The structure of DNA

Grade 9 Science Worksheet - Building Confidence

Circle the best answer.

1. 1.

   Ari labels a paper model of DNA. Arrow W points to the long outer line of the top strand. What is W?

   • The sugar-phosphate backbone, a chain of sugars and phosphates
   • A line of hydrogen bonds holding the two strands together
   • A row of bases, because the bases form the outer lines
   • A stack of base pairs, one resting on the next one
2. 2.

   Esme outlines one unit on the model. Outline Y circles a five-sided shape, a round shape and a lettered block, joined together. What is Y?

   • A base pair: two bases, one from each strand
   • A nucleotide: one sugar, one phosphate and one base
   • A backbone: a long chain of sugars and phosphates
   • A hydrogen bond: a weak link between two bases
3. 3.

   Jun fills in a paper ladder. Which base goes in the blank, directly under adenine (A)?

   • Thymine (T), the partner of adenine
   • Adenine (A), because matching letters go together
   • Guanine (G), because both bases have double rings
   • Cytosine (C), the partner of guanine
4. 4.

   Kira fills in a second paper ladder. Which base goes in the blank, directly under cytosine (C)?

   • Cytosine (C), because matching letters go together
   • Thymine (T), because both bases have single rings
   • Adenine (A), the partner of thymine
   • Guanine (G), the partner of cytosine
5. 5.

   Ladder (model, not to scale). Luz completes the bottom strand under A, C, G. Which three bases fill the blanks, left to right?

   • C, G, T
   • A, C, G
   • T, G, C
   • G, T, A
6. 6.

   Two drawings (not to scale). Milo compares two drawings of the dotted hydrogen bonds between paired bases. Which case is correct?

   • Case P, because the two kinds of pair have the same number of bonds
   • Case Q, because a pair with a double ring has 3 bonds
   • Case P, because A–T has 2 bonds and C–G has 3
   • Case Q, because A–T has 3 bonds and C–G has 2
7. 7.

   Two drawings of rungs (not to scale). Noah checks which drawing pairs the bases correctly. Which case is correct?

   • Case 2, because A pairs with T and C pairs with G
   • Case 1, because bases with matching ring counts pair up
   • Case 1, because A pairs with G and C pairs with T
   • Case 2, because each rung joins two single-ring bases
8. 8.

   “Two drawings (not to scale). Columns:” Pia compares two drawings of a short stretch of DNA. Which case puts the parts in the right places?

   • Case A, because the bases form the long outer lines
   • Case B, because the bases meet inside the chains
   • Case A, because the bases need to be outside to pair up
   • Case B, because the phosphates join the two strands in the middle
9. 9.

   Tala reads the same history card. Where did Watson and Crick get X-ray data for their double-helix model?

   • They used no X-ray data and built the model from parts
   • From the 1962 prize paper, written after the model
   • From Franklin's X-ray work at King's College London
   • From an X-ray photo they took themselves at Cambridge
10. 10.

    Card of base-pair facts (Data: OpenStax Biology 2e). Columns: base pair, hydrogen bonds. A–T: 2. C–G: 3. Zee reads a card of facts about base pairs. How many hydrogen bonds join a C–G pair?

    • 5 hydrogen bonds
    • 1 hydrogen bond
    • 2 hydrogen bonds
    • 3 hydrogen bonds