Using candy to teach DNA structure

The introductory biology course that I teach focuses on cellular biology. I start with an overview of the basic types of biological molecules (carbohydrates, lipids, proteins, and nucleic acids) and the different structures found inside of cells. I then spend several weeks covering energy acquisition (photosynthesis through glycolysis and respiration). A significant portion of the course is then spent discussing inheritance and how genetic information is stored in our DNA. We then end the course with a unit on evolution.

I love teaching cell biology, but one of the challenges with teaching this material is that it can be hard for students to grasp the concepts because they can’t visualize them. I try to use lots of great graphics and we use the microscope in the lab to actually look at cells. Our publisher has produced a wide array of great videos and YouTube is a great resource. These things work well for the visual learner. To meet the needs of my kinesthetic learners, I bring in molecular models and sometimes we act out complicated pathways (electron transport in photosynthesis for example). One thing that worked well during my summer class was to use candy to build models of DNA. For this activity I used bite sized Twizzlers, cherry sours and jelly beans. Toothpicks were used to link together each “molecule”. I have seen similar versions of this activity that used whole Twizzlers to form the backbone of DNA, but I wanted to cover some more of the important details of DNA structure.

Understanding the structure of DNA is essential for understanding the concepts of replication and transcription. Students need to understand that DNA is composed of subunits called nucleotides. Each nucleotide has three components, a phosphate group, a sugar (deoxyribose), and a nitrogen containing base. Cherry sours represented the phosphate group, the Twizzlers represented the sugar, and different colored jelly beans were used to represent each base (Guanine= green, Cytosine=pink, Adenine= yellow, Thymine = orange).

We made a handful nucleotides, making sure that there were equal numbers of complementary bases (Guanine is complementary to Cytosine; Adenine is complementary to Thymine)

The nucleotides were then joined together to form a single stranded nucleic acid. You can point out the repetitive structure of the sugar-phosphate backbone. You can show the students that the chain has directionality, one end has a free phosphate group (5′ end) and one has a free sugar (3′ end). You can also talk about how it is the sequence of the bases that holds the information stored in DNA.

Using the first strand as a template, students can then build a complementary strand of DNA. It is important here that the new strand is oriented in the opposite direction that the template, with the phosphate group at the bottom and the free sugar at the top.

Once the two strands are complete, they can be connected by forming “hydrogen bonds” between the bases. The final result can be seen below. There are two antiparallel strands of DNA held together by the interactions between the bases. This can serve as a great jumping off point to talk about DNA replication, followed by the topics transcription and translation.