Engagement (So, just what does DNA look like and how
does it copy itself?)
1. Ask students to think back on their previous biology
classes. Tell them, “We have spent a lot of time
talking about DNA, but what does DNA look like? Why does
DNA have this specific structure?”
2. Show students the computer simulation of DNA replication
at http://www.ncc.gmu.edu/dna/repanim.htm Draw
to the way DNA splits down the center when it is preparing
to replicate. Note how the leading and trailing edge
copy from different directions.
3. Tell students that they will be replicating a segment
of DNA using licorice, toothpicks, and colored marshmallows.
What will the licorice (sugar-phosphate backbone), toothpicks
(hydrogen bonds), and marshmallows (nucleotide bases)
represent? (Don’t be surprised if these are new
terms to many of your students.)
4. Pass out DNA cards to groups of students. Once they
receive their 10-letter DNA strand, they need to build
the strand out of licorice, toothpicks and marshmallows.
They have the leading edge, which is read left to right.
(The overhead shows an example.)
5. Once students have successfully constructed their
model, have them coil their DNA model. It will curl into
a 1 _ circle. Discuss how this helps DNA be stored more
6. Next, use scissors to cut the hydrogen bonds. They
should begin their cut at the start codon (ATG) and end
at the stop codon (TAG). Remind students that the DNA
is read on the leading edge from left to right.
Once the DNA is separated, students should begin replicating
the DNA starting with the leading edge. Their completed
DNA molecule, with replication from start to stop codon,
should look a bit like an open mouth.
7. Have students explain in their science notebook what
each step in the modeling represents. Diagrams might
be a helpful way to show steps if they are labeled.
8. Have students identify potential places for errors
to occur either in discussion or notebooks. What do they
think the consequences of these errors might be? (Do
not answer or comment on these ideas as they will be
Record concluding thoughts in their science notebooks.
a great time to walk around the room and assess students on
their ability to correctly match base pairs and whether students
start their cut at the start codon and stop at the stop codon.
(If there are two single strand segments on each end then it
is done correctly)