William C. Johnson
San Benito High School
where BBB teaches
You realize by now that the cell is a complex
thing. It "multiplies by dividing". Its not quite like cutting
a cube of butter in two - the cell is at least as complicated as a car,
and you'd hardly take lightly if you saw a car produce a copy of itlelf!
Its amazing how the intertwined, involved structuresseparate neatly and
evenly. Yet the process goes on rappidly, and insome cells, happens
To get a clearer idea of this process we're hoing to
make a movie--the kind where you draw pictures of something happening in
a series of small steps on the edges of cards....and then when you flip
the cards, you have action. Let's make a movie of animal cell
mitosis! We'll use the cells pictrued below as the 6 dey
cards for the stages. In between each pictured card, you will draw
in 3 steps between them. Movies in color are better, so use colored
pencils or pens to jazz it up. You'll need a total of , let's see
6 + 18 equals 24 cards! On the back of each key card tell
what's going on--like interphase-- contrioles form asters and move apart.
You'll also need one card ant the beginning for the movie title, and one
at the end for the credits - the name of the producer. When you're
done with your movie, show it to your teacher, and turn it in with a rubber
band around it.
This is open to your imagination. The basic idea is to get the kids to
see by direct involvement what the process of each type of division is
in terms of the role of chromosomes in each process. Basically, I had several
volunteers come in during lunch (or break) and go through the process of
meiosis. First they chose a partner they wanted to share a homologous chromosome
with. Once I had them in pairs, I gave them each a chromosome one student
with a maternal chromosome (“a” or pink) and their partner with the paternal
homolog (“b” or blue). We then went through the process as outlined below
to prepare for the class presentation.
The students lined up in front of the class, boldly presenting their
chromosome to the class. I mentioned that we were in interphase I
of meiosis and ready to make some gametes. To simulate the
replication of the chromosomes in late interphase, I had them put the matching
half of their assigned chromosome in their back pocket. At the end
of the “S” stage they would pull it out and put them together (velcro or
snap) to make a pair of identical chromatids held together by a centromere.
(I took time in each step to have those not directly invowed tell me what
phase we were in and what was going on.)
At this point imagination will guide you as to how you want to continue
your simulation. I chose two “class organizers” to act as centromeres
they would move to opposite sides of the rom or the area we were in.
they had a cord (nylon string) for the number of pairs of chromosomes in
the simulation (I have 9 pairs in mine, therefor, each centromere has 9
cords). These cords represent the microtubules of the spindle.
After replication of DNA the pairs were told them to 'line up with their
pair. It is vital that everyone understand that the homologs lined up on
the cell equator on a random basis, and that no pair had previously discussed
with each other nor with any other homologous pair just how they were going
to line up during tetrad formation. The next step was to separate the chromatid
pairs from each other. Eadh centromere would hand a cord to the person
(of each pair closest) and “pull that person with their two chromatids
toward the organizer. Make sure they see that the duplicated
chromosomes traveled as a pair, and are not torn apart as in the first
division of mitotic anaphase. Take time to analyze what has occured.
Each organizer has half the chromosomes around them and they represent
a random combination of maternal and paternal chromosomes (Haploid).
The final step is gamete formation. Have each of the students tear
apart the chromatids, putting one in each of 4 boxes provided.
Repeated simulations will reinforce the random assortment of chromosomes.
When dominant and recessive alleles are placed on the chromosomes students
will see the principle of segregation. Make a tally on the
board of the gametes made, and it will show gametes genetically different
from the parent; not only in genotypes, but also in the total number of
chromosomes in the gamete. Go through several rounds, showing that
each round could produce different gametes depending on the random tetrad
formation in metaphase I of meiosis. Finally, cross several gametes
from different rounds to get genetically different individuals. Depending
on background you may want to talk about different genotypes/phenotypes
etc. It is also a good time to compare this to mitosis
which gives genetically identical outcomes to make the point that these
different processes have different results because they have different
purposes. Again, there are many ways to go with this and it is totally
up to your creativity.
Sea urchin project
Here is a page on sea urchin bioasssay
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