Gene / Chromosome - Gene / DNA Relationship Analogies
The following demonstration shows a very simple way to show
the relationship between DNA and a chromosome using a large spool
of light colored thread.
A Spool of 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 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 nucleotide sequence for
a particular gene. If you are using red ink, tell them this 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 and red sections
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:
- The gene is a segment of a chromosome that codes for one
- A gene is a linear sequence on the DNA molecule.
- DNA must be unwound to be able to be transcribed much like
thread must be unwound to be used.