2.4.1 DNA replication and the Meselson and Stahl experiment Quiz: Pearson Edexcel Biology A (Salters-Nuffield), Unit 2

The 20 questions

1. Meselson and Stahl's experiment supported a model of DNA replication. What does 'semi-conservative' mean?

   • The two original strands stay together in one molecule and the second molecule is built entirely from new strands
   • Each daughter molecule contains two strands made from random pieces of the parent molecule
   • New DNA is scattered in short pieces between original and new segments, so every strand is a mixture
   • Each daughter DNA molecule contains one original strand and one newly synthesised strand

2. In Meselson and Stahl's experiment, bacteria were first grown in a medium containing which isotope of nitrogen?

   • Carbon-14 (14C), a radioactive isotope of carbon
   • Nitrogen-15 (15N), a heavy isotope of nitrogen
   • Phosphorus-32 (32P), a radioactive isotope of phosphorus
   • Nitrogen-14 (14N), the common light isotope of nitrogen

3. Which enzyme adds free nucleotides to the growing end of a new DNA strand?

   • DNA ligase, which joins Okazaki fragments on the lagging strand
   • DNA helicase, which unwinds the double helix at the replication fork
   • RNA primase, which makes the RNA primer used to start synthesis
   • DNA polymerase, which adds nucleotides to the free 3' end of a growing strand

4. What is the role of DNA helicase during replication?

   • Joins Okazaki fragments on the lagging strand by forming phosphodiester bonds
   • Breaks hydrogen bonds between complementary bases so that the two strands separate
   • Adds free deoxynucleotides to the 3' end of a growing strand in pairs
   • Removes RNA primers and replaces them with DNA nucleotides

5. In which direction is a new DNA strand always synthesised?

   • In whichever direction is faster on the leading or lagging template
   • In both directions at once, moving outwards from the origin of replication
   • Always 5' to 3', adding nucleotides to the free 3' end of the growing strand
   • Always 3' to 5', adding nucleotides to the free 5' end of the growing strand

6. Which statement correctly contrasts the leading and lagging strands?

   • The leading strand is made continuously towards the replication fork, the lagging strand discontinuously away from it
   • Both strands are made continuously at equal rates, moving in opposite directions along the fork
   • The leading strand is made discontinuously in short fragments, the lagging strand as one continuous strand
   • The lagging strand is made continuously in the direction of unwinding, the leading strand in short fragments

7. Which pair of bases is correctly matched by hydrogen bonding in DNA?

   • Adenine with uracil, and guanine with thymine
   • Adenine with guanine, and thymine with cytosine
   • Adenine with thymine, and guanine with cytosine
   • Adenine with cytosine, and thymine with guanine

8. A bacterium with fully 15N-labelled DNA is transferred to 14N medium. What is seen in the DNA after one generation?

   • Two separate bands, one heavy and one light, in equal amounts
   • A single heavy band, because no 14N has yet been incorporated
   • A single band of intermediate density, halfway between heavy and light DNA
   • A single light band, because all new DNA is made from 14N

9. After two generations in 14N medium, which bands are expected in a density gradient?

   • One band of intermediate density only
   • Three bands: heavy, intermediate and light, in equal amounts
   • Two bands: one intermediate-density band and one light band
   • Two bands: one heavy band and one light band

10. A single double-stranded DNA molecule labelled with 15N is replicated for three generations in 14N. How many of the resulting molecules contain some 15N?

    • 4 molecules
    • 8 molecules
    • 1 molecule
    • 2 molecules

11. A DNA molecule labelled with 15N is replicated for four generations in 14N medium. What percentage of the resulting double-stranded molecules is entirely 14N?

    • 25%
    • 87.5%
    • 12.5%
    • 50%

12. A student observes only an intermediate-density band after one generation of replication. Which model does this rule out?

    • Dispersive replication, which predicts a single intermediate band after one generation
    • Conservative replication, which predicts separate heavy and light bands after one generation
    • Semi-conservative replication, which predicts a single intermediate band after one generation
    • Semi-conservative replication, which predicts only light DNA after one generation

13. In the second generation a light band appears alongside an intermediate band. Which model does this refute?

    • Semi-conservative replication, because it predicts only heavy DNA in generation two
    • Semi-conservative replication, because it predicts a light band in generation two
    • Dispersive replication, because it predicts all DNA would remain intermediate in density
    • Conservative replication, because it predicts only intermediate DNA in generation two

14. A DNA sample contains 30% adenine. What percentage of its bases is cytosine?

    • 20%
    • 30%
    • 10%
    • 40%

15. Why do lagging-strand Okazaki fragments require DNA ligase?

    • To unwind the double helix ahead of the replication fork
    • To break hydrogen bonds between complementary bases
    • To join adjacent fragments by forming phosphodiester bonds between nucleotides
    • To add the RNA primer that starts each fragment

16. Why does DNA polymerase need a primer before it can begin synthesis?

    • It requires a primer to break the hydrogen bonds between complementary bases
    • It can only synthesise DNA that is already single-stranded in both directions
    • It can only add nucleotides to an existing free 3' end
    • It can only add nucleotides to a free 5' end, so a primer must remove one

17. A segment of 100 base pairs contains 40 G-C pairs and 60 A-T pairs. How many hydrogen bonds hold the segment together?

    • 300
    • 200
    • 260
    • 240

18. Which enzyme synthesises the short RNA primer needed to start DNA replication?

    • Topoisomerase
    • Helicase
    • DNA ligase
    • Primase

19. Why does the lagging strand need to be made in short fragments?

    • DNA polymerase can only add nucleotides to the lagging strand in pairs at each step
    • Its template runs antiparallel to the direction the fork opens
    • Its template is shorter than the leading strand template, so fewer nucleotides are needed
    • Helicase unwinds that strand in short bursts, so synthesis must match each burst

20. If one generation of DNA replication were conservative, what would the density gradient show after the first generation?

    • Light DNA only, with no heavy band
    • Heavy DNA only, with no light band
    • Heavy DNA and light DNA as two separate bands
    • A single band of intermediate density only

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