Author: Lynne Gordon
Woodrow Wilson Biology Institute
The following demonstration is a very simple way to show the relationship between DNA and a chromosome using a large spool of light colored thread.
Hold the spool in your hand and ask the students to describe what you have in your hand. Ask them what substances (cotton and wood) are present and what structures (thread and spool) are present. Conclude that the thread of cotton is wound around the spool made of wood. Equate the substance DNA (one molecule per one spool) to thread and the substance wood to the protein (histone) it's wound around (even though DNA is not actually wound around the histone in this manner).
After this is established, unwind a fair amount of thread which accumulates in your hand and proceed to throw it at a student in the second row. Of course, it does not quite get there. Ask them why. It's not a convenient way to transfer the thread (DNA). Rewind the thread (DNA) and then throw the spool of thread (chromosome)to a student in the back row. Why was the spool of thread easier to catch?
Then discuss the importance of "wrapped" DNA (coiled and supercoiled into chromosomes only when DNA needs to be transferred to another part of the cell which happens during mitosis or meiosis only after replication) Stress that when the thread (DNA)is being used (during interphase) it is not so tightly coiled or wound and this is analogous to DNA in a working cell.
Next, take a colored marker and color over a two foot section of the thread. Equate this to the DNA sequence for a particular gene. If you are using red ink, tell them this gene might be an instruction (gene) for the cell to make a red pigment (protein). Continue to unwind another two or three feet of thread and color it blue to represent a gene that might code for the cell to make a blue pigment (protein). Then rewind this thread around the spool so that the blue section and red section appear as part of the linear order of the thread on the spool (linear order of genes on a chromosome).
This adequately conveys the concepts that 1) the gene is a segment of a chromosome that usually codes for one protein, and 2) that a gene is a linear sequence on the DNA molecule that codes for one protein. DNA must be unwound to be able to be transcribed much as thread must be unwound to be used.