Replication Machinery: Origins, Forks and Telomeres · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Copying with one hand tied

Science · Biochemistry & Molecular Biology · ages 19-20
Name ______________________   Date ____________

  1. DNA polymerase can start a brand-new chain from nothing.

    Circle one:   True   False

  2. Why is one new strand built in short pieces?

    • Polymerase builds in only one direction
    • Helicase works too slowly on that side
    • Ligase refuses to visit that side

  3. What are new-strand primers made of?

    • DNA, since polymerase starts with DNA
    • Protein, since clamps are proteins
    • RNA, since primase lays RNA starters

  4. Why do human chromosomes carry up to 100,000 origins?

    • Huge genomes must finish copying in reasonable time
    • Primers only work near origins
    • Circles need more starts than lines

  5. Which enzyme seals the nicks between lagging-strand fragments?

    • Helicase, which unwinds the duplex
    • Ligase, which seals the nicks
    • Topoisomerase, which relieves the strain

  6. Why do bacterial circles avoid the end replication problem?

    • Their telomerase works a hundred times faster
    • Their polymerase builds in both directions
    • They have no ends, so the two forks simply meet

  7. A cell line loses telomerase. What happens over many divisions?

    • Telomeres shorten each round until the ends erode
    • Replication speeds up to cover the loss
    • The circles refuse to divide further

  8. Sam says telomerase shortens telomeres. What is wrong?

    • Telomerase shortens telomeres each round
    • Telomeres shorten from the end problem; telomerase restores them
    • Telomeres lengthen on their own without help
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Answer key

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

Copying with one hand tied W1-mt_v95MJWeV_B-s1

  1. False · Every new strand begins with an RNA primer laid by primase.
  2. Polymerase builds in only one direction · One direction of building forces stop-start work on the backward side.
  3. RNA, since primase lays RNA starters · Primers are short RNA pieces, later removed and replaced.
  4. Huge genomes must finish copying in reasonable time · Many forks working at once share the three billion base pairs.
  5. Ligase, which seals the nicks · Ligase closes the gaps between Okazaki fragments.
  6. They have no ends, so the two forks simply meet · Shortening needs an end, and circles have none.
  7. Telomeres shorten each round until the ends erode · Without restoration, every division costs a little end sequence.
  8. Telomeres shorten from the end problem; telomerase restores them · Loss comes from copying; rescue comes from telomerase.
Worksheet · LightMySky