DNA Structure and Base Pairing · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

The twisted ladder that copies itself

Science · Genetics & Evolution · ages 15-16
Name ______________________   Date ____________

  1. In DNA, which base pairs with G?

    • A
    • T
    • C

  2. Which three parts build one nucleotide?

    • Sugar, phosphate, base
    • Sugar, protein, lipid
    • Base, ribosome, acid

  3. During copying, each old strand acts as a template for a new partner strand.

    Circle one:   True   False

  4. One strand reads GATTACA. What is its partner strand?

    • GATTACA
    • CTAATGT
    • GTAATTG

  5. One strand reads ATCG. What is its partner strand?

    • TAGC
    • ATCG
    • GCTA

  6. Why is only one partner strand possible for GATTACA?

    • Enzymes rewrite any strand until it fits
    • Each base has exactly one partner
    • Both strands always read the same way

  7. After copying, proofreading enzymes fix a wrong base. Why does the copy still end up exact?

    • The template strand still dictates the one right partner
    • Both new strands are built from scratch
    • Wrong bases dissolve on their own

  8. A student copies ACGT as TGCA but writes the last pair as C with T. Where is the mistake?

    • The order should run right to left
    • The final rung breaks the rule, since C pairs with G
    • A and T should swap everywhere
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Answer key

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

The twisted ladder that copies itself W1-mt_PK4uNJeg_k-s1

  1. C · C always pairs with G, and A always pairs with T.
  2. Sugar, phosphate, base · Every nucleotide combines a sugar, a phosphate group and a base.
  3. True · Free nucleotides line up against each old strand by the pairing rule.
  4. CTAATGT · Rung by rung the partners are C, T, A, A, T, G, T.
  5. TAGC · Pair each base: A with T, T with A, C with G, G with C.
  6. Each base has exactly one partner · Fixed partners leave no room for any alternative.
  7. The template strand still dictates the one right partner · The template plus the fixed rule forces the correction to the single right base.
  8. The final rung breaks the rule, since C pairs with G · Checking every rung catches the one pair that breaks the rule.
Worksheet · LightMySky