Practice questions · Topic 7.7

Unit 7 · Practice for the Topic 7.7 end-of-topic test

You’ve gone through everything in this topic. The summary video below recaps it all, so you’re ready for the questions.

Watch first: Common ancestry, summed up

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A shared complicated feature is evidence of one ancestor; a simple one is not. Every eukaryote has membrane-bound organelles, linear chromosomes and genes with introns; a prokaryote almost always has none. A new eukaryote with all three features supports the claim; a eukaryote with a mix challenges it; a prokaryote with one of the three weakens one line; a prokaryote with none neither supports nor challenges it.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. For the free-response questions, write one short sentence for each step of your reasoning, each on its own line, and make every link clear (so, because, therefore). That is what the exam’s ‘paragraph form’ means for you: linked sentences, not bullet points. The first free-response question walks you through one case one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it. Every sequence table and every count in these questions is imagined for the question.

Question 1

Two cells compared: length, where the DNA sits, and any membrane-wrapped bodies inside.

Biologists compare two cells, as the table shows.

Which statement about the two cells is correct?

A. The moss cell is a eukaryote, because it is thirty times as long as the stream cell B. The stream cell is a eukaryote, because its DNA is one closed loop C. The moss cell is a eukaryote, because its DNA sits inside a nucleus D. Both cells are prokaryotes, because both hold DNA

Question 2

The form of the main DNA in four organisms.

Biologists record the form of the main DNA in four organisms, as the table shows.

Which two organisms are eukaryotes?

A. The oak and the mushroom B. The oak and the gut bacterium C. The gut bacterium and the hot-spring archaean D. The mushroom and the hot-spring archaean

Question 3

A biologist watches a newly found single-celled organism copy one gene into RNA. The cell uses the whole copy, cutting nothing out.

Which group does the organism most likely belong to?

A. Eukaryotes B. Prokaryotes C. Both groups equally D. Neither group: a gene without introns marks a new kind of cell

Question 4

Three groups of single-celled organisms from three reservoirs share four features.

Which shared feature is the weakest evidence that the three groups descend from one ancestor?

A. A row of beating hairs on each cell, each hair built from the same forty proteins in one arrangement B. A nucleus wrapped in two membranes of one build C. Mitochondria of one build D. A cell twice as long as it is wide

Question 5

A biologist finds a single-celled organism under the ice of a mountain lake. Each cell has a nucleus. Its main DNA is in several molecules with two free ends. Its genes are copied into RNA and stretches are cut out of the copy.

What does this find do to the claim that all eukaryotes descend from one ancestor?

A. Challenges it, because a cell from under the ice cannot descend from the same ancestor as a warm-water cell B. Supports it, because the cell has all three eukaryote features together C. Proves it, because one more eukaryote with all three features settles the claim D. Challenges it, because the cell has linear chromosomes

Question 6

A biologist finds a single-celled organism in the water of a rice field. Each cell has a nucleus and mitochondria. Its main DNA is one closed loop. Its genes are copied into RNA and stretches are cut out of the copy.

What does this find do to the claim that all eukaryotes descend from one ancestor?

A. Supports it, because the cell has a nucleus B. Supports it, because the cell fits the prokaryote side of the pattern C. Challenges it, because a eukaryote here lacks one of the three features D. Weakens one line a little and leaves the claim standing, because the cell is a prokaryote

Question 7

A seaweed, a spider and a sparrow all keep their DNA as linear chromosomes, one very long molecule wound on proteins along its whole length.

How many times do biologists think the linear chromosome arose?

A. Once, in one ancestor that all three lines descend from B. Three times, once in each of the three lines C. Twice: once in the seaweed’s line and once in an ancestor of spiders and sparrows D. Many times, once in every species that has one

Question 8

A student says: “A newly found organism with a nucleus, whose genes lack introns, would prove that eukaryotes have more than one ancestor.”

Is the student correct?

A. Yes: one eukaryote without introns settles the question for good B. No: a eukaryote without introns fits the pattern, so the find would support the claim C. Yes: genes without introns show the organism is a prokaryote, which disproves the claim D. No: the find would challenge the claim, but one find proves nothing

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Conceptual Analysis · 5 points

A biologist finds a new single-celled organism in the sand of a beach. Inside each cell is a nucleus wrapped in two membranes.

(a) Identify the group, eukaryotes or prokaryotes, to which the organism belongs. (1 point)

Hint: What decides whether a cell is a eukaryote or a prokaryote: its size, its cell wall, or where its DNA sits?

A full-credit answer: The organism belongs to the eukaryotes.
A cell with a nucleus is a eukaryote.

Check the box for each point your answer earns

a eukaryote, because its cells have a nucleus.

(b) Describe the two other features the claim of one eukaryote ancestor predicts the organism’s cells have. (1 point)

Hint: Every eukaryote has three features together. A nucleus is one; name the other two.

A full-credit answer: The claim predicts that the organism keeps its DNA as linear chromosomes, molecules with two free ends.
It also predicts that the organism’s genes hold introns: stretches that are copied into RNA and then cut out of the copy.

Check the box for each point your answer earns

linear chromosomes (two free ends) and genes with introns (stretches cut out of the RNA copy).

(c) The biologist then finds both predicted features in the organism. Explain why that supports the claim. (1 point)

Hint: What pattern does the claim rest on? Does this organism fit it or break it?

A full-credit answer: The claim rests on a pattern: every eukaryote has all three features together.
The organism is a eukaryote with all three.
A eukaryote that fits the pattern supports the claim.

Check the box for each point your answer earns

the organism is a eukaryote with all three features together, which fits the pattern the claim rests on, so the find supports the claim.

(d) From the same sand the biologist takes a second organism. Its cells have no nucleus, but the cell shortens the RNA copy of one of its genes before using it. Predict what this second find does to the claim. (1 point)

Hint: Is the second organism a eukaryote? What is the claim about?

A full-credit answer: The second organism has no nucleus, so it is a prokaryote.
The claim is about eukaryotes, so a prokaryote cannot break it.
A shortened RNA copy has had a stretch cut out: that gene holds an intron.
So introns are not the eukaryotes’ alone, and the intron line of evidence is a little weaker.
So the find weakens one line and leaves the claim standing.

Check the box for each point your answer earns

the claim still stands, because the cell has no nucleus, so it is a prokaryote, and the claim is about eukaryotes. Accept either verdict with that reasoning: the find weakens the intron line of evidence, since introns are not the eukaryotes’ alone; or the find neither supports nor challenges the claim.

(e) Explain why many finds that fit the pattern leave the claim supported rather than proved. (1 point)

Hint: Think about what one future find could do to the pattern.

A full-credit answer: Evidence supports a claim; it does not prove one.
Every find that fits adds support.
A later find could still break the pattern, a eukaryote with a mix of the features.
So the claim stays a supported claim, open to challenge.

Check the box for each point your answer earns

evidence supports but never proves; a later find that breaks the pattern could still challenge the claim.

Free-response score: 0 of 5

Free response 2 · Conceptual Analysis · 4 points

Biologists study three groups of single-celled organisms from three hot springs. Every cell in all three groups carries the same complicated light-sensing spot, built from the same twelve proteins arranged the same way. Every cell in all three groups also has a thick outer coat made of one protein.

(a) Determine which shared feature, the light-sensing spot or the thick coat, is the stronger evidence of one ancestor. (1 point)

A full-credit answer: The light-sensing spot is the stronger evidence.
The spot is complicated, twelve proteins in one arrangement, so it is very unlikely to arise twice.
The coat is a simple feature, one protein, so three groups in hot water could each gain one on their own.

Check the box for each point your answer earns

the spot, because a complicated feature is unlikely to arise separately while a simple feature (one protein) can arise many times.

(b) Explain how the feature you chose in part (a) is evidence that the three groups descend from one common ancestor. (1 point)

A full-credit answer: The spot is a complicated structure: twelve proteins in one arrangement.
Three groups building it separately, in exactly the same form, is very unlikely.
One ancestor having the spot, and all three groups inheriting it, needs the spot to appear only once.
So the simplest explanation is that the three groups inherited the spot from one common ancestor.

Check the box for each point your answer earns

the spot is complicated (many parts in one arrangement), so arising separately three times in the same form is very unlikely, and inheritance from one ancestor is the simpler explanation.

(c) A student says: “The three groups all live in hot springs, so sharing a thick coat shows they inherited it from one ancestor.” Evaluate the student’s claim. (1 point)

A full-credit answer: The claim is not supported.
A thick coat of one protein is a simple feature.
Hot water is the same pressure on all three groups, so each group could gain a coat on its own.
A simple feature shared under the same pressure is weak evidence of one ancestor.

Check the box for each point your answer earns

the claim is not supported, because a simple feature under the same pressure can arise separately, so it is weak evidence of shared ancestry.

(d) Biologists then find a fourth group of hot-spring organisms. Every cell has the same thick coat of one protein, but its light-sensing spot is built from a different set of proteins arranged a different way. Predict what this fourth group does to the conclusion that the three groups share one ancestor. (1 point)

A full-credit answer: The fourth group’s spot has a different build, so it is not the complicated feature the three groups share.
The thick coat is a simple feature, so sharing it is weak evidence of one ancestor.
So the three groups still share one ancestor, and the fourth group is not shown to share it.

Check the box for each point your answer earns

the three groups still share one ancestor, but the fourth group is not shown to, because it lacks the shared complicated spot and the shared coat is weak evidence.

Free-response score: 0 of 4

Multiple choice checked: 0 of 8 correct.

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