Lesson 6.1.2

6.1.2 DNA profiling and PCR amplification Quiz: Pearson Edexcel Biology A (Salters-Nuffield), Unit 6

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 6.1.2, DNA profiling and PCR amplification: 20 multiple choice questions for the Pearson Edexcel Biology A (Salters-Nuffield) (9BI0), Unit 6: Immunity, Infection and Forensics, written with Revision Ninja.

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The 20 questions

  1. Which technique is used to make many copies of a small DNA sample?

    • Gel electrophoresis, which separates DNA fragments according to their length
    • Chromatography, which separates molecules by their charge and size only
    • Restriction digestion, which cuts DNA at specific sequences and does not copy it
    • Polymerase chain reaction (PCR)

  2. Which temperature step in PCR separates the two strands of the DNA double helix?

    • Denaturation at about 95 C, when hydrogen bonds between the strands break
    • Annealing at about 55 C, when primers bind to the single-stranded template
    • Extension at about 72 C, when the polymerase builds new strands from nucleotides
    • Cooling to room temperature, when the enzyme is inactivated so the reaction stops

  3. Why is a heat-stable DNA polymerase, such as Taq polymerase, used in PCR?

    • It cuts the DNA into fragments, which makes the separation by gel electrophoresis easier
    • It survives the high temperatures used in denaturation
    • It works only at room temperature, so the reaction must be kept cool at all times
    • It breaks down the primers, so the reaction produces only single-stranded DNA

  4. What is the role of primers in PCR?

    • They cut the template DNA at specific sites so that fragments of the target can be separated
    • They form the new DNA strands by themselves without needing any enzyme to join nucleotides
    • They label the DNA with a fluorescent dye so that it can be seen after electrophoresis
    • They bind to the template strands and provide a starting point for DNA polymerase to synthesise new DNA

  5. After 20 PCR cycles starting from one double-stranded DNA molecule, and assuming 100% efficiency, how many copies of the target sequence are produced?

    • 1,048,576 copies, since 2^20 = 1,048,576
    • 20,000 copies, since each cycle adds 1,000 new copies to the total
    • 40 copies, since each cycle adds two copies to the original molecule
    • 524,288 copies, since 2^19 is the number produced after twenty complete cycles

  6. Gel electrophoresis separates DNA fragments. In an agarose gel, which statement is correct?

    • DNA is negatively charged, so it moves towards the positive electrode
    • DNA is positively charged, so it moves towards the negative electrode, and longer fragments travel further
    • DNA is neutral, so fragments are separated by their density at the top of the gel
    • DNA does not move in an electric field, so the separation depends only on the gel's colour

  7. A DNA sample is run on a gel and the fragments are compared with a size marker. What does the marker allow?

    • The approximate lengths of the unknown fragments to be estimated from their distance travelled
    • The age of the person who provided the sample to be estimated from fragment size
    • The number of individuals who contributed DNA to the sample to be counted directly
    • The exact base sequence of each fragment to be determined without further testing

  8. DNA profiling is used in forensics mainly because:

    • Short tandem repeat sequences vary between individuals
    • Every individual has the same number of genes, so profiles reveal only gender
    • DNA is destroyed within minutes of death, so profiling is only possible on living people
    • All human DNA is identical, so profiling can only identify species and not individuals

  9. DNA profiling can also be used to determine genetic relationships between organisms. Which is the best example?

    • Testing the pH of a sample to determine whether two plants share a common ancestor
    • Comparing repeat-region profiles of animals to determine whether they belong to the same family group
    • Measuring the colour of a feather to determine how closely two bird species are related
    • Counting the total number of cells in a tissue sample to determine the age of an animal

  10. Which step of PCR allows primers to bind to complementary sequences on the single-stranded template?

    • Annealing, at a lower temperature of about 55 to 65 C
    • Ligation, at about 37 C, when the fragments are joined by a separate enzyme
    • Extension, at about 72 C, when the polymerase synthesises the new strands
    • Denaturation, at about 95 C, when the double helix is separated into single strands

  11. Why does PCR require a high level of care to avoid contamination?

    • Because PCR only works in sterile conditions, so the reaction is always carried out in a vacuum
    • Because contaminating DNA reduces the temperature of the reaction and so stops the cycle
    • Because PCR destroys all DNA in a sample, so any contamination makes the result impossible to read
    • Because even tiny amounts of foreign DNA would be amplified along with the target

  12. A PCR run produces a band on a gel for one sample but not for a negative control. What is the most likely interpretation?

    • The target sequence was present in the sample and amplified, while the control contained no template
    • The target sequence was destroyed by the reaction, so the gel shows its absence
    • The control was contaminated, so both samples show identical bands on the gel
    • The polymerase failed, so the gel contains only primers and no DNA at all

  13. Which is the main limitation of using PCR on a very small or degraded sample?

    • PCR cannot amplify any DNA from a sample unless it contains at least one whole chromosome
    • PCR produces only the primers, so no information about the original sample can be obtained
    • The amplified product may be a mixture or may reflect contamination
    • PCR requires the sample to be cooled to absolute zero, which damages all the DNA in it

  14. Why is the polymerase chain reaction described as exponential?

    • Because the number of copies doubles in each cycle
    • Because the reaction speed slows down as the number of copies increases over time
    • Because the number of copies increases by the same fixed amount in each cycle
    • Because the length of DNA fragments increases by a fixed amount with each cycle

  15. Which of the following is a true statement about DNA profiling?

    • A profile is always certain to identify a suspect, so no statistical interpretation is ever needed
    • A profile can only be produced from blood, so other samples such as hair cannot be used
    • A profile shows the physical appearance of a person, including their height and eye colour
    • A profile is only as reliable as the sample, the markers analysed and the statistical interpretation of matches

  16. Which molecule is copied during PCR?

    • Only the protein products of the target gene, which are copied from its mRNA
    • The entire genome of the organism in a single cycle, regardless of primer design
    • A specific region of double-stranded DNA, defined by the two primers
    • The ribosomes present in the sample, which are used to build new DNA strands

  17. What is the function of magnesium ions and the buffer in a PCR reaction?

    • Magnesium ions act as a cofactor for the polymerase, and the buffer keeps the pH stable
    • Magnesium ions break the hydrogen bonds between strands at room temperature without heating
    • The buffer binds to the primers and prevents them from annealing to the template strands
    • Magnesium ions cut the DNA into shorter lengths so that the fragments can be counted

  18. How many PCR cycles are needed to go from one template molecule to at least 1,000 copies, assuming perfect doubling?

    • 5 cycles, since 2^5 = 32 and this already exceeds 1,000 copies in total
    • 10 cycles, since 2^10 = 1,024 is the first power of two above 1,000
    • 100 cycles, since 1,000 divided by 10 gives the number of cycles needed
    • 3 cycles, since 3 x 1,000 = 3,000 copies is the smallest target that can be reached

  19. Which factor determines how far a DNA fragment travels in an agarose gel during electrophoresis?

    • Its colour, since the stain binds more strongly to longer fragments and slows them down
    • Its number of copies, since samples with more copies always travel further in the field
    • Its temperature, since warm fragments diffuse faster through the gel than cool ones
    • Its length, since shorter fragments pass more easily through the gel pores and move further

  20. In PCR, what is meant by the term template?

    • The banding pattern produced after the amplified product has been run on a gel and stained
    • The plastic tube in which the reaction is placed in the thermal cycler during each run
    • The original DNA strand containing the target sequence, which is copied by the polymerase
    • The mixture of nucleotides that is added to build the new strands during each extension step

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