The structure of DNA

Grade 9 Science Worksheet

Circle the best answer.

1. 1.

   Bottom strand directly beneath it: six bases, ends not labeled. Luz writes the bottom strand under 3′-TACGGA-5′. Which choice gives the bottom strand, read from its own 5′ end?

   • 5′-TCCGTA-3′
   • 5′-CGTAAG-3′
   • 5′-ATGCCT-3′
   • 5′-TACGGA-3′

2. Read this first

   Use this

   Ladder (model, not to scale).

3. 2.

   Milo writes the bottom strand under 5′-GATCCG-3′, left to right as it sits beneath. Which choice is correct, with its end labels?

   • 5′-CTAGGC-3′
   • 3′-AGCTTA-5′
   • 3′-CTAGGC-5′
   • 3′-GATCCG-5′
4. 3.

   Pia completes the bottom strand under 3′-ATGGCA-5′. Which letters fill the blanks, left to right?

   • C, A, G
   • A, G, T
   • A, C, T
   • T, C, A
5. 4.

   Ladder (model, not to scale). Bottom strand directly beneath: five bases, to find. Zee counts the hydrogen bonds in a short stretch whose top strand is 3′-GATTC-5′. How many hydrogen bonds join the five base pairs?

   • 10
   • 12
   • 13
   • 15
6. 5.

   (model, not to scale). Ari studies one nucleotide of the top strand. Arrow Y points to the part that faces the other strand. What is Y?

   • The phosphate, which links to the other strand
   • The base, which pairs with a base on the other strand
   • The sugar, which links to the other strand
   • The hydrogen bond, which is a part of each single nucleotide
7. 6.

   Tala compares two stretches of DNA with the same number of base pairs. Which stretch has more hydrogen bonds, and why?

   • Stretch N, because C–G pairs are the smaller pairs
   • Stretch M, because A–T pairs have 3 bonds each
   • Stretch N, because it has more C–G pairs, with 3 bonds each
   • Neither, because both stretches have 5 base pairs

8. Use this

   Model data card, percent of the bases in a two-stranded sample. A: 28. T: ?. G: ?. C: ?. Total: 100.

9. 7.

   Bo reads a model sample of two-stranded DNA. Adenine is 28% of the bases. What percent of the bases is guanine?

   • 22%
   • 28%
   • 36%
   • 44%
10. 8.

    Model data card, number of bases in a two-stranded sample. A: 240. T: ?. G: ?. C: ?. Total: 1,000. Jun counts the bases in a model sample of two-stranded DNA with 1,000 bases. Adenine is 240 of them. How many bases are cytosine?

    • 260
    • 380
    • 240
    • 520
11. 9.

    Sketch by a classmate (model, not to scale). Noah sketches DNA with the bases on the two outer lines and the sugar-phosphate chains as the crosspieces. What should change?

    • Add a third strand in the middle to hold the sketch together
    • Swap them: the chains form the outer lines and the bases meet in the middle
    • Nothing: the bases must be on the outside to reach each other
    • Pair each A with G and each C with T, to keep the width even

12. Use this

    Who-did-what card (real facts from OpenStax Biology 2e and NIH NLM). Columns: person, place, work. Rosalind Franklin with Raymond Gosling, King's College London, X-ray photo of the B form of DNA in 1952, labeled 51. James Watson and Francis Crick, University of Cambridge, built the double-helix model in 1953 using Franklin's X-ray data.

13. 10.

    Esme says Watson and Crick took the X-ray photo labeled 51. Using the card, which reply is best?

    • Not correct: Franklin joined the work after the 1953 model
    • Not correct: Franklin and Gosling made the photo, and the model used it
    • Not correct: the photo was made in 1962, after the prize
    • Correct: Watson and Crick took it at Cambridge in 1952