Activity 2: DNA Replication
Students will model the semi-conservative replication of DNA.
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)
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).
- Ask students to use the DNA patterns to trace and cut out 16 each of
sugar and phosphate, and 8 of each base.
- 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
- 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.
- Have students create new double-stranded DNA by matching complementary
nucleotides to the bases on each single strand.
- 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.)
- 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.)