Semi-conservative DNA Replication · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Half old strand, half new print

Science · Genetics & Evolution · ages 16-17
Name ______________________   Date ____________

  1. Which enzyme unwinds the double helix?

    • DNA polymerase
    • Ligase
    • Helicase

  2. What does semi-conservative replication mean?

    • Each new molecule keeps one old strand and one newly built strand
    • Each new molecule is built entirely from scratch
    • The parent molecule is fully preserved untouched

  3. Each new DNA molecule is built entirely from scratch.

    Circle one:   True   False

  4. What does primase do, and why is it needed?

    • It unwinds the helix using ATP
    • It lays a short primer because polymerase cannot start from nothing
    • It joins fragments into one strand

  5. Why is one strand built continuously and the other in fragments?

    • One strand has no template to copy
    • Polymerase only adds bases one way, so the other strand loops back in pieces
    • Ligase refuses to work on smooth strands

  6. What seals the gaps between Okazaki fragments?

    • Helicase
    • Primase
    • Ligase

  7. What did the Meselson Stahl result rule out?

    • The use of enzymes in copying
    • Fully kept and fully mixed models of copying
    • The existence of DNA itself

  8. A student says the leading strand is built in fragments. What is wrong?

    • The leading strand grows smoothly while the lagging strand fragments
    • Strands never differ at any fork
    • Fragments only appear outside cells
LightMySky · lightmysky.comW1-mt_wIcBfUw295-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Half old strand, half new print W1-mt_wIcBfUw295-s1

  1. Helicase · Unwinding belongs to helicase, fuelled by ATP.
  2. Each new molecule keeps one old strand and one newly built strand · Half history and half new print is the whole picture.
  3. False · Every daughter inherits one original strand from its parent.
  4. It lays a short primer because polymerase cannot start from nothing · Every new stretch begins with a primer that polymerase extends.
  5. Polymerase only adds bases one way, so the other strand loops back in pieces · Direction forces the lagging side into Okazaki fragments.
  6. Ligase · Joining belongs to ligase, stitching pieces into one strand.
  7. Fully kept and fully mixed models of copying · Only half old, half new matched the shifting density bands.
  8. The leading strand grows smoothly while the lagging strand fragments · The student swapped the two strands: smooth belongs to leading.
Worksheet · LightMySky