Nucleic Acids

Unit 1 · Topic 1.6 end-of-topic test

Suggested time: about 47 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.

Answer every question. When you have answered every question, press Submit the test; the feedback then gives the reasoning for each. For the two 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.

Question 1

One nucleotide, drawn as three joined shapes marked 1, 2 and 3.

The figure shows one nucleotide drawn as three joined shapes, marked 1, 2 and 3. The five-carbon sugar is the middle shape. The shape marked 1 is a group attached to one side of the sugar. The shape marked 3 is a six-sided ring attached to the other side of the sugar.

What are the shapes marked 1 and 3?

A. 1 is a nitrogenous base; 3 is a phosphate group B. 1 is a fatty acid; 3 is a nitrogenous base C. 1 is a phosphate group; 3 is a nitrogenous base D. 1 is a phosphate group; 3 is an amino acid

Question 2

Two nucleotides are taken from different places along the same DNA strand and compared side by side.

Which part of the two nucleotides might be different?

A. The phosphate group B. The nitrogenous base C. The five-carbon sugar D. All three parts

Question 3

A single DNA strand is cut into two shorter pieces. Each piece is still an unbroken chain of nucleotides.

Which bond was broken?

A. A covalent bond between a sugar and the next phosphate B. A hydrogen bond between two neighboring bases C. The bond that holds a base onto its own sugar D. The attraction between the strand and the surrounding water

Question 4

One end of a short single strand of RNA carries a free phosphate group.

What can be said about that end?

A. It is the 3′ end B. Either end could carry the phosphate C. It is an end where a sugar has broken off D. It is the 5′ end

Question 5

A single strand of four nucleotides. X and Y mark its two ends.

The figure shows a single strand of four nucleotides. Phosphate groups are drawn as circles and five-carbon sugars as pentagons. The two ends are marked X and Y.

Which end is the 3′ end?

A. Y, because its sugar carries a free hydroxyl group B. X, because its sugar carries a free hydroxyl group C. X, because it carries the free phosphate group D. Y, because a strand is read from left to right

Question 6

A new DNA strand grows from its first nucleotide until it is 500 nucleotides long.

Where is that first nucleotide now?

A. At the 3′ end, where the new nucleotides are added B. In the middle, because both ends grow equally C. At the 5′ end, which never gains new nucleotides D. Outside the strand, pushed off as it grew

Question 7

A drug binds tightly to the free hydroxyl group at the 3′ end of a growing DNA strand and blocks it.

What happens to that strand?

A. It keeps growing, but from the 5′ end instead B. It stops growing; no new nucleotide can be joined on C. It grows faster, because the blocked end holds units in place D. It falls apart into separate nucleotides

Question 8

A student copies the sequence of a short DNA strand as C-T-T-A-G but forgets to mark which end is the 5′ end. A classmate says the same five bases are there either way, so the mark can be skipped.

Does the missing mark matter, and why?

A. It does not matter; the same bases in the same amounts carry the same information B. It does not matter; a strand can be read from either end and means the same thing either way C. It matters only because the 5′ end is the end that carries the free phosphate D. It matters; the information is the order of the bases from the 5′ end, and from the other end that order differs

Question 9

Two DNA strands are made from the same pool of nucleotides. One reads 5′-G-A-T-T-C-3′. The other reads 5′-C-T-T-A-G-3′.

How do the two strands compare?

A. Different; the bases sit in a different order read 5′ to 3′ B. The same; they are built from the same bases in the same amounts C. The same; one is just the other read backwards D. Different; only one of the two strands has a 3′ end

Question 10

A DNA sample is warmed to about 90 °C. The two strands come apart, but each separated strand is still a complete, unbroken chain of nucleotides.

Which bonds did the heat break?

A. The covalent bonds along each backbone B. The hydrogen bonds between paired bases C. Covalent bonds joining the bases of the two strands D. Both the covalent bonds and the hydrogen bonds

Question 11

A single strand of RNA folds back on itself so that two stretches of its bases pair up, held by hydrogen bonds. One base lies opposite an adenine.

Which base is it?

A. Thymine B. Guanine C. Adenine D. Uracil

Question 12

One stretch of a DNA strand reads G–C–A–T–T. The partner strand lies directly beneath it, base for base.

Written position by position beneath the given strand, what does the partner strand read?

A. G–C–A–T–T B. C–G–U–A–A C. C–G–T–A–A D. T–A–G–C–C

Question 13

The percentage of each base in three nucleic-acid samples. A dash means the base was not present.

The table below gives the percentage of each base measured in three nucleic-acid samples, W, X and Y.

Which sample or samples could be double-stranded, with every base paired with its partner?

A. X only B. W only C. W and X D. W and Y

Question 14

A double-stranded DNA sample from a lake microbe is analyzed. Adenine makes up 32% of the bases and cytosine 18%.

What percentage of the bases is thymine?

A. 32% B. 18% C. 50% D. It cannot be worked out from this

Question 15

A short stretch of DNA drawn untwisted, with the ends of both strands labeled.

The figure shows a short stretch of DNA drawn untwisted, so that the two strands lie side by side. The ends of both strands are labeled.

How are its parts arranged?

A. Backbones outside, bases inside, strands run the same way B. Backbones inside, bases outside, strands run opposite ways C. Backbones inside, bases outside, strands run the same way D. Backbones outside, bases inside, strands run opposite ways

Question 16

A drawing of a piece of double-stranded DNA with the ends of both strands labeled.

The figure shows a drawing of a piece of double-stranded DNA, with the ends of both strands labeled and the paired bases joined by dashed lines.

What, if anything, is wrong with the drawing?

A. Nothing; both strands of DNA run the same way B. The strands are drawn parallel; the strands of DNA are antiparallel C. Both strands should be turned over so that their 3′ ends are at the top D. The dashed lines should be solid; covalent bonds join the two strands

Question 17

One strand of a short piece of DNA reads 5′-T-T-G-C-A-3′.

Read from its own 5′ end, what does the partner strand read?

A. 5′-A-A-C-G-T-3′ B. 5′-A-C-G-T-T-3′ C. 5′-T-G-C-A-A-3′ D. 5′-U-G-C-A-A-3′

Question 18

An unknown nucleic acid is broken down, and the five-carbon sugar released is ribose.

What else should be true of this nucleic acid?

A. It contains uracil and is typically single-stranded B. It contains thymine and is typically double-stranded C. It contains uracil and is typically double-stranded D. It contains thymine and is typically single-stranded

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 or Visual Representation · 4 points

The figure is a schematic of a piece of double-stranded DNA, six nucleotides long. Each nucleotide is drawn as three joined shapes: a small circle, a five-sided shape and a rectangle. Along each strand the circles and five-sided shapes alternate to form the backbone, and each rectangle hangs off a five-sided shape and faces the other strand. In the boxed nucleotide the three shapes are marked: 1 is the circle, 2 is the five-sided shape, and 3 is the rectangle. The bases of the top strand are written as letters, and the top strand's ends are labeled: it reads 5′-A-T-G-C-C-A-3′. The bottom strand's ends are marked X (beneath the top strand's 5′ end) and Y (beneath the top strand's 3′ end), and its bases are left blank. Dashed lines between the rectangles mark where the two strands are held together.

A piece of double-stranded DNA, six nucleotides long. The top strand reads 5′-A-T-G-C-C-A-3′. The bottom strand's ends are marked X (left) and Y (right) and its bases are left blank. One nucleotide is boxed, with its three parts marked 1, 2 and 3.

(a) Identify the parts of the boxed nucleotide marked 1, 2 and 3. (1 point)

A full-credit answer: 1 is the phosphate group, 2 is the five-carbon sugar (deoxyribose), and 3 is the nitrogenous base, here guanine.

Check the box for each point your answer earns

identifying all three: 1 = phosphate group; 2 = five-carbon sugar (deoxyribose); 3 = nitrogenous base (guanine).

Accept: 'sugar' without naming deoxyribose; 'base' or 'G' for the shape marked 3. Working the parts out from their places (the shape that hangs off toward the other strand is the base; the backbone alternates phosphate and sugar; the shape joined to both is the sugar) is the intended route.

Do not award the point if any part is misidentified (for example, 1 as a base or 3 as an amino acid).

Common slip: Reading the three parts in the wrong order, or calling the base an amino acid. Phosphate, then sugar, then base.

(b) Describe the two different kinds of bond in the model: the bonds that join one nucleotide to the next along a strand, and the bonds shown by the dashed lines. (1 point)

A full-credit answer: Along a strand, the sugar of one nucleotide is joined to the phosphate of the next by covalent bonds, forming the sugar-phosphate backbone.
The dashed lines are hydrogen bonds between paired bases, and they hold the two strands together.

Check the box for each point your answer earns

stating BOTH: nucleotides along a strand are joined by covalent bonds (sugar of one nucleotide to phosphate of the next, forming the sugar-phosphate backbone), AND the dashed lines are hydrogen bonds between paired bases that hold the two strands together.

Accept: "weak attractions like those between water molecules" for hydrogen bonds if named as hydrogen bonds somewhere in the answer.

Do not award the point for saying the two strands are joined by covalent bonds, or for naming only one kind of bond.

Common slip: Joining the two strands with covalent bonds. Between the strands there are only hydrogen bonds; the covalent bonds run along each backbone.

(c) Represent the bottom strand: write the sequence of its bases reading from its own 5′ end, and mark which of X and Y is that 5′ end. (1 point)

A full-credit answer: Y is the bottom strand’s 5′ end, because the strands are antiparallel.
Read from Y, the bottom strand is 5′-T-G-G-C-A-T-3′.

Full-credit answer: the bottom strand written in beneath the top strand, Y marked as its 5′ end (X as its 3′ end), and read from Y as 5′-T-G-G-C-A-T-3′.

Check the box for each point your answer earns

the bottom strand written 5′-T-G-G-C-A-T-3′ with Y marked as its 5′ end (so X is the 3′ end; the strands are antiparallel). The end labels and the sequence are one representation: the point is earned when the sequence is written in the direction the labels give.

Accept: the bottom strand written position by position beneath the top strand as 3′-T-A-C-G-G-T-5′, with its ends labeled to match.

Do not award the point for 5′-T-A-C-G-G-T-3′ (right bases, wrong direction), for any uracil, or for X marked as the 5′ end.

Common slip: Writing the partner bases in the same left-to-right order as the top strand, 5′-T-A-C-G-G-T-3′. The bottom strand runs the other way, so its own 5′ to 3′ reading is reversed.

(d) Explain how this model shows that the sequence of one strand carries all the information needed to build the other strand. (1 point)

A full-credit answer: Each base pairs with only one partner, A with T and G with C, so the order of bases on one strand fixes the order on the other.
The bottom strand can be filled in from the top strand alone, base by base; the information is held in that order.

Check the box for each point your answer earns

explaining that each base pairs with only one partner (A with T, G with C), so the order of bases on one strand fixes the order of bases on the other; the information is held in that order of bases.

Accept: "the bottom strand could be filled in from the top strand alone, base by base" together with a reference to the pairing rules.

Do not award the point for statements about copying or how a cell makes a new strand; the point is for the pairing argument.

Common slip: Describing how a cell copies its DNA. The point is the pairing argument: one partner per base, so one strand fixes the other.

Free-response score: 0 of 4

Free response 2 · Conceptual Analysis · 4 points

A laboratory isolates the nucleic acid from a virus. Tests show that it is a single strand, that its sugar is ribose, and that its bases are adenine, uracil, guanine and cytosine. The DNA of the cells the virus infects is double-stranded. The percentage of each base in the virus's strand and in the cells' DNA is in the table.

Percentage of each base in the virus's strand and in the cells' DNA

(a) Identify the kind of nucleic acid the virus carries. Then describe two structural differences between it and the cells' DNA, naming what each of the two has. (1 point)

A full-credit answer: The viral nucleic acid is RNA.
It has ribose as its sugar where DNA has deoxyribose, and it has the base uracil where DNA has thymine.
It is also a single strand, where the cells' DNA is two strands paired together.

Check the box for each point your answer earns

RNA, together with any two of: ribose where DNA has deoxyribose; uracil where DNA has thymine; a single strand where DNA is double-stranded; each difference stated for both molecules.

Accept: 'the sugar differs (ribose in RNA, deoxyribose in DNA)' and 'the base differs (uracil in RNA, thymine in DNA)' in any wording that names both sides.

Do not award the point for RNA alone, for differences in base percentages (those are amounts, not structure), or for two differences with the DNA side missing.

Common slip: Listing the viral features from the description with nothing on the DNA side, or giving the percentage differences. Name the nucleic acid and, for each difference, what DNA has instead.

(b) Explain why the cells' DNA has equal amounts of adenine and thymine and equal amounts of guanine and cytosine, but the virus's strand shows no such matching. (1 point)

A full-credit answer: In double-stranded DNA every adenine is paired across the two strands with a thymine and every guanine with a cytosine, held by hydrogen bonds, so the paired bases must be present in equal amounts.
The viral strand is single, so its bases have no partners and no such matching is required.

Check the box for each point your answer earns

explaining that in double-stranded DNA every adenine is paired with a thymine and every guanine with a cytosine across the two strands (held by hydrogen bonds), so the paired bases must be present in equal amounts, whereas the viral strand is single and its bases are not paired with a partner strand, so no such constraint applies.

Accept: "one-to-one pairing" or "every A has a T opposite it" as the mechanism.

Do not award the point for restating the percentages without the pairing argument.

Common slip: Restating the percentages. The point is the one-to-one pairing across two strands, and its absence in a single strand.

(c) A cell makes a strand of RNA whose bases pair one to one with the bases of the virus's strand along its whole length. Predict the percentage of each base in this new strand. (1 point)

A full-credit answer: Adenine 21%, uracil 27%, guanine 23%, cytosine 29%: each base in the new strand is the partner of the base opposite it in the viral nucleic acid, so it takes that base’s percentage.

Check the box for each point your answer earns

adenine 21%, uracil 27%, guanine 23%, cytosine 29%.

Accept: the same values described in words (uracil takes adenine’s 27%, adenine takes uracil’s 21%, cytosine takes guanine’s 29%, guanine takes cytosine’s 23%).

Do not award the point for any answer containing thymine, or for repeating the viral nucleic acid’s own percentages.

Common slip: Repeating the viral nucleic acid’s own percentages, or bringing in thymine. The new strand is RNA, and each of its bases is the partner of the base opposite.

(d) A student claims that the new strand in part (c) would be an exact copy of the virus's strand. Evaluate the student's claim: judge whether the student is right about the sequence and whether the new strand carries the information needed to rebuild the original strand, and give the ground for each judgement. (1 point)

A full-credit answer: The student is not right about the sequence.
Each position in the new strand holds the partner base: U opposite A, A opposite U, C opposite G, G opposite C.
So the new strand is complementary, not identical.
The student is right about the information.
Each base has only one partner, so the original strand can be read off from the new one, base by base.

Check the box for each point your answer earns

a response that gives BOTH judgements with their grounds: (1) the sequence is NOT identical, because each position on the new strand holds the partner base (U opposite A, A opposite U, C opposite G, G opposite C); AND (2) the new strand DOES carry the information needed to rebuild the original, because each base pairs with only one partner, so the original sequence can be read off from the new strand base by base.

Accept: 'complementary, not identical' for the sequence, together with the rebuild reasoning resting on one base pairing with only one partner.

Accept as an addition, not required: the two strands would run in opposite 5′ to 3′ directions (antiparallel).

Do not award the point for accepting that the two sequences are identical, or for a judgement about the information with no ground from base pairing.

Common slip: Accepting that the two strands are identical, or judging the information claim with no ground. The ground is that each base pairs with only one partner: the sequences differ, yet either strand fixes the other.

Free-response score: 0 of 4

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Multiple choice checked: 0 of 18 correct.

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