Constructing a DNA Paper Helix
Introduction:
DNA is called the blueprint of life. It got 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 up of nitrogen-containing bases. There are four different nitrogencontaining bases in DNA: adenine (A), guanine (G), and thymine (T). These four
bases are of two types: purines and pyrimidines. Purines are large double-ring
structures. Adenine and guanine are purines. Pyrimidines 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 a base sticking out from the opposite sugar-phosphate chain. Only three
rings can fit between the two sugar-phosphate chains, so a pyrimidine (one ring)
and a purine (two rings) form a pair. Because of the chemical structures of the
bases, adenine always pairs with thymine, and cytosine always pairs with guanine.
Activity:
The goal of this lesson is to construct a paper model of a DNA helix. You
will do so by making the fundamental unit of DNA. This unit, called a nucleotide,
consists of one sugar molecule, one phosphate group, and one nitrogenous base.
Each member of the class will make a small segment of a DNA double helix and
then join them to form a large ladder-like helix.
Procedure:
1. Find the patterns of deoxyribose, phosphate groups, and bases according to
the following color code:
Deoxyribose:
Phosphate:
Adenine:
Guanine:
Thymine:
Cytosine:
PURPLE - 4
WHITE - 4
BLUE - 1
GREEN - 1
PINK - 1
YELLOW - 1
2. Cut out the patterns.
3. Glue each nitrogenous base to a deoxyribose molecule by matching up the
dots.
4. Glue each phosphate group to a deoxyribose molecule by matching up the
stars.
*** Now you have 4 separate nucleotides!
5. Now pair up the bases:
Adenine (A) with Thymine (T)
Guanine (G) with Cytosine (C)
*** Notice: to do this you have to turn some of the nucleotides “upside
down” so that the bases can pair up. This accounts for the fact that DNA
is “ANTIPARALLEL” (one strand goes up and one strand goes down.
6. Glue or tape the bases together and the nucleotides together. You end up
with a small piece of DNA that is 2 base pairs in length.
7. Write your name on the back of your piece of DNA. The pieces from the
class will be joined together to form a longer piece of DNA.
Name: __________________________________ Date________ Block________
Constructing a DNA Helix Questions
1.
With what base does adenine pair?
2.
With what base does guanine pair?
3.
What is the smallest unit of DNA called?
4.
What is the shape of the DNA molecule?
5.
Which bases are purines?
6.
Which bases are pyrimidines?
7.
Why must a purine pair with a pyrimidine?
8.
What is the name of the sugar in the DNA backbone?
9. Suppose you know that the sequence of bases on one DNA strand is AGCTCAG.
What is the sequence of bases on the opposite strand?
A G C T C A G
10. Assume that a 100-base pair DNA double helix contains 45 cytosines. How
many adenines are there?