D1.1 DNA replication
Practice exam-style IB Biology questions for DNA replication, aligned with the syllabus and grouped by topic.
Question 1
What is produced by DNA replication in a dividing cell?
A.
One DNA molecule with a new base sequence and one unchanged parental molecule
B.
Two DNA molecules with identical base sequences to the original molecule
C.
Two RNA molecules with base sequences complementary to the original molecule
D.
Protein molecules with amino acid sequences determined by the original molecule
Question 2
In semi-conservative replication, what is found in each daughter DNA molecule?
A.
Two original parental strands
B.
One parental strand and one newly synthesized strand
C.
Two newly synthesized strands
D.
Alternating short sections of parental and newly synthesized DNA in both strands
Question 3
What is the role of helicase during DNA replication?
A.
It stains DNA fragments after gel electrophoresis.
B.
It joins adjacent DNA fragments by forming phosphodiester bonds.
C.
It adds complementary nucleotides to the 5′ end of a growing strand.
D.
It unwinds DNA and separates the strands by breaking hydrogen bonds.
Question 4
What is the general role of DNA polymerase in DNA replication?
A.
Synthesizing a new DNA strand using a template strand
B.
Producing amino acid chains from a base sequence
C.
Breaking the sugar-phosphate backbone of parental DNA
D.
Separating DNA fragments according to their length
Question 5
At which end of a growing DNA strand are nucleotides added by DNA polymerase?
A.
The 3′ end, by joining the 5′ phosphate of the incoming nucleotide
B.
Both ends, because DNA synthesis has no directionality
C.
The 5′ end, by joining the 3′ hydroxyl of the incoming nucleotide
D.
Neither end, because DNA polymerase only separates strands
Question 6
State one reason why DNA replication is required in multicellular organisms.
[1]
State what is meant by an identical base sequence in replicated DNA.
[1]
Question 7
What is the function of primers in PCR?
A.
They break phosphodiester bonds to fragment DNA before amplification.
B.
They move DNA fragments through a gel toward the positive electrode.
C.
They replace thymine with uracil during extension.
D.
They bind to selected DNA sequences and provide a starting point for DNA polymerase.
Question 8
Why is Taq polymerase used in PCR?
A.
It causes DNA fragments to migrate toward the negative electrode.
B.
It remains active after the high temperatures used to separate DNA strands.
C.
It removes RNA primers from Okazaki fragments in prokaryotes.
D.
It stains amplified DNA so that bands fluoresce.
Question 9
In gel electrophoresis of DNA, which fragments travel furthest from the wells in a set time?
A.
The smallest fragments, because they pass most easily through the gel matrix
B.
The fragments with the most adenine, because adenine is positively charged
C.
The fragments with the most thymine, because thymine binds most strongly to the gel
D.
The largest fragments, because they have the greatest negative charge
Question 10
What is a correct description of leading-strand synthesis at a replication fork?
A.
Discontinuous synthesis toward the fork with Okazaki fragments
B.
Continuous synthesis toward the replication fork after one RNA primer
C.
Discontinuous synthesis away from the replication fork using many RNA primers
D.
Continuous synthesis away from the replication fork without a template
Question 11
What is an Okazaki fragment?
A.
A short length of newly synthesized DNA made on the lagging strand
B.
A fluorescent dye used to make bands visible after electrophoresis
C.
A short region of parental DNA removed by helicase
D.
A protein that attaches to the replication fork
Question 12
What is the function of DNA primase in prokaryotic DNA replication?
A.
It seals nicks between adjacent DNA fragments.
B.
It synthesizes short RNA primers on a DNA template.
C.
It removes RNA primers and replaces them with DNA.
D.
It detects DNA bands after gel electrophoresis.
Question 13
Which enzyme is the main prokaryotic enzyme that extends RNA primers during DNA replication?
A.
DNA polymerase I
B.
DNA ligase
C.
DNA polymerase III
D.
DNA primase
Question 14
Define semi-conservative replication.
[1]
Outline how complementary base pairing contributes to accurate DNA replication.
[1]
Question 15
State the role of helicase in DNA replication.
[1]
State the role of DNA polymerase in DNA replication.
[1]
Distinguish the bonds affected by helicase from those formed in the growing DNA strand.
[1]
Question 16
State why DNA fragments move toward the positive electrode during gel electrophoresis.
[1]
Outline how a gel separates DNA fragments of different lengths.
[1]
Question 17
State the chemical difference between the 5′ and 3′ ends of a DNA strand.
[1]
State the direction in which DNA polymerase synthesizes DNA.
[1]
Question 18
The graph shows the temperature of a PCR reaction during several repeated cycles.
Identify the stage labelled X, when the temperature is highest.
[1]
State what happens to DNA strands during stage X.
[1]
Identify the stage in which primers bind to the target DNA.
[1]
Explain why a heat-stable DNA polymerase is required for this process.
[1]
Question 19
Why does using more independent genetic markers improve the reliability of DNA profiling?
A.
It increases the rate at which DNA fragments move through the gel.
B.
It reduces the probability that two unrelated individuals share the same overall profile by chance.
C.
It prevents all mutations from occurring during PCR.
D.
It changes the genetic code so that each person has unique bases.
Question 20
After RNA primers have been replaced with DNA on the lagging strand, which enzyme seals the remaining breaks in the sugar-phosphate backbone?
A.
Taq polymerase
B.
DNA ligase
C.
Helicase
D.
DNA primase
Question 21
During proofreading in prokaryotic replication, what does DNA polymerase III remove?
A.
A mismatched nucleotide from the 3′ terminal of the growing strand
B.
A hydrogen bond between two parental DNA strands
C.
A correctly paired nucleotide from the 5′ terminal of the template strand
D.
An RNA primer from the middle of an Okazaki fragment
Question 22
Why is lagging-strand synthesis discontinuous?
A.
Helicase unwinds only one parental strand at a time, so gaps form randomly.
B.
Complementary base pairing occurs only after the whole parental strand has been copied.
C.
DNA polymerase can synthesize only 5′ to 3′, so synthesis occurs away from the fork in short sections.
D.
DNA ligase prevents continuous replication by breaking phosphodiester bonds.
Question 23
PCR is used to amplify a DNA sequence from a small forensic sample.
State the purpose of denaturation in PCR.
[1]
State the purpose of annealing in PCR.
[1]
Explain why primers determine which DNA sequence is amplified.
[1]
Question 24
A child, the mother and a possible father are tested using DNA profiling.
State why PCR is used before gel electrophoresis in DNA profiling.
[1]
Outline how bands in the child’s profile are used to assess paternity.
[1]
State one application of DNA profiling other than paternity testing.
[1]
Question 25
A DNA profile at one genetic marker matches between a crime-scene sample and a suspect.
State what is meant by a genetic marker in DNA profiling.
[1]
Suggest why a match at one marker alone is weak evidence.
[1]
Suggest how the reliability of the DNA profile could be improved.
[1]
Question 26
State the direction of leading-strand synthesis relative to the replication fork.
[1]
State the direction of lagging-strand synthesis relative to the replication fork.
[1]
Compare the use of RNA primers on the leading and lagging strands.
[1]
Question 27
In prokaryotic DNA replication, several enzymes act on the lagging strand.
State the role of DNA primase.
[1]
State the role of DNA polymerase III.
[1]
Outline the roles of DNA polymerase I and DNA ligase after an Okazaki fragment has been extended.
[1]
Question 28
Define an Okazaki fragment.
[1]
Explain why Okazaki fragments are produced on only one new strand at a replication fork.
[1]
Question 29
State what is meant by antiparallel DNA strands.
[1]
Distinguish the template direction copied by DNA polymerase from the direction of new-strand synthesis.
[1]
Question 30
The gel shows DNA profiles for a mother, a child and three possible fathers at several markers.
Identify one child band that can be attributed to the mother.
[1]
Deduce which possible father is most likely to be the biological father.
[1]
Give evidence from the gel for your answer to (b).
[1]
Suggest why additional markers would be tested before making a final conclusion.
[1]
Question 31
A mixture of DNA fragments of different lengths was separated by gel electrophoresis. The graph shows distance migrated by fragments of known length.
Describe the relationship between fragment length and distance migrated.
[1]
Estimate the length of an unknown fragment from its migration distance using the graph.
[1]
Explain the relationship shown in the graph.
[1]
Question 32
The graph shows the amount of target DNA after successive PCR cycles.
Describe the change in amount of target DNA during the cycles shown.
[1]
Calculate the expected relative amount of target DNA after the stated number of cycles if amplification is perfectly efficient.
[1]
Suggest why the observed amount may be lower than the theoretical amount after many cycles.
[1]
Question 33
The diagram shows a replication fork with parental strand directions and newly synthesized DNA segments.
Deduce which newly synthesized strand is the leading strand.
[1]
Give one reason for your answer to (a).
[1]
Identify an Okazaki fragment in the diagram.
[1]
Explain why repeated RNA primers are required on the other newly synthesized strand.
[1]
Question 34
State one feature of DNA structure that allows each strand to act as a template.
[1]
Explain how semi-conservative replication and complementary base pairing maintain genetic continuity.
[1]
Question 35
State what is meant by DNA proofreading during replication.
[1]
Explain the action of DNA polymerase III during proofreading in prokaryotes.
[1]
State one consequence of proofreading for genetic continuity.
[1]
Question 36
A mutation in a prokaryote reduces the activity of DNA polymerase I but does not affect DNA polymerase III.
State which stage of replication would still occur normally.
[1]
Predict what would happen to RNA primers.
[1]
Explain why DNA ligase could not fully complete lagging-strand processing in this mutant.
[1]
Question 37
Place the following events in order during processing of a prokaryotic Okazaki fragment: DNA ligase seals a nick; DNA polymerase I replaces RNA with DNA; DNA polymerase III extends an RNA primer; primase synthesizes an RNA primer.
State the first event.
[1]
State the second event.
[1]
State the third and fourth events.
[1]
Question 38
The table shows the estimated probability of a random match in DNA profiling as the number of independent genetic markers is increased.
| Independent markers tested | Random-match probability |
|---|---|
| 4 | 1 × 10^-6 |
| 8 | 1 × 10^-12 |
| 12 | 1 × 10^-18 |
| 16 | 1 × 10^-24 |
State the trend shown in the table.
[1]
Calculate the fold decrease in random-match probability between the smallest and largest number of markers shown.
[1]
Explain why independent markers have this effect on reliability.
[1]
Suggest one limitation of using random-match probability alone when evaluating DNA evidence.
[1]
Question 39
A prokaryotic cell extract was tested with four enzyme inhibitors. The table shows the main replication product observed with each inhibitor.
| Inhibitor | Main replication product observed |
|---|---|
| A | Only pre-existing DNA strands lengthened; no new Okazaki fragments formed. |
| B | Okazaki fragments formed, but RNA primer sections remained within the DNA. |
| C | Okazaki fragments formed without RNA primer sections, but adjacent fragments remained unsealed. |
| D | RNA primers formed, but little DNA was added beyond the primers. |
Identify the inhibitor most likely to affect DNA primase.
[1]
Identify the inhibitor most likely to affect DNA polymerase I.
[1]
Explain the product expected when DNA ligase is inhibited.
[1]
State which enzyme should still extend RNA primers if DNA polymerase I is inhibited.
[1]
Question 40
The graph compares the number of replication errors detected after DNA synthesis in two strains of a prokaryote: a wild-type strain and a strain with reduced DNA polymerase III proofreading activity.
State which strain has the higher error frequency.
[1]
Calculate the percentage increase in error frequency in the mutant strain compared with the wild type.
[1]
Explain the difference between the strains in terms of DNA polymerase III activity.
[1]
Question 41
Outline the semi-conservative nature of DNA replication.
[1]
Explain how DNA replication produces accurate copies of a DNA molecule.
[1]
Question 42
Outline the three main temperature-dependent stages of PCR.
[1]
Discuss how PCR and gel electrophoresis can be used together to analyse DNA.
[1]
Question 43
A researcher supplied DNA polymerase with a single-stranded DNA template and four possible primers. The diagram shows which primers were extended.
Identify the primer that could be extended by DNA polymerase.
[1]
State the feature of the primer that allowed extension.
[1]
Explain why the primer with its 5′ end next to the next template base could not be extended in that direction.
[1]
Deduce the direction in which the template strand is read during extension.
[1]
Question 44
Newly synthesized DNA was briefly labelled with a radioactive nucleotide. The diagram shows the distribution of label close to a replication fork after a short pulse.
Identify which side of the fork shows lagging-strand synthesis.
[1]
Give evidence from the distribution of label for your answer.
[1]
Explain why labelled DNA appears in short separated sections on that side.
[1]
Question 45
Outline how DNA profiles are produced from selected regions of DNA.
[1]
Evaluate the use of DNA profiling in forensic investigations.
[1]
Question 46
State two biological contexts in which DNA replication is required.
[1]
Explain the roles of helicase, DNA polymerase and complementary base pairing in copying DNA before cell division.
[1]
Question 47
State the direction in which DNA polymerases synthesize DNA and the end of the growing strand that is extended.
[1]
Compare and contrast replication on the leading strand and the lagging strand at a replication fork.
[1]
Question 48
Outline the role of RNA primers in prokaryotic DNA replication.
[1]
Explain the functions of DNA primase, DNA polymerase III, DNA polymerase I and DNA ligase in replication of the lagging strand.
[1]
Question 49
Outline how complementary base pairing contributes to fidelity during DNA replication.
[1]
Discuss how directionality and proofreading together influence the accuracy and organization of prokaryotic DNA replication.
[1]
Question 50
A prokaryotic mutant has normal helicase activity but reduced DNA ligase activity.
Predict two immediate effects on DNA replication products in this mutant.
[1]
Evaluate how reduced DNA ligase activity would affect leading-strand and lagging-strand replication, using the roles of other prokaryotic replication enzymes in your answer.
[1]
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