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Ask your students what a chromosome looks like and they will probably describe an X-shaped object similar to what is shown in Figure 1 (page 48). But how much do they really know about chromosomes?
?¢ Do your students know that the X-shaped chromosomes result from DNA replication?
?¢ Can they make a connection between chromosomes during mitosis and DNA replication?
?¢ Do they realize that chromosomes are only in that replicated form if the cell is preparing for and then going through cell division and that during the majority of a cell's life cycle the chromosomes are not shaped like an X?
This article describes a partner activity and then a whole-class activity that use modeling to teach DNA replication, connect it to the shape of chromosomes during mitosis, and help students understand how daughter cells have the same DNA.
Modeling is integral to science, helping students understand phenomena that may not be seen. Yes, students might extract DNA in a lab activity, but they cannot zoom in to the molecular level to directly observe it.
Before engaging students in this activity about DNA replication, I incorporate lessons about DNA structure using online resources, handouts, body modeling, and chenille stem modeling (Robertson 2016b). I also have students create paper chromosomes (Robertson 2016a) to familiarize them with the terms genome, chromosome, gene, gene locus, and allele. (You need at least one of the paper chromosome models for the whole-class activity described later.)
DNA replication
DNA replication is complicated, involving several different enzymes. Before replication, the cell accumulates DNA nucleotides (the building blocks of DNA) used in the process. These nucleotides are readied for replication by the addition of phosphate groups and are then referred to as deoxyribonucleoside 5´-triphosphates (dNTPs). Most of the early research on DNA replication was done with E. coli, but replication in eukaryotes is now also well known (Cooper 2000). History of the research and explanation of DNA replication is available online (see "On the web").
Simply put, DNA replication is semi-conservative, meaning that the double helix is essentially split in half by breaking apart the hydrogen bonds that hold together the base pairs of each step. Since the base pairs combine only in certain ways (adenine with thymine, and guanine with...