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?

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

   Ladder (model, not to scale). Bo keeps labeling the paper model. Arrow X points to one short crosspiece between the two strands. What is X?

   • A gene, a long stretch that holds the plan for a protein
   • A piece of the backbone, because the sugars and phosphates form the rungs
   • A nucleotide, a sugar with a phosphate and a base
   • A base pair, two bases joined by hydrogen bonds
3. 3.

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

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

   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
   • G, T, A
   • T, G, C
   • A, C, G
5. 5.

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

   • Case Q, because A–T has 3 bonds and C–G has 2
   • Case P, because A–T has 2 bonds and C–G has 3
   • 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
6. 6.

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

   • Case 1, because A pairs with G and C pairs with T
   • Case 2, because each rung joins two single-ring bases
   • Case 1, because bases with matching ring counts pair up
   • Case 2, because A pairs with T and C pairs with G
7. 7.

   “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 B, because the bases meet inside the chains
   • Case A, because the bases need to be outside to pair up
   • Case A, because the bases form the long outer lines
   • Case B, because the phosphates join the two strands in the middle
9. 8.

   Ren reads a history card. According to the card, who made the X-ray photo labeled 51?

   • James Watson and Francis Crick, at Cambridge
   • Rosalind Franklin, with Raymond Gosling
   • Raymond Gosling, working alone
   • Maurice Wilkins, who received the 1962 prize
10. 9.

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

    Use the picture in question 8.

    • 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
11. 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?

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