One of the
students I tutor had to build a DNA model for an assignment for 9th
grade biology. We built our model, photographed it and posted it on Facebook.
Some friends saw the picture and asked me to post instructions, so they could
build similar models with their kids.
with the basics.
of several units:
and ribose molecules comprise the backbone. The phosphates and riboses
strands of the phosphate-ribose backbone are antiparallel. That means the two
strands go in opposite directions.
cross-links are the nucleotide base pairs.
nucleotides are: Adenine (A), Guanine (G), Cytosine (C) and Thymidine (T).
Guanine are purines. Cytosine and Thymidine are pyrimidines.
pairing is A with T and G with C. (Figure 1)
pairs are connected by two hydrogen bonds.
G-C base pairs are connected with three hydrogen bonds. (Figure 1)
DNA forms a
right-handed helix. In other words, when you look down from the end, the twist
occurs about every ten base pairs.
assignment stated that the model must be made with household items, not a
molecular model kit. We chose wire and beads. The assignment also had several
criteria to make the finished model as realistic as possible. The criteria
phosphate-ribose backbones must be antiparallel. This means that if you start
with a ribose on one strand, start with a phosphate on the opposite strand. In
our model, the ribose molecules were five-pointed stars, so we were able to
emphasize the directionality with the star beads. (Figure 1)
molecules must be larger than the phosphate molecules.
and phosphate molecules must be different shapes or different colors.
must be correct. (A with T and G with C)
and G) must be larger than pyrimidines (C and T).
must be attached to the ribose molecules only.
bonds must be smaller than the nucleotides.
must include at least 15 base pairs.
must include at least two twists.
plastic-coated steel wire for the phosphate-ribose backbone. (This wire is
sturdy enough to support the structure, yet malleable enough to bend and
dark annealed steel wire for the cross-links. (This wire was bendable and
worked for the project. However, the dark coating rubbed off on our hands.
Another type of thin malleable wire might work better.)
for the phosphate molecules – at least 60.
crafting beads for the ribose molecules – at least 60. (The ones I bought were
plastic with a metallic-sheen coating.) Make sure the hole in the bead is big
enough to allow both the 18 gauge and 22 gauge wires to go through at the same
of assorted shapes and colors for the nucleotides. You’ll need four types of
beads and approximately 15 beads of each. The A and G beads must be larger than
the C and T beads. The A beads should be a different color than the T beads.
The G beads should be a different color than the C beads. Make sure the 22
gauge wire will go through the hole in the beads.
straws in two colors
(We used a piece of Styrofoam. Clay would also work. Alternately, the model can
hang from a string.)
Cut two 2.5-foot
lengths of the 18-gauge wire.
Bend a small
loop in one end of each wire so the beads don’t slide off.
strand with a phosphate (pony bead). Alternate riboses (stars) and phosphates
(pony beads) until you have 30 beads on the wire.
strand must be antiparallel. That means you need to start with a ribose (star)
and follow with a phosphate (pony bead). Make sure the stars point in opposite
directions on the two strands. String 30 beads on the wire.
Bend a small
loop in the far end of the wire so no beads slide off.
nucleotides must be connected to the ribose molecules. (The nucleotides are
supposed to be connected only to the ribose molecules, but we twisted the 22-gauge wire around the 18-gauge wire for stability. This slight structural convenience
is hidden by the beads.)
Use the sharpie
to label the wooden beads with A, G, C or T. Make sure the As and Gs are larger
than the Cs and Ts.
drinking straws. Make two vertical lines on both sides on one straw with the marker. Make three
vertical lines on both sides of the other straw.
into short pieces (smaller than the nucleotide beads). The pieces of straw are
Cut 30 seven-inch
pieces of 22-gauge wire.
Loop the 22-gauge wire once or twice around the 18-gauge wire between the phosphate and the
Twist the 22-gauge wire around the arms of the ribose star. Do not use all of the seven
nucleotide bead on the 22-gauge wire.
appropriate hydrogen bond (straw pieces) next. Remember A-T base pairs have 2
hydrogen bonds and C-G base pairs have 3 hydrogen bonds.
appropriate nucleotide bead to pair with the first on the cross-bridge. (If the
first was a C, string a G. If the first was a T, string an A, etc.)
end of the 22-gauge wire around the star on the opposite strand to secure.
all of the ribose molecules (stars) on the two strands have cross-bridges.
You may have
extra 18-gauge wire at the top of the phosphate ribose backbones that needs to
one-inch loops at the top to insert into the Styrofoam platform. Other types of
platforms may require different wire accommodations.
figure out how much wire you’ll need to attach the DNA to the platform, trim
the 18-gauge wire and make loops in the ends.
Look at the
DNA molecule end-on so the loops in the wire are facing you.
structure counterclockwise to create a right-handed helix.
Try to get
about ten base pairs per turn.
structure to the platform or hang it with string.