Constructing a Paper Helix
Introduction:
DNA is called the BLUEPRINT OF LIFE. It gets this name because it contains the
instructions for making every protein in your body. Why are proteins important?
Proteins are important because they are what your muscles and tissues are made of;
they synthesize the pigments that color your skin, hair, and eyes; they digest your food;
they make (and sometimes are) the hormones that regulate your growth; they defend
you from infection. In short, proteins determine your body’s form and carry out its
functions. DNA determines what all of these proteins will be.
The DNA molecule is a double helix. Think of it as a ladder that has been twisted into a
spiral. The outside of the ladder is made up of alternating sugar and phosphate groups.
The sugar is called deoxyribose. The rungs of the ladder are made of nitrogencontaining bases. There are four different bases in DNA: adenine (A), guaning (G),
cytosine (C), and thymine (T). These four bases are of two types: purines and
pyrimidines. The purines (adenine and guanine) are larger molecules with a double-ring
structure. The pyrimidines (cytosine and thymine) are smaller single- ring structures.
Cytosine and thymine are pyrimidines.
Inside the DNA ladder, two bases pair up to make a “rung.” One base sticks out from
each sugar-phosphate chain toward the inside of the ladder. It forms a pair with its
complementary base sticking out from the opposite sugar-phosphate chain. There is
room for only three molecular rings between the two sugar phosphate chains, so a
pyrimidine and a purine form a pair. Because of the chemical structures of the bases,
adenine always pairs with thymine, and cytosine always pairs with guanine.
The goal of this activity is to construct a paper model of a DNA molecule. You will do so
by making the smallest subunit (monomer) of DNA. This subunit is called a nucleotide.
It consists of one sugar molecule, one phosphate molecule, and one nitrogenous base
(A, T, C, or G). You will then join your nucleotides together to make a ladder-like helix.
Procedure:
1. Your group should have 2 pages of phosphate molecules, 2 pages of sugar
molecules, and one page each of A, T, C, and G molecules, as well as scissors
and tape or glue.
2. If needed, color your molecules as instructed by your teacher. Cut out each of
the molecules.
3. Work with your group to figure out how to correctly assemble these pieces into a
DNA helix that is 6 base-pairs long. Use the information in the introduction to
help you figure out how to put your DNA together.
4. Tape or glue your DNA molecule together and put all group members’ names on
the back. Turn in to your teacher and answer the questions below.
Questions:
1. What base does adenine always pair with?
2. What base does guanine always pair with?
3. What is the smallest subunit (monomer) of DNA called? What are the three parts
of this monomer?
4. What is the shape of the DNA molecule?
5. Which bases are PURINES?
6. Which bases are PYRIMIDINES?
7. Why must a purine always pair with a pyrimidine?
8. What is the name of the sugar molecule in DNA?
9. Suppose you knew that the sequence of bases on one strand of DNA is:
AGCTCAG. What is the complementary sequence of bases on the opposite
strand?
10. Assume that in a 100 base-pair DNA double helix, there are 45 cytosines. How
many adenines are there?