End-of-topic test: Common Ancestry

Unit 7 · Topic 7.7 end-of-topic test

Suggested time: about 44 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.

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. Then open the scoring guide and mark your own work against it. Every sequence table and every count in this test is imagined for the question.

Question 1

What the cells of four newly found single-celled organisms hold.

Biologists look inside the cells of four newly found single-celled organisms, as the table shows. They propose that the pond organism and the leaf organism share a more recent common ancestor with each other than either shares with the soil organism or the hot-spring organism.

Which feature shared by the pond organism and the leaf organism is the strongest evidence for the proposal?

A. Ribosomes in their cells B. A nucleus and mitochondria in their cells C. A cell wall around each of their cells D. Ribosomes and a cell wall in their cells, like the soil organism’s

Question 2

Five organisms compared: where the DNA sits, where most ATP is made, and any photosynthetic parts.

Biologists compare five organisms, as the table shows.

Which claim does the pattern in the table support?

A. The pond amoeba, the bread mold and the green alga inherited their membrane-bound organelles from one shared ancestor B. The pond amoeba, the bread mold and the green alga each built their membrane-bound organelles separately C. The green alga is more closely related to the photosynthetic bacterium than to the bread mold, because both photosynthesize D. The salt-lake archaean descends from the pond amoeba and lost its nucleus

Question 3

The form of the main DNA in six organisms.

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

Which conclusion does the table support?

A. The archaean is a eukaryote, because it lives in a hot spring B. Linear chromosomes arose separately in the yeast, the pine and the frog C. The yeast, the pine and the frog keep linear chromosomes, as one ancestor of the eukaryotes did D. The gut bacterium, the soil bacterium and the archaean inherited their closed loop from the eukaryotes’ ancestor

Question 4

A student reads that a bacterium’s main DNA is one closed loop, and says: “This is a linear chromosome, because the DNA is one long line.”

Is the student correct?

A. Yes: any chromosome that is one long molecule is a linear chromosome B. No: the loop’s two ends are joined, so the DNA is a circular chromosome C. Yes: a bacterium’s chromosome is linear and a eukaryote’s is circular D. No: a bacterium’s main DNA is a plasmid, not a chromosome

Question 5

One gene for the same enzyme in four organisms: its length in the DNA and the length of the RNA copy the cell uses; the lengths are imagined for this question.

A lab measures one gene for the same enzyme in four organisms: the length of the gene in the DNA, and the length of the RNA copy the cell uses to make the enzyme. The table gives the lengths.

What is the pattern in the table evidence of?

A. Three eukaryotes that each need a longer gene to make the same enzyme B. A eukaryote ancestor of the bacterium that lost every intron from its genes C. Separate gains, in the yeast, the moss and the fruit fly, of the ability to cut stretches out of an RNA copy D. One ancestor of the yeast, the moss and the fruit fly whose genes held stretches cut out of the RNA copy

Question 6

One gene in three eukaryotes: the length of its protein and where its two introns sit; the numbers are imagined for this question.

A lab reads one gene in a moss, a moth and a turtle. In all three the gene codes for a protein 164 amino acids long, and in all three the gene holds two introns at the same two places, as the table shows.

Why are the matching intron positions stronger evidence of one ancestor than the matching protein length?

A. Protein length does not fix where introns sit, so matching positions were inherited from one gene B. Introns are longer than the stretches that code for the protein, so they carry more of the gene’s DNA C. A protein of one length constrains its gene’s introns to the same positions D. Two introns at the same two places make the three proteins fold the same way

Question 7

What each of four newly found single-celled organisms does with the RNA copy of one gene before using it.

Biologists watch what four newly found single-celled organisms do with the RNA copy of one gene before using it, as the table shows.

Which organism is most likely a prokaryote?

A. The pond organism B. The bark organism C. The mud organism D. The reef organism

Question 8

Two groups of single-celled organisms from two lakes share four features.

Which shared feature is the strongest evidence that the two groups descend from one ancestor?

A. A thick outer coat one protein thick around every cell B. A rounded cell shape, the same in both groups of cells C. A pale stripe along one side of the cell, the same in both groups D. A nucleus wrapped in two membranes, with pores of the same build

Question 9

Two groups of desert plants share a thick water-storing stem. Their cells also share a water-storing cell type built from thirty kinds of protein.

Which shared feature is evidence of one common ancestor, and why?

A. The cell type, because a complicated feature is very unlikely to arise twice in the same form B. The thick stem, because a feature that the desert selects for is inherited from the group’s ancestor C. The thick stem, because a feature every member has must have come from one ancestor D. The water-storing cell type, because any feature made of proteins is inherited from one ancestor

Question 10

A single cell from a drainage ditch is 200 μm long. It holds ribosomes and one closed loop of DNA, and no body wrapped in its own membrane. A student says: “A cell this large must be a eukaryote.”

Is the student correct?

A. Yes: every cell over 100 μm long is a eukaryote B. No: the cell has no nucleus, so it is a prokaryote whatever its size C. Yes: a closed loop of DNA that large is a nucleus D. No: the cell is neither a prokaryote nor a eukaryote, because it is so large

Question 11

Three newly found single-celled organisms: a nucleus, the form of the main DNA, and introns in the genes.

Biologists find three new single-celled organisms and record three features of each, as the table shows.

Which find challenges the claim that all eukaryotes descend from one ancestor?

A. The desert-crust cell B. The tree-bark cell C. The lake-bottom cell D. The desert-crust cell and the tree-bark cell

Question 12

Four newly found single-celled organisms: a nucleus, the form of the main DNA, and what happens to the RNA copy of a gene before the cell uses it.

Biologists collect four newly found single-celled organisms and look inside their cells. The table shows what they see.

Which find supports the claim that all eukaryotes descend from one ancestor?

A. The snowfield cell B. The estuary cell C. The gutter cell D. The compost cell

Question 13

Over twenty years, biologists look inside the cells of 2,000 newly found eukaryotes, and every one has a nucleus, linear chromosomes and genes with introns.

What have the 2,000 finds done to the claim that all eukaryotes descend from one ancestor?

A. They have supported it more strongly, and a later find could still challenge it B. They have proved it, so no later find can challenge it C. They have left it exactly where it was, because no find challenged it and none added to it D. They have weakened it, because 2,000 finds show the features are common

Question 14

Three newly found single-celled organisms: stretches cut out of RNA copies, the form of the main DNA, and membrane-bound organelles.

Biologists record three features of three newly found single-celled organisms, as the table shows.

Which two organisms descend from the one ancestor of all eukaryotes?

A. The marsh organism and the reef organism B. The marsh organism and the dune organism C. The reef organism and the dune organism D. The marsh organism, the reef organism and the dune organism

Question 15

Two single-celled organisms, one from a peat bog and one from sea ice, both keep their DNA as linear chromosomes inside a nucleus and both hold genes with introns.

What are these shared features evidence of?

A. A recent common ancestor of these two organisms alone, shared with no other eukaryote B. Linear chromosomes and introns arising twice, once in the bog and once in the sea ice C. One ancestor of all eukaryotes that had these features, from which both descend D. A prokaryote ancestor of both organisms that kept its DNA as linear chromosomes

Question 16

A biologist reads one gene from a newly found single-celled organism and finds that the cell cuts four stretches out of every RNA copy of it.

Which two other features would the biologist predict the organism’s cells have?

A. A nucleus and linear chromosomes B. A cell wall and ribosomes C. A nucleus and one closed loop of DNA D. One closed loop of DNA and no membrane-bound organelle

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 · 4 points

Biologists find a new single-celled organism in a tidal pool on a rocky shore. Inside its cells they see a nucleus and mitochondria. Its DNA is in several molecules, each with two free ends. When the cell copies a gene into RNA, it cuts stretches out of the copy before using it.

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

A full-credit answer: The organism belongs to the eukaryotes.
Its cells have a nucleus, and a nucleus makes a cell a eukaryote.

Check the box for each point your answer earns

a eukaryote, because its cells have a nucleus (accept: membrane-bound organelles).

(b) Explain how the organism’s linear chromosomes and its genes with introns are evidence that it shares an ancestor with every other eukaryote. (1 point)

A full-credit answer: Every eukaryote keeps linear chromosomes and has genes with introns that are cut out of the RNA copy.
Linear chromosomes and introns are both complicated features.
Separate lines building the same complicated features in the same form is very unlikely.
So the organism inherited them from the one ancestor of all eukaryotes, as every other eukaryote did.

Check the box for each point your answer earns

linear chromosomes and genes with introns are complicated features shared by every eukaryote, so they were inherited from one ancestor rather than arising separately.

(c) Biologists claim that the organism shares a more recent common ancestor with a fish than with a sea-floor bacterium. Support the claim with evidence from the organism’s cells. (1 point)

A full-credit answer: The organism has a nucleus, linear chromosomes and genes with introns.
A fish has all three; a bacterium has none: its DNA is one closed loop, free in the cytosol, and its genes almost always lack introns.
The three shared complicated features mark the eukaryotes’ one ancestor.
So the organism and the fish descend from that ancestor, and the bacterium does not.

Check the box for each point your answer earns

the organism shares the three eukaryote features (nucleus, linear chromosomes, introns) with the fish and none of them with the bacterium, and those shared complicated features mark descent from the eukaryotes’ one ancestor.

(d) A second find from the same pool has no nucleus and no other membrane-bound organelle, but its DNA is in several molecules with two free ends. Predict what this second find would do to the claim that all eukaryotes descend from one ancestor. (1 point)

A full-credit answer: The second find has no nucleus, so it is not a eukaryote.
The claim is about eukaryotes, so this cell cannot break it.
Its linear chromosomes show that the linear form is not the eukaryotes’ alone, so that one line of evidence is weaker.
So the find weakens one line a little and leaves the claim standing.

Check the box for each point your answer earns

the find does not challenge the claim, because a cell with no nucleus is not a eukaryote and the claim is about eukaryotes. Accept with or without: it weakens the linear-chromosome line, since that form is not the eukaryotes’ alone.

Common slip: Reading any linear chromosome outside the eukaryotes as a challenge; the claim is about what every eukaryote has, not about what no prokaryote has.

Free-response score: 0 of 4

Free response 2 · Conceptual Analysis · 4 points

Biologists compare the cells of a sponge, a mushroom and a moss. In their cells, biologists see a body wrapped in two membranes, with pores of one build, that holds the DNA. In all three, the RNA copy of a gene is often shortened before the cell uses it, and the main DNA is in several separate molecules rather than one closed loop.

(a) Describe the pattern in the three groups’ cells, naming each of the three eukaryote features these observations show. (1 point)

A full-credit answer: The body wrapped in two membranes that holds the DNA is a nucleus, a membrane-bound organelle.
Main DNA in several separate molecules, not one closed loop, is linear chromosomes.
An RNA copy shortened before use has had stretches cut out: the genes hold introns.
So the three groups share all three eukaryote features together.

Check the box for each point your answer earns

naming the three features — membrane-bound organelles (a nucleus, mitochondria), linear chromosomes, genes with introns.

(b) Explain how a complicated structure of one build, shared by all three groups, is evidence that they descend from one ancestral cell. (1 point)

A full-credit answer: A nucleus wrapped in two membranes with pores of one build is a complicated structure, built from hundreds of proteins in one arrangement.
Three lines building it separately in the same form is very unlikely.
One ancestral cell having it, and all three inheriting it, needs the nucleus to appear once.
So the shared nucleus is evidence that the three descend from one ancestral cell.

Check the box for each point your answer earns

the nucleus is complicated (many proteins in one arrangement), so arising separately three times in the same form is very unlikely, and inheritance from one ancestral cell is the simpler explanation.

(c) A student says: “The sponge’s line evolved the two-membrane body that holds its DNA on its own, because animals are so different from fungi and plants.” Evaluate the student’s claim. (1 point)

A full-credit answer: The claim is not supported.
The sponge’s nucleus has the same build as the mushroom’s and the moss’s: two membranes with pores of one build.
A complicated structure of the same build in three lines was inherited, not built three times.
How different the groups look today does not change what their cells share.

Check the box for each point your answer earns

the claim is not supported, because the sponge’s nucleus has the same complicated build as the other two, which is explained by inheritance from one ancestor, not by a separate origin.

(d) Biologists then find a fourth organism in a mountain lake: a single cell whose DNA sits inside a body wrapped in two membranes. Predict the form of its main DNA and what happens to the RNA copies of its genes. (1 point)

A full-credit answer: A nucleus makes the cell a eukaryote.
Every known eukaryote has all three features together.
So biologists predict that its main DNA is in linear chromosomes, molecules with two free ends, and that stretches are cut out of the RNA copies of its genes: its genes hold introns.

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), because the three eukaryote features travel together.

Free-response score: 0 of 4

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Multiple choice checked: 0 of 16 correct.

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