Lesson 3.1.5.2.1

3.1.5.2.1 Semi-conservative DNA replication Quiz: AQA Biology, Unit 1

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.1.5.2.1, Semi-conservative DNA replication: 20 multiple choice questions for the AQA Biology (7402), Unit 1: Biological molecules, written with Revision Ninja.

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The 20 questions

  1. Which enzyme unwinds the DNA double helix and breaks the hydrogen bonds between bases during replication?

    • ATP hydrolase
    • ATP synthase
    • DNA polymerase
    • DNA helicase

  2. Which enzyme joins adjacent nucleotides together during DNA replication?

    • Lipase
    • Amylase
    • DNA helicase
    • DNA polymerase

  3. What does semi-conservative replication mean?

    • Each new DNA molecule contains two original strands and no new ones
    • The original DNA molecule is cut in half and each half is copied separately
    • Each new DNA molecule contains one original strand and one newly synthesised strand
    • Each new DNA molecule contains two newly synthesised strands and no original ones with no exceptions

  4. Which bonds are broken when DNA is unwound during replication?

    • Glycosidic bonds between the sugars in the strands
    • Peptide bonds between amino acids in the enzymes
    • Phosphodiester bonds in the sugar-phosphate backbone
    • Hydrogen bonds between complementary base pairs

  5. What is the role of the template strand in DNA replication?

    • It provides the energy needed to break the hydrogen bonds between bases
    • It acts as the enzyme that joins nucleotides into a new strand
    • Its exposed bases determine which free nucleotides pair with it to form the new strand
    • It is cut into short fragments that are joined to form the new strand

  6. Which bond does DNA polymerase form between nucleotides?

    • Peptide bond formed between two amino acid residues
    • Glycosidic bond formed between two monosaccharide units
    • Hydrogen bond formed between complementary base pairs
    • Phosphodiester bond

  7. Which scientists' experiment with nitrogen isotopes provided evidence for semi-conservative DNA replication?

    • Hershey and Chase with no exceptions
    • Meselson and Stahl
    • Griffith
    • Mendel

  8. A DNA molecule has 1000 base pairs and replicates once. How many new nucleotides must be assembled in total?

    • 2000
    • 500
    • 4000
    • 1000

  9. A DNA molecule labelled with heavy nitrogen (15N) is replicated for two generations in light nitrogen (14N). What fraction of the resulting DNA molecules contain only 14N?

    • 0
    • 1/2
    • 3/4
    • 1/4

  10. A DNA molecule replicates three times. How many DNA molecules are produced from one starting molecule?

    • 3
    • 6
    • 8
    • 16

  11. A drug blocks DNA helicase in a cell about to divide. What is the most likely consequence?

    • The cell produces extra RNA, because the template strands are not unwound
    • DNA polymerase is inhibited, so the nucleotides are not linked into a strand
    • The DNA strands cannot separate, so new nucleotides cannot pair with the template strands
    • The cell's ribosomes produce more protein to replace the lost DNA

  12. During replication of a DNA molecule with 100 A-T base pairs, how many hydrogen bonds must be broken to separate the strands?

    • 200
    • 300
    • 400
    • 100

  13. A student claims that a new DNA strand is made by using another new strand as its template. Which evaluation is correct?

    • Incorrect, because each new strand is synthesised using an original template strand
    • Correct, because both new strands are made from each other in alternating steps
    • Incorrect, because new strands are made from amino acids rather than nucleotides
    • Correct, because the template is the sugar-phosphate backbone rather than the bases

  14. Which statement correctly describes the direction in which DNA polymerase adds nucleotides?

    • Nucleotides are added to the middle of the strand, so the chain grows in both directions
    • Nucleotides are added to the 3' end of the growing strand, in the 5' to 3' direction
    • Nucleotides are added to the phosphate group, so the direction is set by the base
    • Nucleotides are added to the 5' end of the growing strand, in the 3' to 5' direction

  15. In the Meselson and Stahl experiment, what result after one generation in light nitrogen ruled out the conservative replication model?

    • A single band of intermediate density was seen, not separate heavy and light bands
    • Two bands were seen, one heavy and one light, which matched the conservative model
    • Only heavy DNA was seen, which showed that all new DNA was made from the old strands
    • No band was seen, so the DNA was not replicated at all

  16. A DNA molecule of 500 base pairs (1000 nucleotides in total) replicates four times in a medium containing all required nucleotides. How many new nucleotides are assembled overall?

    • 14000
    • 15000
    • 16000
    • 1000

  17. Explain why replication is described as semi-conservative and how this helps genetic continuity between cell generations.

    • Each daughter molecule is made entirely from original strands, so no new genetic information is added
    • Each daughter molecule is made from RNA templates, so the information is preserved without DNA
    • Each daughter molecule keeps one original strand as a template, so the base sequence is copied accurately into each new cell
    • Each daughter molecule contains only new strands, so mutations are removed in each generation

  18. Explain how complementary base pairing allows the sequence of one DNA strand to determine the sequence of the other.

    • Adenine pairs with any base that happens to be nearby, so the sequence of one strand has no effect on the other
    • Guanine pairs with thymine and adenine pairs with cytosine, so the sequence of the partner strand is reversed
    • Adenine always pairs with thymine and cytosine always pairs with guanine, so each base on one strand specifies its partner
    • The phosphate groups determine which base pairs with each base on the opposite strand, regardless of its identity in every case

  19. Why did the Meselson and Stahl experiment use nitrogen isotopes and density gradient centrifugation?

    • Centrifugation removes the nitrogen atoms from the DNA, leaving pure carbon-based molecules for analysis
    • Light nitrogen forms covalent bonds that make the DNA soluble in the medium used in the centrifuge tube
    • Heavy nitrogen breaks the hydrogen bonds between the bases, so the two strands separate during centrifugation
    • The heavy and light isotopes of nitrogen give DNA molecules different densities, so they can be separated by centrifugation

  20. A student says that the Watson-Crick model predicts that each daughter DNA molecule must be identical to the parent. Which evaluation is correct?

    • Partly correct, because semi-conservative copying gives identical sequences only if no errors occur during replication
    • Incorrect, because the Watson-Crick model says that DNA does not replicate at all
    • Correct, because complementary base pairing guarantees perfect copies in every case
    • Incorrect, because the Watson-Crick model predicts that daughter molecules are entirely different from the parent in every case

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