Practice questions · Topic 6.5

Unit 6 · Practice for the Topic 6.5 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: Regulation of gene expression, summed up

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Which genes and how much decides the phenotype; constitutive and inducible genes; regulatory sequences and proteins; the lac and trp operons; epigenetic tags; one genome, many cell types; transcription factors in sequence; genes regulated together.

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 silent gene 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.

Question 1

One foot cell of a cone snail, tested for the venom enzyme's gene, the gene's mRNA and the enzyme.

A cone snail's venom-gland cells make an enzyme in its venom. A biologist tests one of the snail's foot cells for the enzyme's gene, for the gene's mRNA and for the enzyme. The table gives the results.

Which of the following describes the enzyme's gene in the foot cell?

A. The foot cell does not carry the gene B. The foot cell carries the gene and is not transcribing it C. The foot cell carries the gene and transcribes it, and no enzyme is built from the mRNA D. The foot cell carries the gene and expresses it: the enzyme is built

Question 2

One gene of a bacterium in two broths.

Read a gene's level across conditions: about the same in every condition means constitutively expressed; high in one condition only means inducible, and that condition holds its signal. Suppose a biologist measures a bacterium's gene in a broth with no zinc and in the same broth with zinc added. Its levels are drawn.

Which kind of gene is it?

A. Constitutively expressed B. Inducible, with zinc as its signal C. Inducible, with the absence of zinc as its signal D. Neither: its level is too low to sort

Question 3

A soil bacterium's gene codes for an enzyme. A biologist studies two strains, each with one change in its DNA. The first strain makes the normal amount of the enzyme's mRNA, 100 units, and its enzyme does not work. The second strain makes 5 units of the mRNA, and the enzyme it does make works normally.

Which stretch of DNA did the second strain's change most likely alter?

A. A codon in the middle of the gene B. The gene's start codon C. The gene's last codon, before the stop codon D. The regulatory sequence beside the gene

Question 4

The mannose operon in a cell with no mannose.

In the drawing the block is the repressor, and the oval is RNA polymerase, off the DNA. The drawing shows a bacterium's operon for breaking down mannose, a sugar, in a cell with no mannose. The operon works like the lac operon.

Which lettered part is the operator?

A. Q B. U C. E D. None of Q, U and E

Question 5

mRNA of the mannose operon's genes in four cultures.

A biologist measures the mRNA of the mannose operon's genes in four cultures of the bacterium: with and without mannose in the broth, in a normal strain and in a strain that makes no repressor. The table gives the results.

Which of the following is mannose, for this operon?

A. An activator B. A corepressor C. An inducer D. A repressor

Question 6

Suppose a bacterium's operon holds the genes for building the amino acid lysine. Right now the cell holds very little lysine.

Which of the following describes the operon's genes and its repressor right now?

A. Not transcribed, with the repressor on the operator B. Not transcribed, with the repressor off the DNA C. Transcribed, with the repressor on the operator D. Transcribed, with the repressor off the DNA

Question 7

E. coli cells grow in a broth of glucose plus lactose. A technician removes the glucose from the broth and leaves the lactose.

An hour later, how much of the lactose enzymes are the cells making, compared with before?

A. Far more B. About the same C. Far less D. None at all

Question 8

Cells from an eel grow in a dish. Two chemicals each silence the same gene in the cells. A technician exposes one dish to the first chemical and another dish to the second for two days, washes each chemical out, and grows the cells on for a month, through many divisions. After the month, the first dish's cells transcribe the gene again; the second dish's gene stays silent. The gene's base sequence is unchanged in both dishes.

Which of the following explains the two results?

A. In both dishes, enzymes removed the tags the chemical added B. In both dishes, the cells copied the tags to their daughter cells at each division C. In the first dish, enzymes removed the tags; in the second, the cells copied the tags to their daughter cells at each division D. In the first dish, the cells copied the tags to their daughter cells at each division; in the second, enzymes removed the tags

Question 9

A cell in a scorpion's claw controls how often RNA polymerase transcribes one of its genes.

Which of the following is a transcription factor?

A. The enzyme bound to the promoter, copying the gene into RNA B. The stretch of DNA beside the gene where a regulatory protein binds C. A kinase in the cytosol adding a phosphate group to a regulatory protein D. A protein bound to the DNA beside the gene, helping RNA polymerase bind the promoter

Question 10

A biologist measures a gene in the cells of a sea slug's skin. The cells hold 210 units of the gene's mRNA and 0 units of its protein.

Which of the following can explain the absence of the protein?

A. RNA polymerase is not transcribing the gene B. The gene's promoter carries many methyl groups C. Ribosomes are not translating the mRNA D. The cells have lost the gene

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

Two cells of the iris of one person's eye each carry a copy of the same gene. In the first cell RNA polymerase transcribes the gene steadily. In the second cell the gene is silent, and its promoter carries many methyl groups.

(a) Describe what the methyl groups on the promoter do to the winding of the second copy's DNA. (1 point)

Hint: Think of the DNA as thread wound on histone beads. Do the methyl groups pack the beads together or set them apart?

A full-credit answer: The methyl groups on the promoter make the DNA wind tightly around its histones, so the beads pack together.

Check the box for each point your answer earns

the DNA winds more tightly (the histones pack together; the DNA coils tight).

(b) Describe how the base sequences of the two copies compare. (1 point)

Hint: A methyl group sits on a cytosine base. Is the cytosine still a cytosine?

A full-credit answer: The two copies have the same base sequence: a methyl group sits on top of a base and changes none of them.

Check the box for each point your answer earns

the two copies have the same base sequence (the methyl groups change no base).

(c) Explain why the second copy stays silent. (1 point)

Hint: Which kind of winding can RNA polymerase reach: loose or tight?

A full-credit answer: The second copy stays silent because its DNA is wound tightly around its histones.
RNA polymerase can only reach a gene that is loosely wound.
So RNA polymerase cannot reach the second copy's promoter.

Check the box for each point your answer earns

the tightly wound DNA keeps RNA polymerase from reaching the gene (its promoter), so it is not transcribed.

Restating part (a), that the DNA is tightly wound, earns nothing on its own: the point wants RNA polymerase (or a transcription factor) kept off the promoter.

(d) Predict what happens to the silent gene in the two daughter cells when the second cell divides. (1 point)

Hint: At S phase the old strand keeps its methyl groups. What do enzymes do to the new strand?

A full-credit answer: When the second cell divides, both daughter cells keep the gene silent.
At S phase the old strand keeps its methyl groups, and enzymes add methyl groups to the new strand to match.
So each daughter cell receives a promoter with the same methyl pattern.

Check the box for each point your answer earns

the gene stays silent in both daughter cells, because the methyl pattern is copied onto the new DNA at S phase (the daughters inherit the parent's tags).

(e) Explain how the silent gene could be switched on again in the second cell. (1 point)

Hint: Which enzymes act on the methyl groups, and what does their loss do to the winding?

A full-credit answer: The gene could switch on again if enzymes remove the methyl groups from the promoter.
With the methyl groups gone, the DNA winds more loosely.
So RNA polymerase can reach the promoter and transcribe the gene.

Check the box for each point your answer earns

enzymes remove the methyl groups, so the DNA winds more loosely and RNA polymerase can reach (transcribe) the gene.

Free-response score: 0 of 5

Free response 2 · Analyze Data · 4 points

A biologist plans to measure how a pesticide changes gene expression in the gill cells of a fish. To compare a gene's level between treated and untreated fish, she needs a reference gene: a gene whose level stays the same whatever the fish's conditions, so that every other gene can be measured against it. She measures three candidate genes in gill cells of fish kept in clean water, in water with arsenic and in water with a weedkiller. The table gives the results.

Three candidate genes in the gill cells of fish kept in three waters.

(a) Identify the gene with the lowest level in the gill cells of fish kept in clean water. (1 point)

A full-credit answer: Gene Q, at 40 units.

Check the box for each point your answer earns

gene Q.

(b) Identify the gene whose level varies most across the three waters. (1 point)

A full-credit answer: Gene U: it reads 280 units in clean water, 115 with arsenic and 25 with the weedkiller.

Check the box for each point your answer earns

gene U.

(c) The biologist chooses gene E as her reference gene for the pesticide experiment. Evaluate her choice using the data. (1 point)

A full-credit answer: Her choice is supported.
Gene E reads 88, 90 and 87 units across the three waters: about the same level in every condition, so it is constitutively expressed.
A gene whose level does not change with the fish's conditions is a fair yardstick for the genes that do.

Check the box for each point your answer earns

supported, because gene E's level is about the same in every condition (88 / 90 / 87 units; constitutively expressed), so it can serve as the unchanging reference.

Common slip: Choosing by level: a reference gene is chosen for a level that does not change, not for a high or a low level.

(d) Gene Q's level rises in water with arsenic. Explain how arsenic could raise transcription of gene Q. (1 point)

A full-credit answer: Arsenic switches on a transcription factor in the gill cell, for example by binding it and changing its shape.
The switched-on factor binds the regulatory sequence beside gene Q.
Bound there, the factor helps RNA polymerase bind gene Q's promoter, so RNA polymerase transcribes gene Q more often.

Check the box for each point your answer earns

arsenic switches on a transcription factor (regulatory protein) that binds the DNA beside gene Q and helps RNA polymerase bind the promoter, so gene Q is transcribed more often. Accept: arsenic acts through a signal pathway that switches the factor on.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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