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Ellen S. Lamb


Students will show how the basic structure of DNA is important in the process of DNA replication

Materials (per class of 30):

Twizzlers candy or other similar licorice stick candy:
red, 3 large packages, each length of candy cut into four pieces
black, 2 large packages, each length of candy cut into six pieces

(Note: 24 pieces of each color are needed per student group; although you will be able to handle more student groups with less candy if the pieces are smaller; beware of cutting them too small to accept the toothpicks; cut so that the red and the black pieces are different sizes to allow for easier identification by students.)

An alternative available in some locales is the candy sold in small pieces, usually called "Bits" or something like that.

Colored marshmallows or gumdrops OR any small, soft candy available in 4 colors. (you will need the 4 colors to show replication. ) Make sure that the candy Is large enough to accept the toothpicks.
(Note: you will need 24 pieces of candy (6 of each of the four colors) for each student group)

Plain wooden toothpicks , two boxes of 500, broken in halves (just a few of the second box need to be prepared for a class of 30)


Note: Be sure that working surfaces and hands have been cleaned before starting this activity, if you intend to consume your models after finishing.

1. Assign one nitrogen base to each of the four colors. For example: green = adenine, blue = thymine, red = cytosine, yellow = guanine.

2. Distribute 24 pieces each of red and black Twizzlers, assorted colors of marshmallows, and 72 toothpick halves to each team. (Note to teacher: There should be equal numbers of each color of marshmallows to insure that each base will be able to match with its complementary base. it may be convenient to provide 6 marshmallows in random colors at first, and then to allow students to get the necessary color matches to make the complementary strand.)

3. Prepare six individual nucleotides: use toothpicks to connect one black to one red twizzler piece. Then add one marshmallow perpendicularly to the black candy. (Note to students: what DNA component is represented by the black twizzler piece? by the red twizzler piece? by the marshmallow?)

4. Assemble nucleotides into a polynucleotide strand by connecting the red piece of one nucleotide to the black of another. Continue until a strand of six nucleotides has been constructed. (Note to teacher: it may be useful to have students draw a DNA molecule that can then be used as a model for this construction.) Once this is completed, all of the reds and blacks should be in alternation in a straight line; the marshmallows should be sticking out at a perpendicular angle from the blacks.

5. Assemble a strand which is complementary to the strand which you have already built. Place the second strand next to the first so that the "bases" touch.

6. To demonstrate replication, first make 12 more nucleotides with the same nitrogen bases as the first two strands.

7. "Unzip" the DNA double strand. Assemble the proper nucleotides, one by one, showing the 5'-3' order for each strand: one side should be assembled in order, the other should be assembled showing the "lag" and formation of Okazaki fragments.

8. After you demonstrate this to your teacher, you may dispose of your models. This is one case where you may eat your science project, if you have kept everything clean and your teacher gives permission. Be sure to remove toothpicks before you eat!!! Clean up, being sure that no toothpicks or sticky residue is left behind. Wash your hands!

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