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

   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 nucleotide: one sugar, one phosphate and one base
   • A base pair: two bases, one from each strand
   • A hydrogen bond: a weak link between two bases
   • A backbone: a long chain of sugars and phosphates
4. 4.

   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
5. 5.

   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 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
   • 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 2, because A pairs with T and C pairs with G
   • Case 2, because each rung joins two single-ring bases
   • Case 1, because A pairs with G and C pairs with T
   • Case 1, because bases with matching ring counts pair up
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 A, because the bases form the long outer lines
   • Case A, because the bases need to be outside to pair up
   • Case B, because the bases meet inside the chains
   • 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.

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

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