History and Discovery of Genes
Activity Building the DNA molecule / DNA replication.
As a means of review and reinforcement have two or three students relate to the rest of the class about the work of Watson and Crick, in their discovery to modern genetics.
Point out that chromosomes are made of molecules of DNA. The DNA molecules are the genes that control all traits passed on from parents to offspring. DNA molecules are composed of the same elements but their arrangement varies. Emphasize to students that this variation in the arrangement of these elements (bases) determines individual heredity.
Explain to students that each unit of DNA has a phosphate, a sugar and a nitrogen base. There are four different types of nitrogen bases which combine in very specific ways. In the DNA, which takes on the shape of a twisted ladder, Adenine (A) always pairs with thymine (T) and Guanine (G) always pairs with Cytosine (C).
Materials (per class of 24, working in teams of four).
6 sets paper clips: 40 black
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40 white
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32 red
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32 green
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string or yarn
masking tape
magic marker
Directions Have students get into groups of four before distributing the different colors of paper clips and remaining materials to them.
Provide students with a key to designate the nucleotide bases. For example:
COLOR
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BASE
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Black
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Adenine (A)
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Green
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Guanine (G)
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White
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Thymine (T)
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Red
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Cytosine (C)
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PART A
Ask students to construct one DNA strand by linking the colored paper clips together in the following way:
1. Link 3 black paper clips to each other followed by 2 green, 1 white, 1 red, 1 white, 2 red, 1 white, 1 red, 1 white, 2 black, 2 white, 2 green, 1 white, 1 red, 1 white, 2 red, 2 white, 1 black, 2 green, 1 white, 1 red, 1 white, 2 red, 2 white.
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The students should realize that this strand of DNA begins with three Adenine bases and ends with two Thymine bases.
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2. Use a small piece of masking tape labeled #1 and attach it to the AAA (three black paper clips) end of the strand to identify this strand.
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3. Now construct the complementary strand of DNA that would pair with strand #1. You may want to monitor the groups to make sure that they are using the key properly and matching the base pairs correctly.
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Remind students that Thymine(T) bonds with Adenine(A), and Guanine(G) bonds with Cytosine(C).
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4. Attach another piece of masking tape labeled #2 to the TTT end of the complementary strand.
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5. Ask students to place both strands alongside each other making certain that they have constructed the proper sequence of nucleotide bases in #2 strand.
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6. Use the piece of yarn or string provided to tie or join the first paper clips in #1 strand (AAA) with the first paper clips in strand #2 (TTT).
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7. Repeat step 6 for the paper clips on the other end of strands #1 and #2.
PART B
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8. Place the completed double-stranded DNA on his/her desk or table so that both sides are in a straight line opposite each other. (Do not untie the string at the end.)
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9. Count 22 paper clips beginning from the AAA end of the strand and tie this 22nd pair of paper clips together as done before on the ends of the molecule.
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10. Untie the string holding the two ends of the molecule together at the AAA and TTT end of the strand. Strands will separate to form a “Y” or forked shape.
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11. At this point, explain to students the process of DNA replication.
If time permits, ask students to construct new complementary DNA strands for strands #1 and #2 to show or to illustrate DNA replication.
Home-work Assignment Describe how DNA replication makes it possible to produce two identical cells from one parent.
Questions for Discussion
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1. Which base pairs up with the adenine during replication?
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2. Which base pairs up with cytosine during replication?
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3. What would happen if the bases did not pair up this way?
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4. Compare the two new double-stranded molecules that you have just completed. How are they similar to the original DNA molecule containing strands #1 and #2? How are they different?
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5. Describe the differences in the way strand #1 and #2 are replicated.