Lesson 6.1.3a

6.1.3a DNA sequencing and DNA profiling Quiz: OCR Biology, Unit 7

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 6.1.3a, DNA sequencing and DNA profiling: 20 multiple choice questions for the OCR Biology (H020), Unit 7: Genetics and evolution, written with Revision Ninja.

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The 20 questions

  1. Which modified nucleotides terminate chain extension in Sanger (dideoxy) sequencing?

    • Ribonucleotides
    • Free deoxyribose sugars
    • Dideoxynucleotides
    • Amino acids

  2. Which region of the genome is primarily analysed during DNA profiling?

    • Ribosomal RNA genes
    • Structural centromeres
    • Protein-coding exons
    • Short tandem repeats

  3. What is the correct order of stages in a single PCR cycle?

    • Denaturation, annealing, extension
    • Denaturation, extension, annealing
    • Extension, annealing, denaturation
    • Annealing, denaturation, extension

  4. By which physical property are DNA fragments separated during standard gel electrophoresis?

    • Charge
    • Base composition
    • Size
    • Shape

  5. Where do restriction endonucleases cut double-stranded DNA molecules?

    • Specific recognition sites
    • Terminal telomeres
    • Non-coding introns
    • Random base pairs

  6. Which heat-stable DNA polymerase is commonly used in the polymerase chain reaction?

    • Taq polymerase
    • RNA polymerase
    • Reverse transcriptase
    • DNA ligase

  7. What is the main purpose of adding primers in PCR?

    • Join DNA fragments
    • Provide a 3' end
    • Denature target DNA
    • Hydrolyse nucleotides

  8. A single DNA molecule is subjected to 5 cycles of PCR, with each cycle doubling the DNA. How many copies are produced?

    • 32
    • 25
    • 10
    • 5

  9. In gel electrophoresis, which DNA fragments move furthest towards the positive electrode?

    • Largest fragments
    • Smallest fragments
    • Single-stranded fragments
    • Circular plasmids

  10. If ten independent loci each have a 1 in 10 match probability, what is the combined match probability?

    • 1 in 10 billion
    • 1 in 100
    • 1 in 100 thousand
    • 1 in 1 million

  11. A linear DNA molecule is cut by a restriction enzyme at 3 sites. How many fragments are produced?

    • 3
    • 2
    • 4
    • 6

  12. A forensic sample contains only about 1 ng of DNA. Which technique is used to increase the amount of DNA for profiling?

    • Western blotting
    • Electrophoresis alone
    • Ultracentrifugation
    • PCR

  13. What happens when a dideoxynucleotide is incorporated into a growing DNA strand during sequencing?

    • Chain termination occurs
    • Transcription begins
    • Strand unwinding starts
    • Mutation rate increases

  14. In Sanger sequencing, why do synthesised DNA fragments terminate at different lengths?

    • Primer degradation
    • Random dideoxynucleotide incorporation
    • Variable gel temperature
    • Enzyme denaturation

  15. Why can a matching DNA profile not guarantee absolute 100% certainty of identity?

    • Relatives share alleles
    • Enzymes lack specificity
    • DNA degrades instantly
    • STRs are non-variable

  16. Why is interpreting a DNA profile difficult when a sample contains DNA from multiple individuals?

    • Gel melts
    • Primers fail
    • DNA unwinds
    • Peaks overlap

  17. A PCR begins with 4 cycles producing 16 copies. A sample starts with 500 copies and undergoes 6 cycles. How many copies result?

    • 32000
    • 6400
    • 16000
    • 3000

  18. Why are short tandem repeats (STRs) useful in human DNA profiling?

    • Found only on ribosomes
    • Variable repeat numbers
    • Code for vital enzymes
    • Identical in all humans

  19. Why is contamination particularly problematic when preparing samples for PCR amplification?

    • Taq polymerase inhibits
    • Contaminating DNA amplifies
    • Primers digest sample
    • DNA fragments dissolve

  20. Which region of human DNA is primarily analysed during standard DNA profiling?

    • Exons coding proteins
    • Mitochondrial coding genes
    • Ribosomal RNA genes
    • Non-coding repeat sequences

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