The structure of DNA

Grade 9 Science Worksheet

Circle the best answer.

1. Read this first

   Use this

   Ladder (model, not to scale).

2. 1.

   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′-GATCCG-5′
   • 3′-AGCTTA-5′
   • 3′-CTAGGC-5′
3. 2.

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

   • A, G, T
   • A, C, T
   • T, C, A
   • C, A, G
4. 3.

   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
   • 15
   • 12
   • 13
5. 4.

   (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 sugar, which links to the other strand
   • The base, which pairs with a base on the other strand
   • The phosphate, which links to the other strand
   • The hydrogen bond, which is a part of each single nucleotide
6. 5.

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

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

7. Use this

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

8. 6.

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

   • 28%
   • 44%
   • 22%
   • 36%

9. Use this

   Model data card, percent of the bases in a two-stranded sample. A: ?. T: 31. G: ?. C: ?. Total: 100.

10. 7.

    Ren reads a second model sample of two-stranded DNA. Thymine is 31% of the bases. What percent of the bases is cytosine?

    • 31%
    • 19%
    • 69%
    • 38%
11. 8.

    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?

    • Pair each A with G and each C with T, to keep the width even
    • 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

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. 9.

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

    • 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
    • Not correct: Franklin joined the work after the 1953 model

14. Use this

    History card (real dates). 1952: Rosalind Franklin with Raymond Gosling, at King's College London, make an X-ray photo of the B form of DNA, labeled 51. 1953: James Watson and Francis Crick, at the University of Cambridge, publish a double-helix model built with Franklin's X-ray data on 25 April; Franklin and Gosling publish a paper in the same issue of Nature. 1962: Francis Crick, James Watson and Maurice Wilkins receive the Nobel Prize in Physiology or Medicine; Franklin is no longer living, and the prize is not awarded after death.

15. 10.

    Kira says Franklin missed the 1962 prize because her data were not used. Using the card, which reply is best?

    • Not correct: her data were used, and the prize is not awarded after death
    • Correct: Watson and Crick did the X-ray work themselves
    • Correct: her X-ray work was not needed for the model
    • Correct: the prize was given before her 1952 photo