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Practice questions · Topic 6.8

Unit 6 · Practice for the Topic 6.8 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: Biotechnology, summed up

Video coming soon

The gel sorts by size; reading the ladder and the fingerprint; the PCR cycle and its doubling; putting a plasmid into a bacterium and selecting the cells that took it up; sequencing and comparison; from a band or a plate to the biology it shows.

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 cut plasmid one step 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 technician sorts a mixture of DNA fragments by size on a gel.

Which of the following steps comes first?

Question 2
A gel with wells along its top edge, a minus sign at the top and a plus sign at the bottom. The ladder lane holds seven dark bars with their sizes in base pairs written to the left, from 3,000 at the top to 300 near the bottom. Lane 2 holds one dark bar, between the 3,000 and 2,000 bars and nearer the 3,000 bar3,000 bp2,000 bp1,500 bp1,000 bp700 bp500 bp300 bpladderlane 2−+
The ladder and lane 2, after the run.

In the drawing below, the small boxes along the top edge are the wells, and each dark bar is a band. The first lane is the ladder. Lane 2 holds one band.

About how long are the fragments in lane 2’s band?

Question 3
A gel with five lanes. The ladder lane holds seven dark bars with their sizes written to the left. The lane named droppings holds two bars; the lanes named vole, rat and shrew each hold two bars at various heights3,000 bp2,000 bp1,500 bp1,000 bp700 bp500 bp300 bpladderdroppingsvoleratshrew−+
The droppings beside the same stretch from three small mammals.

In the drawings below, the small boxes along the top edge are the wells, and each dark bar is a band. The first lane is the ladder. Suppose droppings are found in a barn. One enzyme cuts the same stretch of DNA from the droppings and from three small mammals, and the fragments are loaded beside the ladder.

Which animal has the same band pattern as the droppings?

Question 4
A straight horizontal line labeled a straight piece of DNA, 5,650 bp, with two short lines across it; under the first the words 1,650 bp from the left end, under the second the words 4,500 bp from the left enda straight piece of DNA, 5,650 bpcut site: 1,650 bp from the left endcut site: 4,500 bp from the left end
The straight piece with its two cut sites.

A straight piece of DNA is drawn as one straight line, and a cut site as the same short line across it. Suppose a straight piece of DNA 5,650 base pairs long carries an enzyme’s cut sequence at two places, 1,650 and 4,500 base pairs from its left end, as drawn. The enzyme cuts at both.

Which list gives the sizes of the pieces in base pairs, largest first?

Question 5

A machine heats and cools a tube of DNA, two primers, free nucleotides and a heat-stable DNA polymerase through one cycle of PCR.

Which of the following is the order of the three events in the cycle?

Question 6

Suppose a tube holds one copy of a target at the start, with both primers, free nucleotides and the heat-stable DNA polymerase. The machine heats and cools the tube for 13 cycles.

How many copies of the target does the tube hold after the 13 cycles?

Question 7

A laboratory has used PCR to copy one stretch of a long DNA molecule. Now it wants to copy a different stretch of the same molecule instead.

Which of the following must the laboratory change?

Question 8

Suppose a laboratory tests a scraping from a garden bird feeder for the DNA of one kind of bacterium that sickens birds, with primers that match a stretch found only in that bacterium. It sets up the scraping’s tube and a tube with water in place of a sample, and forgets the tube with a little of the bacterium’s own DNA. After 30 cycles the scraping’s lane and the no-sample lane are both blank.

Which of the following can the laboratory conclude?

Question 9

Suppose a technician seals a walnut tree’s gene for a leaf protein into a plasmid that carries a gene for resistance to an antibiotic and mixes the plasmid with cells of a bacterium she has chilled in a salt solution and warmed for a moment: the treated cells. She spreads treated and untreated cells on jelly with the antibiotic and on plain jelly: four plates.

Which plate shows that the salt solution and the warming left the cells alive?

Question 10

Suppose a biologist finds a large egg on a shore, reads a stretch of DNA from the membrane inside its shell, and sets the read against the same stretch from the sea birds nesting nearby, to find which species laid it.

Which of the four jobs is this?

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 Model · 4 points
In the drawing below, the circle is the plasmid, drawn as one closed line. A short line across the circle marks a place where the enzyme cuts: a cut site. Suppose a circular plasmid 6,150 base pairs long carries an enzyme’s cut sequence at the cut sites drawn below. Two of the arcs between neighboring cut sites are labeled with their lengths, 1,550 base pairs and 2,650 base pairs. The enzyme cuts at every site, and the pieces are loaded in one lane of a gel beside a ladder.
A circle drawn as one line, labeled 6,150 bp, with three short lines across it, each labeled cut site. Outside the circle, the arc from the first site to the second is labeled 1,550 bp and the arc from the second to the third 2,650 bp; the third arc carries no labelcut sitecut sitecut site1,550 bp2,650 bp6,150 bp
The plasmid with its cut sites.

(a) Determine how many pieces the enzyme makes. (1 point)

Hint: A circle has no ends. What does the first cut do, and what does every cut after it do?

Write down the values in the question

a closed circle; count the cut sites on the drawing: 3

Write down the equation

pieces=cuts

Substitute in the values, and calculate

pieces=3

A full-credit answer: The enzyme makes three pieces.

(b) Calculate the size of the third piece, in base pairs. (1 point)

Hint: The pieces are all the DNA there was. What do their sizes add up to?
bp

Write down the values in the question

plasmid = 6,150 bp
piece 1 = 1,550 bp
piece 2 = 2,650 bp

Write down the equation

piece 1+piece 2+piece 3=plasmid

Substitute in the values, and calculate

piece 3=6,150−1,550−2,650=1,950bp

A full-credit answer: The third piece is 1,950 base pairs long.

(c) Predict the number of bands the lane shows if a substitution destroys the cut site between the 1,550 and 2,650 base-pair arcs before the technician adds the enzyme. (1 point)

Hint: With that site gone, how many places does the enzyme cut, and how many pieces does a circle cut that many times give?

A full-credit answer: The lane shows two bands.

Check the box for each point your answer earns

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

Hint: Which two pieces have become one, and how long is the joined piece?

A full-credit answer: The enzyme cuts only where its sequence occurs, so with one site destroyed it cuts the circle at two sites.
A circle cut at two sites gives two pieces.
The 1,550 and 2,650 base-pair pieces have become one piece of 4,200 base pairs, beside the 1,950 base-pair piece.
Two pieces of different sizes make two bands.

Check the box for each point your answer earns

Free-response score: 0 of 4
Free response 2 · Analyze Data · 4 points
Suppose an old insect collection holds a weevil, a small beetle, whose label has been lost. A biologist reads a 30-base stretch of one of its genes and sets the read against the same stretch from three weevil species. In this gene, members of one species match at every base. The table gives the count of matching bases for each species.
A table with two columns, the weevil species and the bases of its stretch that match the collection weevil’s read, of 30, and three rows: the copper weevil, 30; the striped weevil, 26; the gray weevil, 19weevil speciesbases matching the read (of 30)the copper weevil30the striped weevil26the gray weevil19the same 30-base stretch read in each
The bases of each species’ stretch that match the collection weevil’s read.

(a) Identify the species the collection weevil belongs to. (1 point)

A full-credit answer: The copper weevil.

Check the box for each point your answer earns

(b) Using the table, describe the difference between the copper weevil’s count of matching bases and the striped weevil’s count. (1 point)

A full-credit answer: The copper weevil’s stretch matches the read at all 30 bases.
The striped weevil’s stretch matches at 26 bases, four fewer.

Check the box for each point your answer earns

(c) A student claims that the striped weevil is a closer relative of the collection weevil than the gray weevil is. Support the claim using the table. (1 point)

A full-credit answer: The striped weevil’s stretch matches the read at 26 of 30 bases, and the gray weevil’s at 19.
DNA is copied from parent to offspring, and changes build up over the generations, so DNA with more matching bases came from a closer relative.
So the striped weevil is the closer relative.

Check the box for each point your answer earns

(d) Explain why biologists chose, for identifying species, a stretch whose bases are the same in every member of one species and differ between species. (1 point)

A full-credit answer: Because the stretch is the same in every member of one species, a read from any member of that species matches the species’ sequence at every base.
Because the stretch differs between species, a read from another species differs somewhere along it.
So a match at every base means one species, and a difference means another.

Check the box for each point your answer earns

Free-response score: 0 of 4
Multiple choice checked: 0 of 10 correct.
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