Practice questions · Topic 1.6

Unit 1 · Practice for the Topic 1.6 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: nucleic acids, summed up

The nucleotide and its three parts, the strand and its two ends, the second strand and its pairing rules, antiparallel strands, and DNA against RNA.

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 choose one answer 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. Use a hint if you are stuck, then open the full-credit answer and score your own work against it.

Question 1

One RNA nucleotide drawn as three joined shapes: a small circle X, a five-sided shape Y and a six-sided shape Z.

The drawing shows one RNA nucleotide as three joined shapes. X is a small circle, Y is a five-sided shape and Z is a six-sided shape.

Which part is the nitrogenous base, the part that gives the nucleotide its letter?

A. X, the small circle B. Y, the five-sided shape C. Z, the six-sided shape D. X and Y together

Question 2

A single strand of RNA is 75 nucleotides long. Nucleotide 20 and nucleotide 21 sit next to each other along the strand.

Which bond joins nucleotide 20 to nucleotide 21?

A. A hydrogen bond between their two bases B. A covalent bond between the sugar of one and the phosphate of the next C. A covalent bond directly between their two bases D. A covalent bond from the sugar of one to the sugar of the next

Question 3

One end of a single DNA strand has a free hydroxyl group, –OH, on its last sugar, joined to nothing further.

Which end of the strand is this?

A. The 5′ end B. Either end; both ends carry a free hydroxyl group C. The 3′ end D. The middle; a free hydroxyl group marks a break in the strand

Question 4

A DNA strand is being built one nucleotide at a time. A new nucleotide is about to be joined on.

Which group on the growing strand does the incoming nucleotide’s phosphate bond to?

A. The free hydroxyl group on the 3′ carbon at the strand’s 3′ end B. The free phosphate at the strand’s 5′ end C. The base of the last nucleotide on the growing strand D. The hydroxyl group on the sugar at the strand’s 5′ end

Question 5

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

Do the two strands carry the same information?

A. Yes; they contain the same four bases in the same amounts B. Yes; one is the other read from the opposite end, so the message is the same C. No; only one of the two strands has a 5′ end D. No; the order of bases read 5′ to 3′ differs, and the information is in that order

Question 6

The two strands of a double-stranded DNA sample are separated at room temperature. Each separated strand is still a complete, unbroken chain of nucleotides.

Which attraction was overcome?

A. The hydrogen bonds between the paired bases B. The covalent bonds along each sugar-phosphate backbone C. The covalent bonds that join the bases of one strand to the bases of the other D. The bonds that hold each base onto its sugar

Question 7

One stretch of a DNA strand reads C–A–G–T–T–C. 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. C–A–G–T–T–C B. G–U–C–A–A–G C. G–T–C–A–A–G D. G–T–G–A–A–G

Question 8

Double-stranded DNA from a beetle is analyzed. Guanine makes up 21% of its bases.

What percentage of the beetle’s bases is adenine?

A. 21% B. 29% C. 58% D. 79%

Question 9

A piece of double-stranded DNA. The left strand’s ends are labeled 5′ at the top and 3′ at the bottom. The right strand’s top end is marked with a question mark and its bottom end is blank. Dashed lines join the paired bases.

The drawing shows a piece of double-stranded DNA. The left strand is labeled 5′ at the top and 3′ at the bottom. The right strand’s top end is marked with a question mark.

Which label belongs at the question mark?

A. 3′, because the strands are antiparallel B. 5′, because paired strands run the same way C. 3′, because a strand is always read from the top D. 5′, because every strand is drawn with its 5′ end at the top

Question 10

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

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

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

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

A short strand of DNA, strand 1, reads 5′-A-C-G-T-T-A-3′. It pairs along its whole length with a second strand, strand 2. The paired strands are then heated to 95 °C and separate again.

(a) Identify the three parts that make up any nucleotide, and state which part differs from one nucleotide to the next. (1 point)

Hint: Which parts of a nucleotide repeat unchanged along the whole strand?

A full-credit answer: Every nucleotide has a phosphate group, a five-carbon sugar (deoxyribose in DNA) and a nitrogenous base; only the base differs from one nucleotide to the next.

Check the box for each point your answer earns

phosphate group, five-carbon sugar and nitrogenous base, with the base as the part that differs.

Accept: ‘deoxyribose’ for the sugar; letters A, C, G, T for the bases.

Do not award: a list missing one part, or the sugar named as the part that differs.

Common slip: Naming the sugar as the part that changes. The sugar and the phosphate repeat unchanged; the base is the only part that differs.

(b) Describe the bond that joins one nucleotide to the next along strand 1, and the bonds that hold strand 1 to strand 2. (1 point)

Hint: One kind of bond runs along a backbone; the other kind runs between the two strands.

A full-credit answer: Along the strand, a covalent bond joins the sugar of one nucleotide to the phosphate of the next, making the sugar-phosphate backbone; the two strands are held together by hydrogen bonds between paired bases, weak one at a time but strong in number.

Check the box for each point your answer earns

covalent bonds sugar to phosphate along the backbone, and hydrogen bonds between paired bases holding the two strands together.

Accept: ‘strong bonds along the backbone, weak attractions between the bases’ if both kinds are also named.

Do not award: covalent bonds between the strands, or hydrogen bonds along the backbone.

Common slip: Joining the two strands with covalent bonds. The strands are held by many weak hydrogen bonds; the covalent bonds run along each backbone.

(c) Write the bases of strand 2, position by position beneath strand 1. (1 point)

Hint: Which base is each of A, C, G and T always found opposite in DNA?

A full-credit answer: Beneath A-C-G-T-T-A, strand 2 reads T-G-C-A-A-T.

Check the box for each point your answer earns

T-G-C-A-A-T written beneath strand 1, position by position.

Do not award: any uracil, or a base left unchanged.

Common slip: Using uracil opposite adenine. Strand 2 is DNA, so adenine pairs with thymine.

(d) Describe the order of the bases in strand 2 when it is written from its own 5′ end, and explain why it comes out in that order. (1 point)

Hint: How do the directions of the two strands in double-stranded DNA compare?

A full-credit answer: From its own 5′ end, strand 2 reads 5′-T-A-A-C-G-T-3′, because the two strands are antiparallel, so strand 2’s 5′ end lies beneath strand 1’s 3′ end, at the right, and reading from there gives the partners in reverse order.

Check the box for each point your answer earns

5′-T-A-A-C-G-T-3′, with the order explained by the strands being antiparallel (running in opposite 5′ to 3′ directions).

Accept: ‘strand 2’s 5′ end is at the other end, so you read it the other way’.

Do not award: 5′-T-G-C-A-A-T-3′ (right bases, wrong direction), or the order with no reason.

Common slip: Writing the partners in the same left-to-right order as strand 1. Strand 2 runs the other way, so from its own 5′ end the order is reversed.

(e) The paired strands are heated to 95 °C and separate. Predict what each strand looks like afterward, and justify your prediction from the two kinds of bond. (1 point)

Hint: Compare the strength of one hydrogen bond with the strength of a covalent bond.

A full-credit answer: After heating, each strand is a complete, unbroken chain of six nucleotides, because the heat broke only the hydrogen bonds between the paired bases, which are weak, while the covalent bonds along each backbone, which are far stronger, held.

Check the box for each point your answer earns

each strand stays whole; heat breaks the weak hydrogen bonds between bases but leaves the strong covalent bonds of the backbones intact.

Accept: ‘the strands unzip but do not break’ with both bonds named.

Do not award: strands broken into nucleotides, or the strands staying paired.

Common slip: Predicting that the strands fall apart into nucleotides. The heat is enough to break hydrogen bonds, which hold the strands together, but far short of what a covalent backbone bond needs.

Free-response score: 0 of 5

Free response 2 · Analyze Data · 4 points

A virus that infects cattle carries its instructions in RNA. Most RNA is a single strand, but this virus is different: careful tests show that its RNA is two strands paired along their whole length. Its bases are adenine 22%, uracil 22%, guanine 28% and cytosine 28%.

(a) Identify the sugar in this virus’s RNA, and describe two ways in which RNA differs in structure from typical DNA. (1 point)

A full-credit answer: The sugar is ribose.
Unlike typical DNA, RNA has the base uracil rather than thymine, and it is usually a single strand where DNA is two paired strands; its sugar, ribose, also differs from DNA’s deoxyribose.

Check the box for each point your answer earns

ribose, together with any two of: uracil in place of thymine; ribose in place of deoxyribose; usually a single strand where DNA is two paired strands.

Accept: ‘this virus’s RNA is unusual in having two strands’ as an extra remark; the two structural differences must still be given.

Do not award: deoxyribose, or only one difference.

Common slip: Naming deoxyribose because the molecule is double-stranded. The sugar of RNA is ribose however many strands it has; the number of strands is what makes this virus unusual.

(b) Evaluate the claim that this virus’s RNA is two paired strands, using its base percentages. (1 point)

A full-credit answer: The claim is supported by the percentages.
In two paired strands every base is paired one-to-one with its partner: adenine with uracil and guanine with cytosine.
So adenine must equal uracil and guanine must equal cytosine.
Here adenine and uracil are both 22%, and guanine and cytosine are both 28%, so the data fit two paired strands.
A single strand has no partner, so its bases would not have to match.

Check the box for each point your answer earns

the judgement AND the ground for it: the claim is supported, because one-to-one pairing across two strands (A with U, G with C) forces the paired bases to be equal, and the data show A = U (22%) and G = C (28%).

Accept: ‘every A on one strand is matched by a U on the other, so the amounts are equal’.

Do not award: the judgement with no ground; the percentages read as adding to 100%; or equal amounts explained with no pairing.

Common slip: Saying the claim is supported because the percentages ‘add up correctly’. Any set of four percentages adds to 100%; the ground is that the two partners in each pair are equal.

(c) A different virus also carries double-stranded RNA. In its RNA, guanine makes up 30% of the bases. Calculate the percentage of adenine. (1 point)

A full-credit answer: In this second virus, cytosine equals guanine, so cytosine is 30% and the two together are 60%.
That leaves 40% for adenine and uracil, which are equal, so adenine is 20%.

guanine = 30%

cytosine = guanine

adenine = uracil = \frac{100 % - guanine - cytosine}{2}

cytosine = guanine

cytosine = 30 %

adenine = uracil = \frac{100 % - guanine - cytosine}{2}

adenine = \frac{100 % - 30 % - 30 %}{2}

adenine = \frac{40 %}{2}

adenine = 20 %

Check the box for each point your answer earns

20% adenine, from cytosine = 30% and the remaining 40% shared equally by adenine and uracil.

Accept: 20% shown as half of the 40% left after guanine and cytosine.

Do not award: 30%, 40% or 70%.

Common slip: Stopping at 40%. That is adenine and uracil together; they are equal, so adenine is half of it.

(d) One stretch of one strand of the first virus’s RNA reads 5′-A-G-U-C-3′. Write the partner strand from its own 5′ end, and justify the order you wrote it in. (1 point)

A full-credit answer: The bases opposite A-G-U-C are U-C-A-G, but the two strands are antiparallel, so the partner’s 5′ end lies beneath the given strand’s 3′ end.
Read from its own 5′ end the partner is 5′-G-A-C-U-3′.

Check the box for each point your answer earns

5′-G-A-C-U-3′, with the reversal justified by the strands running in opposite directions (antiparallel).

Accept: the partner written 3′-U-C-A-G-5′ beneath the given strand, with its 5′ end marked at the right.

Do not award: 5′-U-C-A-G-3′ (right bases, wrong direction), or any thymine.

Common slip: Using thymine opposite adenine, or writing the partners in the same left-to-right order. This is RNA, so adenine pairs with uracil, and the partner runs the other way.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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