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 line of hydrogen bonds holding the two strands together
   • A row of bases, because the bases form the outer lines
   • The sugar-phosphate backbone, a chain of sugars and phosphates
   • A stack of base pairs, one resting on the next one
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 piece of the backbone, because the sugars and phosphates form the rungs
   • A base pair, two bases joined by hydrogen bonds
   • A gene, a long stretch that holds the plan for a protein
   • A nucleotide, a sugar with a phosphate and a base
3. 3.

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

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

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

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

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

   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 Q, because a pair with a double ring has 3 bonds
   • Case P, because the two kinds of pair have the same number of bonds
   • Case P, because A–T has 2 bonds and C–G has 3
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 phosphates join the two strands in the middle
   • 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 bases meet inside the chains
9. 8.

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

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

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