Question 1
What is produced by DNA replication before a body cell divides?
A.
Two DNA molecules with unrelated base sequences produced by recombination
B.
Two DNA molecules with identical base sequences to the original DNA
C.
Two RNA molecules with complementary base sequences to the original DNA
D.
Two protein molecules with amino acid sequences copied from DNA
Question 2
What is the role of helicase at a DNA replication fork?
A.
It cuts covalent bonds in the sugar-phosphate backbone.
B.
It unwinds DNA by breaking hydrogen bonds between bases.
C.
It adds DNA nucleotides to the growing complementary strand.
D.
It joins adjacent DNA fragments by forming phosphodiester bonds.
Question 3
A double-stranded DNA molecule replicates once by a semi-conservative mechanism. What is the composition of each DNA molecule produced?
A.
Two newly synthesized strands joined together
B.
One original parental strand and one newly synthesized strand
C.
Two original parental strands joined together
D.
Short mixed sections of original and newly synthesized DNA in both strands
Question 4
DNA fragments are placed in wells at the negative end of an agarose gel and an electric field is applied. What result is expected after gel electrophoresis?
A.
Smaller fragments travel furthest toward the positive electrode.
B.
All fragments travel the same distance because they have the same charge.
C.
Fragments remain in the wells unless a DNA ladder is present.
D.
Larger fragments travel furthest toward the negative electrode.
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Question 5
What describes the directionality of DNA synthesis by DNA polymerase?
A.
Nucleotides are added to the end, so the new strand grows to .
B.
Nucleotides are added to both ends, so the new strand grows in both directions.
C.
Nucleotides are added randomly, so strand direction is not biologically important.
D.
Nucleotides are added to the end, so the new strand grows to .
Question 6
What is the role of DNA polymerase I in prokaryotic DNA replication?
A.
It seals remaining breaks by forming phosphodiester bonds between fragments.
B.
It synthesizes short RNA primers on the DNA template.
C.
It extends RNA primers to form most of the new DNA strand.
D.
It removes RNA primers and replaces them with DNA nucleotides.
Question 7
Cells in the root tip of a growing onion, Allium cepa, divide frequently by mitosis.
State the term for the process that produces copies of DNA before cell division.
[1]
Outline why DNA replication is required before mitosis in a root tip cell.
[2]
Question 8
Two enzymes act at a replication fork during DNA replication.
Outline the role of helicase in DNA replication.
[2]
State the general role of DNA polymerase in DNA replication.
[1]
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Question 9
A PCR cycle includes denaturation, annealing and extension. What happens during the annealing step?
A.
The two DNA strands separate as hydrogen bonds are broken.
B.
Taq polymerase is permanently denatured by the high temperature.
C.
DNA fragments move through a gel toward the positive electrode.
D.
Primers bind to complementary sequences on single-stranded DNA.
Question 10
A forensic DNA profile from one marker matches a suspect. The investigator repeats the comparison using many independent markers. What is the main reason for using more markers?
A.
It allows DNA fragments to move toward the negative electrode.
B.
It reduces the probability of a false match between unrelated individuals.
C.
It changes the base sequence of the suspect's DNA to match the sample.
D.
It removes the need for PCR before gel electrophoresis is performed.
Question 11
What is the correct sequence of enzyme actions during synthesis and processing of an Okazaki fragment in prokaryotic DNA replication?
A.
DNA ligase, DNA primase, DNA polymerase III, DNA polymerase I
B.
DNA primase, DNA polymerase III, DNA polymerase I, DNA ligase
C.
DNA polymerase I, DNA ligase, DNA primase, DNA polymerase III
D.
DNA polymerase III, DNA primase, DNA ligase, DNA polymerase I
Question 12
During prokaryotic DNA replication, DNA polymerase III inserts a nucleotide with a mismatched base at the end of a growing strand. What happens during proofreading?
A.
DNA polymerase III removes the nucleotide from the terminal and replaces it with a correctly matched nucleotide.
B.
DNA polymerase I seals the mismatch by forming a phosphodiester bond in the backbone.
C.
DNA ligase removes the nucleotide from the terminal and replaces it with an RNA primer.
D.
DNA primase removes the template strand and synthesizes a new strand with random bases.
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Question 13
The diagram shows a simplified model of DNA replication. Newly synthesized strands are shown differently from parental strands.
State what is meant by semi-conservative replication.
[2]
Explain how complementary base pairing contributes to the accuracy of DNA replication.
[2]
Question 14
Polymerase chain reaction (PCR) can amplify a selected DNA sequence from a very small biological sample.
State the function of primers in PCR.
[1]
Describe the role of temperature changes in one PCR cycle.
[3]
Question 15
DNA fragments from a PCR product were separated by gel electrophoresis. Wells were placed near the negative electrode.
State the direction in which DNA fragments move during electrophoresis.
[1]
Explain why DNA fragments move in this direction.
[1]
Deduce which fragments, larger or smaller, have moved furthest through the gel. Give a reason.
[2]
Question 16
A short DNA strand is shown with chemically different ends.
Distinguish between the and ends of a DNA strand.
[2]
State which end of the growing strand is extended by DNA polymerase.
[1]
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Question 17
A thermocycler was programmed for PCR amplification of a selected DNA sequence. The graph shows the temperature changes during repeated cycles, and the amplification curves show the amount of product obtained with different reaction mixtures.
Cycle | Stage 1 / | Stage 2 / | Stage 3 / | Complete mix / copies | No primers / copies | No template / copies |
|---|---|---|---|---|---|---|
0 | 95 | 55 | 72 | 1 | 0 | 0 |
1 | 95 | 55 | 72 | 2 | 0 | 0 |
2 | 95 | 55 | 72 | 4 | 0 | 0 |
3 | 95 | 55 | 72 | 8 | 0 | 0 |
4 | 95 | 55 | 72 | 16 | 0 | 0 |
5 | 95 | 55 | 72 | 32 | 0 | 0 |
6 | 95 | 55 | 72 | 64 | 0 | 0 |
7 | 95 | 55 | 72 | 128 | 0 | 0 |
8 | 95 | 55 | 72 | 256 | 0 | 0 |
Identify the PCR stage occurring at the highest temperature in each cycle.
[1]
Calculate the maximum number of copies of one starting DNA molecule after eight cycles, assuming perfect doubling in each cycle.
[1]
Explain why the reaction without primers does not produce the selected PCR product.
[2]
Question 18
PCR products from three samples were separated by gel electrophoresis. A DNA ladder containing fragments of known length was loaded in the first lane.
State the direction in which DNA fragments moved during electrophoresis.
[1]
Deduce which sample contains the shortest DNA fragment.
[1]
Explain why smaller DNA fragments travel further through the gel in a fixed time.
[2]
Question 19
A replication fork is moving to the right. The upper template strand runs to from left to right and the lower template strand runs to from left to right. Which diagram correctly shows synthesis of the leading and lagging strands?
A.
B.
C.
D.
Question 20
Which diagram shows the way DNA polymerase extends a growing DNA strand?
A.
B.
C.
D.
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Question 21
A forensic laboratory compared DNA from a crime scene with DNA from three suspects using several genetic markers. The gel shows the banding patterns for one marker.
State how PCR is useful before gel electrophoresis in DNA profiling.
[1]
Explain how banding patterns can be used to compare the crime scene DNA with the suspects' DNA.
[2]
Suggest why more than one independent genetic marker should be analysed.
[1]
Question 22
The diagram shows a replication fork in a prokaryotic chromosome. The parental template strands are antiparallel.
Identify the newly synthesized strand that is made continuously at the replication fork.
[1]
Compare replication on the leading strand and the lagging strand.
[3]
Question 23
In prokaryotes, several enzymes are required to complete replication of the lagging strand.
Outline the function of DNA primase during replication.
[1]
Outline the functions of DNA polymerase III, DNA polymerase I and DNA ligase during prokaryotic lagging strand replication.
[3]
Question 24
During replication, a DNA polymerase III molecule has just added a nucleotide whose base does not correctly pair with the template base.
State the name of the error-correction process carried out by DNA polymerase III.
[1]
Explain how DNA polymerase III corrects the mismatched nucleotide.
[2]
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Question 25
Bacteria were grown for many generations in a medium containing a heavy isotope of nitrogen and were then transferred to a medium containing the common lighter isotope of nitrogen. DNA was extracted after successive rounds of replication and separated by density-gradient centrifugation.
Describe the position of the DNA band after one round of replication in the light nitrogen medium.
[1]
Deduce the mode of DNA replication supported by the results.
[1]
Explain how the results after two rounds of replication provide evidence against the conservative and dispersive models.
[2]
Question 26
An investigation tested two inhibitors of DNA replication in cultured cells. Fluorescent probes were used to measure the length of newly separated parental DNA and the amount of newly synthesized DNA at replication forks.
Identify the dependent variable used to measure DNA synthesis in this investigation.
[1]
Deduce which inhibitor is most likely to inhibit helicase.
[1]
Explain the effect expected when helicase is inhibited.
[2]
Question 27
PCR was used to test patient samples for a DNA sequence specific to a bacterial pathogen. The PCR products were separated by gel electrophoresis with positive and negative controls.
Identify the patients with a positive result for the pathogen-specific DNA sequence.
[1]
Explain the purpose of the negative control in this PCR test.
[1]
Suggest two reasons why a patient infected with the pathogen might show no band in the gel.
[2]
Question 28
A short primer was annealed to a DNA template in a replication assay. The diagram shows the chemical direction of the primer and template and two possible sites where a new nucleotide could be added.
Identify the end of the primer to which DNA polymerase can add the next nucleotide.
[1]
State the direction in which the new DNA strand is synthesized.
[1]
Explain why DNA polymerase cannot extend the primer from its end.
[2]
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Question 29
A hypothetical DNA polymerase is found that can add nucleotides only to an existing end, but it has lost the ability to bind to an RNA primer.
Explain why a normal DNA polymerase cannot start synthesis of a new DNA strand from single-stranded DNA alone.
[2]
Suggest the effect of the hypothetical polymerase on DNA replication in a prokaryotic cell.
[2]
Question 30
A replication fork is shown with the direction of fork movement and the polarity of the two parental template strands.
Deduce which template strand will be used to synthesize the leading strand. Give a reason.
[2]
Explain why Okazaki fragments are produced on the other template strand.
[2]
Question 31
A forensic laboratory estimated the probability of a false match when different numbers of independent DNA markers were used in DNA profiling.
Number of independent DNA markers | Estimated false-match probability |
|---|---|
1 | 0.10 |
2 | 0.01 |
3 | 0.001 |
4 | 0.0001 |
Describe the relationship shown between the number of independent markers and the probability of a false match.
[1]
Calculate the probability of a false match using four independent markers if the probability of a match at one marker is .
[1]
Explain why using more markers increases the reliability of DNA profiling.
[2]
Question 32
The diagram shows a replication fork moving along a DNA molecule. Newly synthesized DNA is shown on both template strands, but the leading and lagging strands are not named.
Identify the strand, A or B, that is the lagging strand.
[1]
Compare the use of RNA primers on the leading and lagging strands.
[1]
Explain why Okazaki fragments are formed on only one of the two newly synthesized strands.
[2]
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Question 33
A prokaryotic cell-free replication system was prepared with all required replication enzymes. In separate treatments, one enzyme was omitted. The products of replication were analysed for RNA primers, short DNA fragments and sealed sugar-phosphate backbones.
Omitted enzyme | RNA primers detected | Short DNA fragments detected | Sealed backbones detected |
|---|---|---|---|
X | No | No | No |
Y | Yes | Yes | No |
Z | No | Yes | No |
W | Yes | No | No |
Deduce which omitted enzyme is DNA ligase.
[1]
Identify the enzyme responsible for removing RNA primers and replacing them with DNA in prokaryotic replication.
[1]
Explain why no Okazaki fragments would be produced if DNA primase were omitted.
[2]
Question 34
Two strains of bacteria were compared. The mutant strain produces DNA polymerase III that can add nucleotides but has reduced proofreading activity. Replication errors were measured after one round of DNA replication.
Describe the effect of reduced proofreading activity on mismatch frequency.
[1]
Calculate the fold increase in mismatch frequency if the mutant value is and the wild-type value is .
[1]
Explain how DNA polymerase III proofreading reduces the number of replication errors.
[2]
Question 35
The diagram shows part of a newly replicated prokaryotic lagging strand after DNA polymerase III has extended an RNA primer to form an Okazaki fragment.
Identify the enzyme that removes the RNA primer and replaces it with DNA nucleotides in prokaryotes.
[1]
Identify the enzyme that seals the remaining nick in the sugar-phosphate backbone.
[1]
Explain why DNA polymerase III extends from an RNA primer rather than initiating a new DNA strand by itself.
[2]
Question 36
A researcher grew bacteria for many generations in a medium containing a heavy isotope of nitrogen and then transferred them to a medium containing the normal isotope. DNA was extracted after successive rounds of replication and separated by density.
Replication round | Heavy band / % | Intermediate band / % | Light band / % |
|---|---|---|---|
0 | 100 | 0 | 0 |
1 | 0 | 100 | 0 |
2 | 0 | 50 | 50 |
3 | 0 | 25 | 75 |
Explain how the results after one round of replication support semi-conservative replication.
[3]
Complementary base pairing allows accurate copying of DNA.
I.
State the complementary base pair for adenine in DNA and the complementary base pair for cytosine in DNA.
[1]
II.
Explain why an incorrect nucleotide is less likely to be retained in the new strand.
[1]
Discuss the importance of accurate DNA replication for multicellular organisms.
[2]
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Question 37
The diagram shows a replication fork in a eukaryotic chromosome. The parental DNA strands are separated and new strands are being synthesized.
Identify the role of the enzyme positioned at the point where the two parental strands separate.
I.
State the name of this enzyme.
[1]
II.
Outline its action on the DNA molecule.
[1]
Explain the role of DNA polymerase in producing a new strand at the replication fork.
[2]
Compare the bonds broken by helicase with the bonds formed by DNA polymerase during replication.
[2]
Question 38
Cells in the lining of the small intestine divide frequently to replace cells lost from the surface. Before each mitotic division, the DNA in each cell must be replicated.
Explain why DNA replication is required before mitosis in this tissue.
I.
State what is produced by DNA replication.
[1]
II.
Explain the importance of this before cell division.
[1]
Explain how the base sequence of a parental DNA strand determines the base sequence of a new strand.
[2]
Discuss one possible consequence if replication was not sufficiently accurate in dividing intestinal cells.
[2]
Question 39
Radioactively labelled nucleotides were supplied to replicating bacterial DNA for a very short pulse and then replaced with unlabelled nucleotides for a chase period. Autoradiographs were used to detect labelled DNA fragments.
Deduce which newly synthesized strand first contains many short labelled DNA fragments.
[1]
State the name of the short fragments produced on this strand.
[1]
Explain why the labelled short fragments become part of longer DNA during the chase period.
[2]
Question 40
A clinic uses PCR followed by gel electrophoresis to test patient samples for a DNA sequence found only in a bacterial pathogen. The target sequence is amplified using primers specific to the pathogen.
Explain why primers are essential in this PCR test.
I.
State the role of primers in PCR.
[1]
II.
Explain how primers contribute to the specificity of the test.
[1]
Explain the purpose of temperature changes during PCR.
[3]
Evaluate the reliability of using the gel result alone to diagnose infection.
[3]
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Question 41
DNA profiles from a child, the mother and two possible fathers were produced using PCR and gel electrophoresis at several genetic markers.
Explain how PCR and gel electrophoresis are used together to produce a DNA profile.
I.
State the function of PCR in DNA profiling.
[1]
II.
Explain how gel electrophoresis separates the PCR products.
[2]
Discuss how the profiles can be used to decide whether either possible father is excluded.
[2]
Evaluate why using more genetic markers increases the reliability of the conclusion.
[2]
Question 42
A student amplified DNA from two plant species using the same pair of primers. The PCR products were analysed by gel electrophoresis to determine whether the amplified region differed in length between the species.
Explain why Taq polymerase is used rather than most other DNA polymerases in PCR.
I.
State the property of Taq polymerase that is useful in PCR.
[1]
II.
Explain why this property is needed during PCR cycling.
[1]
Explain how the gel can be used to compare the lengths of PCR products from the two plant species.
[3]
Evaluate whether a faint extra band in one lane necessarily invalidates the conclusion.
[3]
Question 43
The diagram shows a replication fork moving along a prokaryotic chromosome. The parental strands are antiparallel and DNA polymerases are shown extending new strands.
Use the directionality shown in the diagram to identify the leading and lagging strands.
I.
State the direction in which DNA polymerase synthesizes new DNA.
[1]
II.
Identify which new strand is leading and which is lagging, using the diagram.
[1]
Compare and contrast replication on the leading and lagging strands.
[3]
Explain why antiparallel DNA strands make discontinuous synthesis necessary at one side of the fork.
[2]
Question 44
A mutation in a prokaryotic cell reduces the activity of one enzyme involved in processing Okazaki fragments. Newly synthesized lagging strands contain many short sections separated by unsealed nicks in the sugar-phosphate backbone.
Explain the sequence of enzyme actions required to make a continuous lagging strand in prokaryotes.
I.
State the role of DNA primase.
[1]
II.
State the role of DNA polymerase III.
[1]
III.
State the role of DNA polymerase I.
[1]
IV.
State the role of DNA ligase.
[1]
Deduce which enzyme is most likely affected in the mutant cell. Give a reason.
[2]
Discuss why this defect would affect the lagging strand more obviously than the leading strand.
[2]
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Question 45
A synthetic nucleotide analogue has a normal nitrogenous base but lacks the chemical group at the end needed for DNA polymerase to extend a growing strand. The analogue can sometimes be incorporated during DNA synthesis.
Explain why DNA polymerase requires an existing end to synthesize DNA.
I.
State the direction in which DNA polymerase adds nucleotides to a growing strand.
[1]
II.
Explain the chemical basis of extension at the end of the growing strand.
[2]
Deduce the effect of incorporating the analogue into a growing strand.
[2]
Discuss why such an analogue would affect both leading and lagging strand synthesis, but the pattern of interruption might differ.
[2]
Question 46
Two strains of Escherichia coli were compared. Strain A has normal DNA polymerase III. Strain B has DNA polymerase III that can add nucleotides but has reduced ability to remove mismatched nucleotides from the terminal of the growing strand.
Explain the normal proofreading action of DNA polymerase III.
I.
State where a newly added mismatched nucleotide is detected during proofreading.
[1]
II.
Describe the correction process after a mismatch is detected.
[2]
Use the graph to evaluate the effect of reduced proofreading on genetic continuity.
[3]
Suggest why complementary base pairing alone is not sufficient to prevent all mutations during replication.
[2]
Question 47
During replication of a prokaryotic chromosome, newly synthesized DNA was briefly labelled with a radioactive nucleotide. Shortly after labelling, much of the radioactivity on one side of each replication fork was found in short DNA fragments. Later, radioactivity was found in longer continuous DNA molecules.
Explain why short labelled fragments appear soon after the pulse.
I.
Name the short fragments made during lagging-strand synthesis.
[1]
II.
Explain why these fragments are produced on the lagging strand.
[2]
Explain why labelled DNA is later found in longer continuous molecules.
[3]
Compare the use of RNA primers on the leading and lagging strands.
[2]
Question 48
An antibiotic being developed for bacterial infections inhibits prokaryotic DNA primase. Researchers measured DNA synthesis at replication forks after adding the antibiotic.
Explain why inhibiting primase reduces DNA synthesis.
I.
State the product made by DNA primase.
[1]
II.
Explain why DNA polymerase III depends on this product.
[2]
Use the graph to discuss why lagging-strand synthesis is more strongly affected than leading-strand synthesis.
[3]
Evaluate one advantage and one limitation of targeting primase with an antibiotic.
[2]
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