membrane.
Question 2 (multiple choice · core)
A newly described single-celled organism, Z, lives in mud with very little oxygen. Z has a nucleus, several linear chromosomes with telomeres, and genes with introns removed by a spliceosome. It has no mitochondria. However, its nuclear DNA contains several genes whose closest relatives are genes that work inside mitochondria in other eukaryotes.
Which conclusion is best supported?
- (A) Z is a eukaryote whose ancestors had mitochondria; its lineage later lost them.
- (B) Z is a prokaryote, because it has no mitochondria.
- (C) Z’s eukaryotic features arose separately from those of other eukaryotes, without any endosymbiosis.
- (D) Z shows that the nucleus evolved from a mitochondrion.
Answer and explanation
Answer: (A). Z has all three shared eukaryotic features, so it belongs with the eukaryotes. Its mitochondria-related genes are leftovers from ancestors that had mitochondria. Losing an organelle in a low-oxygen habitat is a change within a lineage.
- (B) ignores the nucleus, linear chromosomes and spliceosome. Prokaryotes have none of these. Lacking one organelle does not make a cell prokaryotic.
- (C) ignores the mitochondria-related genes, which show that an endosymbiosis did take place in Z’s ancestry. A separate origin of three matching features is also far less likely than inheritance.
- (D) is not supported: nothing in the data links the origin of the nucleus to mitochondria.
Question 3 (multiple choice · core)
Yeast cells normally use the enzyme telomerase to rebuild the repeated DNA at the ends of their chromosomes after replication. A mutation inactivates telomerase in a yeast strain. Which outcome is most likely?
- (A) The chromosome ends shorten a little at each replication, and after many generations the cells stop dividing normally.
- (B) The chromosomes become circular straight away, like a bacterial chromosome.
- (C) The cells can no longer remove introns from pre-mRNA.
- (D) There is no effect, because only animal chromosomes have telomeres.
Answer and explanation
Answer: (A). Linear chromosomes lose a little DNA from their ends each time they are copied. Telomerase normally adds the repeats back. Without it, telomeres shorten gradually, and once they are too short the chromosomes are no longer protected and division fails.
- (B) A mutation in one enzyme does not join the ends of every chromosome; the effect is gradual shortening.
- (C) mixes up telomere maintenance with splicing, which is done by a different machine, the spliceosome.
- (D) Telomeres cap linear chromosomes in eukaryotes generally, including yeast; this is one of the shared features of eukaryotes.
Question 4 (data analysis · core)
Researchers examined 200 protein-coding genes in each of five organisms and counted how many contain at least one spliceosomal intron.
| Organism | Group | Genes examined | Genes with ≥1 intron |
|---|---|---|---|
| P | animal | 200 | 172 |
| Q | plant | 200 | 158 |
| R | fungus (a yeast) | 200 | 9 |
| S | bacterium | 200 | 0 |
| T | archaeon | 200 | 0 |
Each of R’s 9 intron-containing genes has a single intron, and 7 of these 9 introns sit at exactly the same positions as introns in the matching genes of P and Q.
(a) Calculate the percentage of genes with introns for P, Q and R. (b) Describe the overall pattern in the data. (c) A student says: “R has so few introns that it cannot share an ancestor with P and Q.” Evaluate this claim using the data. (d) Explain why the 0% values for S and T do not show that bacteria and archaea are unrelated to eukaryotes.
Worked solution and suggested Marlbridge rubric
(a) P: 172 ÷ 200 × 100 = 86%. Q: 158 ÷ 200 × 100 = 79%. R: 9 ÷ 200 × 100 = 4.5%.
(b) All three eukaryotes have genes with spliceosomal introns, although the proportion varies widely (4.5% to 86%). The two prokaryotes have none.
(c) The claim is not supported. R still has spliceosomal introns, a eukaryote-only feature. More tellingly, 7 of its 9 introns (about 78%) sit at the same positions as in P and Q. Matching positions are very unlikely to arise separately, so they were probably inherited from a shared ancestor. R’s low number fits a lineage that lost many introns over time.
(d) Introns are a feature of eukaryotes, so their absence only shows that S and T are not eukaryotes. Bacteria, archaea and eukaryotes still share features found in all life, such as DNA, the genetic code and ribosomes, which point to a more distant common ancestor of all three.
| Point | What earns it |
|---|---|
| 1 | All three percentages correct |
| 1 | Pattern: introns in all eukaryotes, variable amount, none in prokaryotes |
| 1 | Rejects the claim using shared intron positions (7 of 9) and intron loss |
| 1 | Absence of introns explained as a eukaryote-only feature, with a universal shared feature named |
Question 5 (constructed response · core)
A student claims: “Membrane-bound organelles evolved separately in animals, plants and fungi, because each group needed to keep different reactions apart.”
(a) Identify the kind of evidence that would support the student’s claim of separate origins. (b) Use two pieces of evidence about organelles to argue against the claim. (c) Explain why the fact that the same organisms also share linear chromosomes and split genes strengthens your argument.
Model answer and suggested Marlbridge rubric
(a) Separate origins would be supported if the organelles of each group did the same jobs but were built from unrelated molecules, or differed in basic structure (for example, a nucleus with a single membrane in one group and a double membrane in another).
(b) Any two:
- The nucleus in all three groups has a double membrane with pores, and the pore proteins are related (homologous) in yeast, plants and animals.
- Mitochondria in all three groups have a double membrane, their own DNA and bacteria-like ribosomes, consistent with a single endosymbiosis in a shared ancestor.
- The endomembrane system (endoplasmic reticulum, Golgi, vesicles) works in the same way and uses related proteins in all three groups.
(c) Nothing about having organelles requires a cell to have linear chromosomes or introns. When several unrelated features appear together in exactly the same organisms, the simplest explanation is that one ancestor had the whole package and passed it on. Separate origins would need each feature to arise again, in each group, in the same detailed form.
| Point | What earns it |
|---|---|
| 1 | Says what separate origins would look like (same job, different build) |
| 1 | First valid piece of organelle evidence, stated as shared detail or homology |
| 1 | Second valid piece of evidence |
| 1 | Package argument: unrelated features occurring together are best explained by one ancestor |
Question 6 (constructed response · core)
Doctors often find it harder to develop safe drugs against fungal infections than against bacterial infections. A fictional compound, F, blocks a protein in the spliceosome of a disease-causing fungus.
(a) Predict the effect of compound F on the fungus, and explain how this effect would arise. (b) Predict the effect of compound F on bacteria living in the patient’s gut. Justify your prediction. (c) Predict a risk of compound F for the patient, and explain it using common ancestry. (d) Suggest what kind of target would make a safer antifungal drug.
Model answer and suggested Marlbridge rubric
(a) The fungus can no longer splice its pre-mRNA correctly. Unspliced or wrongly spliced mRNA gives too little working protein, so the fungal cells grow and divide less, or die.
(b) Little or no effect. Bacteria have no spliceosome, so compound F has nothing to bind to.
(c) Human cells also have spliceosomes, inherited from the common ancestor of fungi and animals, and their proteins are homologous to the fungal ones. Compound F may bind the human version too, disrupting splicing in human cells and harming tissues, especially those with rapidly dividing cells.
(d) A structure or molecule that fungi have and human cells lack, for example something in the fungal cell wall, or an enzyme that makes a membrane component found only in fungi. Equally acceptable: a spliceosome protein region that differs clearly between the fungus and humans.
| Point | What earns it |
|---|---|
| 1 | Fungus: faulty splicing linked to less functional protein and reduced growth |
| 1 | Bacteria unaffected, because they lack a spliceosome |
| 1 | Human risk explained by homologous spliceosomes inherited from a shared ancestor |
| 1 | Sensible safer target: present in the fungus, absent or clearly different in humans |
Do not award the third point for “humans are also eukaryotes” without linking it to the shared, inherited machinery.
Question 7 (constructed response · stretch)
The table compares four organisms.
| Feature | Plant | Fungus | Bacterium | Animal |
|---|---|---|---|---|
| Cell wall | yes, made of cellulose | yes, made of chitin | yes, made of peptidoglycan | no |
| Nucleus with double membrane | yes | yes | no | yes |
| Linear chromosomes with telomeres | yes | yes | no (one circular chromosome) | yes |
| Genes with spliceosomal introns | yes | yes | no | yes |
A student argues: “Plants, fungi and bacteria all have cell walls, so they are more closely related to each other than any of them is to animals.”
(a) Explain why the cell wall is weak evidence of common ancestry here. (b) Use the table to argue for a different grouping. (c) Researchers compare the DNA sequence of a gene for a spliceosome protein in the plant, fungus and animal. Predict what they will find, and state what result would weaken your argument.
Model answer and suggested Marlbridge rubric
(a) The three cell walls are built from different molecules (cellulose, chitin, peptidoglycan). They do a similar job, support and protection, but their build differs, so they are probably not homologous. They may have arisen separately in each lineage.
(b) Plant, fungus and animal share three features the bacterium lacks: a nucleus with a double membrane, linear chromosomes with telomeres, and spliceosomal introns. These are complex and built the same way in each group. The best-supported grouping is plant + fungus + animal (the eukaryotes), with the bacterium outside.
(c) Prediction: the three sequences will be clearly similar to one another (homologous), with differences that build up over time since the lineages split. There will be no matching gene in the bacterium. The argument would be weakened if the plant, fungus and animal versions turned out to be unrelated sequences doing the same job, which would suggest separate origins of splicing.
| Point | What earns it |
|---|---|
| 1 | Different wall molecules identified, so similar function but not homologous |
| 1 | Three shared eukaryote features used to group plant, fungus and animal |
| 1 | Explains why several shared, complex features outweigh one similar function |
| 1 | Prediction of homologous sequences and a result that would weaken it |
How did you do?
- Q1 or Q7 wrong: re-read “Background: unity across all of life” and “Why shared features point to one ancestor” in the study guide.
- Q2 or Q5 wrong: re-read “Membrane-bound organelles”, and Topic 2.10 on endosymbiosis.
- Q3 wrong: re-read “Linear chromosomes” and look again at Figure 1.
- Q4 wrong: rework Worked example 1 and Figure 2, especially the part on intron loss.
- Q6 wrong: rework Worked example 2 on predicting the effect of blocking splicing.
Then tick off the topic checklist.