Practice questions · Topic 6.6

Unit 6 · Practice for the Topic 6.6 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: Gene expression and cell specialization, summed up

Video coming soon

Transcription factors and RNA polymerase at the promoter; enhancers near or far; upstream and downstream of the start site; repressors block; activator or repressor from the data; different reading makes a cell’s products; small RNAs silence a message; an siRNA treatment and a dose series show what a gene does.

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 dose series 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

A biologist compares transcription in a bacterium and in a yeast cell, a eukaryote. In each cell, RNA polymerase is about to transcribe a gene.

In which of the two cells does RNA polymerase bind the promoter directly, with no other protein bound there first?

A. The bacterium only B. The yeast cell only C. Both cells D. Neither cell

Question 2

Which of the following is an enhancer?

A. The stretch just before a gene where the transcription factors bind and RNA polymerase docks B. A stretch inside a gene’s second intron whose bound proteins loop the DNA to the promoter C. A stretch of DNA beside a gene that a repressor binds, so the DNA around the gene winds tight D. The stretch of an mRNA that a short RNA pairs with

Question 3

On the gene map of a eukaryotic gene, a first regulatory sequence lies 5,200 bp before the transcription start site, beyond the promoter. A second lies 1,200 bp beyond the gene’s last exon.

Which of the following describes the two positions?

A. Both sequences lie downstream of the start site B. The first lies downstream and the second upstream C. Both sequences lie upstream of the start site D. The first lies upstream and the second downstream

Question 4

Four constructs from near the rhinoceros beetle’s horn gene, and the glow of each.

In the drawings, an open box is a piece of DNA joined in, and the filled box is the reporter gene. The arrow inside the stretch’s box shows which way round the stretch was joined. A team tests a stretch of DNA from near a rhinoceros beetle’s horn gene. The table gives four constructs and the glow of each.

In which kind of cell does the stretch raise transcription?

A. In antenna cells only B. In horn cells only C. In both kinds of cell D. In neither kind of cell

Question 5

Four constructs from near the rhinoceros beetle’s horn gene, and the glow of each.

In the drawings, an open box is a piece of DNA joined in, and the filled box is the reporter gene. The arrow inside the stretch’s box shows which way round the stretch was joined. A team tests a stretch of DNA from near a rhinoceros beetle’s horn gene, in the four constructs the table gives. A student claims that the stretch raises transcription of the reporter gene.

Which construct, compared with the promoter alone in the same kind of cell, best supports the claim?

A. Construct 2: the promoter alone, in antenna cells B. Construct 3: the stretch with the promoter, in horn cells C. Construct 4: the stretch with the promoter, in antenna cells D. Constructs 3 and 4 together

Question 6

In a cell of a hazel tree, a repressor binds a short stretch of DNA far from a gene’s promoter and brings in enzymes. The gene falls silent.

What do the enzymes the repressor brings in do?

A. Adding acetyl groups to the histones B. Cutting the gene’s mRNA into pieces C. Taking acetyl groups off the gene’s histones D. Taking the transcription factors off the promoter

Question 7

In a stoat’s cell, a repressor keeps one gene silent. Now imagine a change in the repressor’s gene gives the repressor a binding site that fits only a sequence found nowhere in the cell’s DNA.

What happens to transcription of the gene?

A. Transcription switches on B. Transcription switches off C. Transcription stays as it was D. Transcription of every gene in the cell switches on

Question 8

In a tench’s cell, a biologist removes the gene for one regulatory protein from the DNA, so the cell makes none of the protein. The gene beside the protein’s binding site is now transcribed six times as often as in a normal tench cell.

Which kind of protein was it, for that gene?

A. A repressor B. An activator C. Not a regulatory protein D. An enhancer

Question 9

Which of the following is a microRNA?

A. A short RNA an enzyme lays down at a replication fork to start a new DNA strand B. A folded RNA that carries an amino acid to the ribosome C. A short RNA a biologist supplies to cells, which pairs with one gene’s mRNA D. A short RNA a cell transcribes from one of its own genes, which pairs with an mRNA

Question 10

Four lines of results for the alga’s cells, control and siRNA.

Single cells of a pond alga glide across the bottom of a dish. A biologist gives half the cells an siRNA against one gene, whose job is unknown. The table gives, for control cells and siRNA cells, transcription of the gene, its mRNA, its protein and what the cells do.

Which of the following does the result show about the gene?

A. The siRNA cells transcribe the gene less often than the control cells B. The gene is transcribed only while the cells glide C. The gene’s protein is needed for gliding D. The gene’s protein stops the cells gliding

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 · Analyze Data · 5 points

A biologist grows three lines of cells from one kind of sea cucumber. Every cell carries the gene for a protein that cuts one particular mRNA: the cutting gene. The lines hold two, one and no working copies of the cutting gene; the biologist broke copies of the gene in two of the lines and left the third line as it was. The table gives the target mRNA in each line as a mean in arbitrary units with its standard error (SE); a ±2SE range runs from two SE below the mean to two SE above.

Target mRNA, with its SE, in the three lines of sea cucumber cells.

(a) Identify the control line. (1 point)

Hint: Which line holds the cells nobody changed?

A full-credit answer: The control line is the line with two working copies: the normal cells, which nobody changed.

Check the box for each point your answer earns

the two-copy line (the normal, unchanged cells).

(b) Describe the relationship between the number of working copies and the target mRNA. (1 point)

Hint: Read the three means in the order of the table.

A full-credit answer: As the working copies fall from two to none, the target mRNA rises: 12 units, then 48, then 84.

Check the box for each point your answer earns

the target mRNA rises as the working copies fall (the inverse relationship), with the direction stated.

(c) Predict how much cutting protein the one-copy cells make, compared with the two-copy cells, and justify your prediction. (1 point)

Hint: A cell makes the protein from every working copy it holds. How many does each of the two lines hold?

A full-credit answer: The one-copy cells make about half as much cutting protein as the two-copy cells.
A cell makes the cutting protein from each working copy of the cutting gene.
One working copy is half as many as two.

Check the box for each point your answer earns

about half as much cutting protein, because the protein is made from each working copy and the line holds one copy instead of two.

(d) Describe what the pattern shows about the effect of the cutting protein on the target mRNA. (1 point)

Hint: Put the three lines side by side: the dose of protein in each, and the target mRNA left in each.

A full-credit answer: The more cutting protein a line makes, the less target mRNA it holds: the two-copy cells make the most protein and hold 12 units, and the zero-copy cells make none and hold 84.
So the cutting protein destroys the target mRNA.

Check the box for each point your answer earns

more cutting protein, less target mRNA, so the protein destroys (cuts up) the target mRNA; nobody changed the target gene.

(e) Explain what the ±2SE ranges of the one-copy line and the zero-copy line show about the difference between them. (1 point)

Hint: Work each range: two SE below the mean to two SE above. Do the two ranges share any value?

A full-credit answer: The two ranges share no value.
The one-copy line’s mean is 48 units with SE 3, so its ±2SE range is 42 to 54 units.
The zero-copy line’s mean is 84 with SE 4, so its range is 76 to 92 units.
So the two true means are very unlikely to be the same: the lines differ.

Check the box for each point your answer earns

the two ±2SE ranges, 42 to 54 and 76 to 92 units, do not overlap, so the true means differ (the difference is very unlikely to be chance).

Free-response score: 0 of 5

Free response 2 · Analyze Data · 4 points

Two strains of one kind of bacterium live in oily soil. A biologist tests a cell of each strain for two genes: a gene for an enzyme that breaks down oil, and a gene for an enzyme that builds the cell wall. The table gives, for each cell, whether each gene is in the DNA and the level of its mRNA and its protein. In the table, present or absent says whether the gene is in the cell’s DNA; the levels are in arbitrary units.

Two genes in a cell of each strain: the gene, its mRNA and its protein.

(a) Identify the gene that both strains transcribe. (1 point)

A full-credit answer: The cell-wall enzyme gene.

Check the box for each point your answer earns

the cell-wall enzyme gene.

(b) A student claims that the two strains carry the same genes and differ only in which genes they express. Evaluate the claim using the table. (1 point)

A full-credit answer: The claim is not supported.
The oil-enzyme gene is present in the first strain’s DNA and absent from the second’s.
So the two strains differ in the genes they carry, and the second strain’s missing mRNA and enzyme follow from the missing gene.

Check the box for each point your answer earns

not supported, because the DNA row shows the oil-enzyme gene present in the first strain and absent from the second, so the strains differ in their genes (the mRNA and protein rows follow from that).

Common slip: Reading the oil-enzyme mRNA row, 240 against 0 units, as a difference in expression alone. Check the DNA row first: a gene that is absent cannot be expressed at all.

(c) The biologist gives a cell of the second strain a copy of the oil-enzyme gene, joined to a promoter that the cell’s RNA polymerase binds. Predict what the cell holds a day later. (1 point)

A full-credit answer: A day later the cell holds oil-enzyme mRNA and the oil enzyme itself.

Check the box for each point your answer earns

the cell holds the oil-enzyme mRNA and the oil enzyme (it now expresses the gene).

(d) Justify your prediction in part (c). (1 point)

A full-credit answer: The cell now carries the oil-enzyme gene with a promoter its RNA polymerase binds.
So RNA polymerase transcribes the gene into mRNA.
Ribosomes build the enzyme from that mRNA.
So the cell holds both the mRNA and the enzyme.

Check the box for each point your answer earns

with the gene and its promoter in the cell, RNA polymerase transcribes the gene into mRNA and ribosomes build the enzyme from it. Accept reasoning consistent with the prediction in part (c).

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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