Cell Division and Mitosis

Nicholas DiGiovanni            Naperville Central High School
1340 Wilshire Dr               440 W Aurora 
Naperville IL 60540            Naperville IL 60540
708-357-8856                   708-420-6417

Objectives:

1. For grades 9-12 (may be modified for younger). 
2. Students will make a slide of onion root tips and observe different phases of 
mitosis.  
3. Students will be able to state why cells divide.
4. Students will draw the different stages of mitosis and try to arrange them in 
order of division. 
5. Students will demonstrate each phase of mitosis and the main characteristics 
of each. 

Materials:

1. Prepared onion root tip mitosis slides.
2. In place of these and to promote interest, students may set up their own 
slides.  Materials needed per slide are: fresh grown onion root tip, 5-10ml 
distilled water, 5ml 6M HCl, 1 ml Feulgen reagent in a vial, 5 ml 45% acetic 
acid, dropper pipette per solution, beaker, slide, coverslip, and a pencil with 
eraser or small cork to squash the slide.  Materials needed per class are: 5-10 
ml Carnoy's solution (1 glacial acetic acid : 3 absolute alcohol) in vial, 2-3 
cups and onions, and toothpicks.
3. A microscope per student or pair.
4. Two different colored pipe cleaners cut at varied lengths to represent 
chromosomes for assessment (4 chromosomes per cell).  I use 2 long and 2 short 
blue pipe cleaners and 2 long and 2 short red pipe cleaners connected with beads
which represent the centromeres.  This model would use the long pipe cleaners 
as a homologous pair of chromosomes and the short as another pair.  Two pipe 
cleaners of each size and color are used to model replication.  Each chromosome 
is composed of two chromatids. 

Strategy:

This lesson would be used after cells have been observed with microscopes in the
lab.  After a review of cell parts, the teacher would exhibit the onion (or 
prepared slides) and explain that root tips have been cut for the students to 
observe.  Explain that the stain to be used in this preparation is different 
because something will be noticeable that could not be seen in past labs 
(chromosomes).  Try to have the students guess this by questioning them about 
the cells they are observing - i.e. dividing cells - and how these are different
than previous cells observed.  Nuclei were observed before but not the 
chromosomes. 

LAB
Students are more eager to learn how to set up their own slides and observe them 
than to use a prepared slide with which they may be unfamiliar.  Steps 1-8 
explain slide preparation.  Go to #9 if prepared slides are being used. 

1. Advance preparation: 
a. Take an ordinary yellow onion.  Cut off any old root growth.  Place the onion 
in a cup of water so that only the root portion is under water.  To do this, 
push toothpicks into the side of the onion which extend outward and hold it on 
the rim of the cup.  New roots should grow within two days. 
b. Cut off .5-1 cm of growth at the root tip - enough for all the students.
c. Transfer immediately to Carnoy's solution.  After 24 hours, roots should be 
stored in 70% ethanol in a refrigerator.  This stops cell division.

(Steps 2-9 to be completed by students)
2. Obtain a root tip.
3. After obtaining the root tip, pour off the fixative and replace it with 2-5 
ml distilled water.  Solutions may be poured into a beaker or down the drain. 
4. After 1 minute remove the water with a pipette and add 2-5 ml 6M HCl.
5. After 3 minutes carefully remove the acid and wash tissue off with distilled 
water.  Agitate the vial for 1-2 minutes.  Discard the water.
6. Use forceps to transfer the tissue to a vial containing 1-2 ml Feulgen 
reagent.  The reagent may be added to this vial if desired.  (CAUTION: this dye 
will stain hands and clothes permanently.)  After 20 minutes use forceps to 
transfer the tissue to a vial containing 5 ml 45% acetic acid. 
7. Place 1-2 drops of acetic acid onto a microscope slide and transfer the 
tissue to the drop.  Using dissecting pins and razor blades tease and macerate 
the tissue into tiny pieces.
8. Place a coverslip over the macerated tissue trying not to get air bubbles 
under the coverslip.  Press down firmly onto the coverslip with a small cork or 
pencil eraser to spread the cells in a very thin layer.  Push down in a 
perpendicular direction and the coverslip should not break.
9. Once the slide has been prepared or obtained from the teacher, observe it and 
draw all the different views of cells present under high power.  Be careful to 
observe the nucleus and chromosomes since this is what was not observed 
previously. 

Discussion

1. During the 20 minute stain time (Step 6) it is important to have the students 
discuss what the cells are doing.  Since these cells are in the root tip, they 
are rapidly dividing.  During normal cell activity the chromosomes are unwound 
and too thin to be seen.  During cell division, chromosomes thicken, take up 
stain and can be easily observed.  The students should also try to come up with 
reasons why cells divide (possible answers: to grow, to repair or replace 
damaged cells, to reproduce, or to differentiate in the cell cycle of 
multicellular organisms).  Although it is better to have the students elicit 
these, it may be necessary to give them some of the reasons since this is an 
introduction.  Also at this time explain what is happening in interphase.  Cells 
need to replicate the chromosomes before dividing to ensure that the newly 
formed cells contain the same genetic material (chromosomes). 

2. What the students are observing is an ordered process by which the cells 
divide the chromosomes so that one copy of each goes to each new cell. Once they 
have drawn all the different views of cells they have observed, they should 
share them with their lab groups, the teacher and perhaps the class.  Hopefully 
all the stages of mitosis have been observed and drawn.  These can be put on the 
board or overhead so that the entire class can see all the phases/views and copy 
them onto their papers.  It is not necessary to name the observed cells with a 
phase of mitosis, but it may be easier so that students can more easily 
differentiate them and relate them to a new vocabulary term.  Once all the 
students have drawings of all the phases of mitosis, ask them to arrange the 
pictures in a way which would show a logical sequence of cell division. 

3. Have one member from each group explain what order they put the drawings and 
why they did it that way.  After all students have explained their process, 
students may wish to choose the model that they think is best.  Students should 
now look into their textbooks to find out what is happening in each phase and 
how the text organizes the phases of mitosis. 

Assessment:

1. Students are to make a model of a cell which is in the process of mitosis and 
cell division.  They should draw on their paper the border of a cell which is 
dividing.  These need to be big enough so that the pipe cleaners can be put 
inside.  There needs to be a cell border for interphase, prophase (perhaps one 
for early and late prophase), metaphase, a partially dividing cell for anaphase, 
a nearly totally divided cell for telophase, and two new cells for the daughter 
cells. 

2. Students will take the pipe cleaners and place them in the "cells".  They 
should arrange four "chromosomes" properly in the various phases of mitosis.  
The pipe cleaners may be taped or glued on the paper and a description written 
which describes the events occurring within the cell.  The pipe cleaners may be 
manipulated and each phase described to the teacher.  Students may work in 
groups of four and only one student from each group would be tested to save time 
evaluating the mitosis demonstration. 

3. The teacher should determine the grading method, points or letter grade, but 
a recommended method would be: 5 for all the phases demonstrated exemplary in 
proper order with a detailed description of each.  4 for correct demonstration 
of phases and clear description.  3 for a generally correct model but lacking 
some clarity and detail.  2 for a partial demonstration, perhaps the phases are 
out of order or improperly shown and the description is incoherent.  1 for an 
incorrect demonstration of model and explanation. 

Other notes:

1. For a multicultural emphasis on teaching, students can observe that different 
organisms have cells which look very similar.  Different races and nationalities 
of people must have cells which are not distinguishable from each other.

2. It may not be necessary to use a visible pipe cleaner model to better 
understand mitosis, but when teaching meiosis the model is very good to 
demonstrate crossing over and variable gamete formation as well as differences 
from mitosis.