gloves should be extended in opposite directions. Or students
could touch the second finger of the left glove with the forefinger
of the right glove, modeling A-T or G-C bonding. Students can
switch fingers of each glove to continue to model the activity of
the bonding of nitrogen bases. Other fingers not being used in the
model are folded back into the palm of the glove. If students practice
this activity, they will begin to see how nitrogen base sizes affect
the bonding patterns of the nucleotides and shape of the DNA
molecule. With the help of other students, a portion of a DNA
molecule can be modeled by students sitting on opposite sides
of a long table and reaching across the table until their hands
meet. However, this is not the intent of the model. The model
is designed to be a quick reference for individual students to supplement their understanding of DNA nucleotide structure and
DNA Nucleotide Bonding Pattern
James Watson and Francis Crick were aided by the data of Rosalind
Franklin (Franklin & Gosling, 1953) and Erwin Chargaff (Chargaff
et al., 1950) in determining the structure of the DNA molecule.
Franklin’s data showed that the DNA was a double helix and water
molecules were on the outside of the molecule, not on the inside
as Watson and Crick had originally thought. Chargaff’s data established that the concentration of adenine to thymine was equal in
all samples of DNA, as was the concentration of cytosine to guanine.
This data helped Watson and Crick determine the bonding pattern
Hydrogen Bonding of Nucleotides
of the nitrogen bases to be A-T and C-G. Additionally, the
diagrammatic model (Figure 6) shows how this bonding pattern
establishes an equal distance across the width of the molecule along
its entire length. That pattern is created by a longer purine base
bonding to a shorter pyrimidine base, and no matter where that
occurs, the distance across the molecule is the same.
For an in-depth understanding of nucleotide bonding, students can
add dots to represent sites of weak bonds called hydrogen bonds that
bond the nucleotides to each other. C (pyrimidine) and G (purine)
each have three sites for hydrogen bonding. This results in the bonding of C to G. Similarly, adenine (purine base) and thymine (
pyrimidine base) each has two sites for hydrogen bonding. This results in
the bonding of A to T. Students will place three horizontal dots on
the tips of glove fingers that represent C or G for the three hydrogen
bonds participating in the C-to-G bonds (Figure 7). Students will
mark the tips of the glove fingers representing A or T nitrogen bases
with two horizontal dots, representing the two hydrogen bonds of A
and T that allow them to bond to each other. This illustrates that only
C can bond with G, and only A can bond with T, due to the matching up of
the hydrogen bonds. This was a key factor that contributed to the
description of the structure of the DNA molecule. At this point, you
can have students construct a grid to show the relationships of the
DNA nitrogen bases (Table 1).
Figure 6. DNA nucleotide bonding patterns (© https://
britannica.com/science/DNA). By courtesy of Encyclopaedia
Britannica, Inc., copyright 1998; used with permission.
Figure 7. DNA nucleotide bonding of cytosine to adenine,
showing three hydrogen bonds.
Table 1. DNA nitrogen base grid.
Nitrogen Base Kind of Base Bonds with Number of Rings
Relative Size of
A Purine T 2 Long 2
G Purine C 2 Long 3
C Pyrimidine G 1 Short 3
T Pyrimidine A 1 Short 2