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Activity 2: DNA Replication


Students will model the semi-conservative replication of DNA.

Background Information

Although the pairs formed between the nucleotide bases in DNA are very specific (A with T and G with C) there is no restriction to the order in which the bases are arranged on a particular DNA strand. This allows a tremendous amount of information to be stored in the long molecule of DNA. The sequence of bases can be varied in countless ways to form the code for genetic information. When DNA replicates, a particular sequence, or code, serves as a template for the information that is passed on.

The replication process begins when the hydrogen bonds between the base pairs are broken by a special enzyme and the two halves of the molecule start to unwind. The individual strands act as templates along which complementary nucleotides bond. The chains of nucleotides are then linked together by sugar and phosphoric acid chemical groups to create two daughter DNA molecules that exactly resemble the parent material. Each daughter molecule consists of one-half of the original DNA chain, and one-half new material.


For each group

  • DNA patterns (see preparation)
  • Paper
  • Pencil
  • Scissors


Create and duplicate sample patterns of the follow-ing parts of the DNA molecule: the four bases (A, C, T, G), a sugar (S), and a phosphate (P).


  1. Ask students to use the DNA patterns to trace and cut out 16 each of sugar and phosphate, and 8 of each base.
  2. Have students build a model of a segment of a DNA molecule. The segment should contain 4 base "rungs." Any bases can be used for the sequence, as long as the appropriate complementary bases are used for the pairs.
  3. Once students have made their models, ask them to separate the models down the middle so that there are now two single strands of DNA.
  4. Have students create new double-stranded DNA by matching complementary nucleotides to the bases on each single strand.

Discussion Questions

  1. Compare the two new strands of DNA. Are they the same or different? (The same.) Why? (Because each strand contains complementary pairs, so that each daughter molecule consists of one-half of the original DNA chain, and one-half new material.)
  2. How does the structure of a DNA molecule help account for the great variety of life that exists on earth? (The seguence of the base pairs determines how the organism will form. The variety in the sequence of the base pairs accounts for the variety of life forms.)


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