An Innovative Model System for Teaching DNA Replication
By Vernon W. Bauer, PhD
Associate Professor of Biology, Francis Marion University
Teaching biology to incoming freshmen over the past 10 years has shown me that many students are weak in molecular biology. As a geneticist, I know how important it is for students to understand the fundamental mechanisms of DNA replication before they are asked to apply them in a practical application like PCR.
These molecular concepts are visual, meaning students need to “see” the process unfold to really appreciate it. Various model sets have been offered. We have used at least 3 different ones at Francis Marion and were frustrated by all of them. They were easy for students to assemble improperly, not durable, slow to reset between labs, and difficult to understand for colleagues who were not molecular biologists.
Managing these sets, i.e., helping students assemble them, repairing broken pieces, resetting them between labs, and helping colleagues understand them, ate into our teaching time. We needed a set that eliminated the frustration and allowed us to focus on teaching rather than lab management; so I developed one that did—the DNA Synthesis Set model system (Fig. 1).
The set allows students to build the DNA model by themselves. This is critical when managing up to 24 students per lab. Each component is made out of a single piece of nearly indestructible plastic with highly visible, scratch-resistant labeling. The set is ready to use right out of its compartmented box. An embedded magnet system helps guide students to the correct molecular orientations for each component. For example, the set will not allow a student to build an A = C base pair—only A = T and G = C base pairs will stay together. (Loose magnets are easily repaired with superglue; just pay careful attention to orientation before gluing.) To reset for the next class, students simply return the pieces to the box.
The most important thing is that the set helps teach the core concepts of DNA structure and replication, and that it does so in a very practical way. Each piece clearly shows the 3 components of nucleotide structure (Fig. 2). Phosphate groups, deoxyribose sugars, and the cyclic bases each get their own color for easy identification.
The chemical positions of the 5 carbons in the sugar are labeled to help students understand the biochemical basis for the DNA chain’s 5' to 3' orientation. They can see how the lack of a hydroxyl group at the 2' position gives deoxyribose its name. Students can also see what makes purines (double ring) and the pyrimidines (single ring) different.
One of the set’s strongest features is that students can visualize the dynamic nature of hydrogen bonding potential and complementarity between bases. After using the kit they should be able to tell you why A does not pair with C, for example. Regarding DNA replication, students can observe DNA synthesis direction, the concepts of leading and lagging strands, and the semiconservative nature of the whole process.
All of these are hard for a teacher to describe without having a visible model or picture in front of the students as they do it. In addition, we usually couple this set with the popular pop-bead mitosis kits to tie these events of S phase into the bigger picture of the cell cycle. To illustrate the 3-D aspects of the DNA double helix and concepts like the major and minor grooves, supplement the set with a 3-D model of DNA.
The set is easy for teachers to use. An extensive instruction manual helps introduce concepts and guides students through the exercise step by step. A separate teacher’s manual contains answers to the student questions, a worksheet copy master, and a vocabulary list.
Because each piece in the set is clearly marked, teachers do not need to be molecular biologists to teach students what terms like deoxyribose, cyclic base, hydrogen bonding, and 5' really mean. So, does the kit make things easier for teachers? Yes, but more importantly, it is a valuable tool for teaching students some very difficult molecular concepts. Our department is very pleased with how it has worked for us.
I’m pleased that this set has increased our actual teaching time and eliminated the frustrations we used to experience during this lab. It consistently outperformed other model sets in lab-by-lab comparisons and students have given us excellent feedback on its usefulness. Try this durable, visually appealing model in your classroom and I think you, too, will be pleased at how it facilitates teaching difficult molecular concepts.