MYSTERIES OF MITOSIS 

MASTER TEACHER   Veronica Zonick

GRADES 9-10

OVERVIEW 
This lesson provides students with an understanding of the process of cell division and its importance to all living beings.  This lesson uses microscopy techniques to prepare and examine actively dividing onion cells.  Sampling techniques are used to estimate the relative number of cells in the interphase or mitosis portion of the cell cycle.  These data are manipulated to produce percentages and expressed graphically as a pie chart.   The use of video is intended to link the static images of the phases of mitosis, as seen through the microscope, with the dynamic processes of the cell cycle as seen in living cells. 

ITV SERIES
Our Human Body from Science Source:  What Are Cells Like?

Learning Objectives
Students will be able to:
*describe the four stages of mitosis
*explain the importance of organized division of the nucleus
*identify mitotic stages in dividing cells
*calculate the percentage of the cell cycle spent in each phase of mitosis and interphase
*use data to construct a pie chart showing the relative amount of time spent in each phase of mitosis and interphase

MATERIALS 
(Per class)
Overhead projector, screen and thick (Vis-a-Vis)
   transparency markers
Transparency "Cell division in Onion Root Tips"
Transparency "The Cell Cycle:  Relative amount of
   time spent in interphase and mitosis"
Transparency "Phases of Mitosis"
Model chromosomes:
  (Photocopy chromosome master sheets onto 3 
   sheet of yellow, pink  and blue copier paper.
   Laminate. 
Attach chromosomes to chalk board using
   self-adhesive magnetic tape or double-sided tape.)
chalkboard and chalk
6 yards of yellow, pink or blue yarn
electric drill
Ethanol: Acetic Acid Fixative  (Mix 300 mL of 95%
   Ethanol with 100 mL of Glacial acetic acid)

(Per pair of students) 
1 plastic film canister containing 5 to 10 mL of Ethanol: Acetic Acid fixative 
   (film canisters are available free of  charge from
     many film processors)
1 pearl onion
1 Styrofoam cup
4 toothpicks
1 pair of safety glasses/goggles
1 compound microscope
1 microscope slide
1 plastic coverslip
1 dropper bottle of Toluidine blue O or
     Hematoxylin stain
Prepared slide of the phases of mitosis
Forceps
Beaker of distilled water
Eyedropper 
Pencil
Handout "Mysteries of Mitosis"
Handout " Steps of Mitosis"
1 sheet notebook paper

VOCABULARY 
cell cycle                anaphase
mitosis                   prophase
chromosome          telophase
meristem                metaphase
interphase              supercoiling

PREVIEWING ACTIVITIES 
Begin this portion of the lesson at least two to three days before the lab portion. 
Explain to your students that in the coming days the class will be studying how cells divide.  Tell the students, "If we set an onion in water, its cells will respond by dividing and producing roots." Explain, "Plant cells have two times during the day when they divide: most often between noon and 1:00 p.m. and between 12 noon and 1:00 a.m." 

Direct students to lab stations containing one pearl onion, one Styrofoam cup, one film canister containing fixative, and four toothpicks for each pair of students.   Ask students to work in pairs to set a pearl onion just above water in the Styrofoam cup.   Instruct the students to take the onion set-up home.  In three days, they must harvest the very ends of the root tips, between midnight and 1:00 a.m. Say, "The film canister contains alcohol and acid which will kill and preserve the cells.  Any cells that are dividing will be stopped in their tracks.  As soon as you harvest the root tips, drop them into the liquid.  Bring the canister to school the next day."

Three days later. Ask, "When you started out your life, how many cells did you contain?"  (One or egg and sperm cells.)  Ask, "How did you go from one cell to millions of different cells with different jobs?"  (The fertilized egg cell must divide over and over, to produce so many cells.)  Say,  "But you have so many different cells in your body: skin cells, blood cells, muscle cells.  How can one cell produce all of these different types of cells?"  (The first cell, the fertilized egg, has all of the information that will be used by every cell in your body.)  This information is stored within the cells DNA.  In fact, every cell with DNA contains all of this information.  Once the cell has divided the daughter cells will begin to use that information to do the different jobs within our bodies.  The important thing is making sure 
that each cell gets a complete set of this information. 

Take the length of colored yarn, and wad it loosely into a ball.  Say,  "Each cell in your body contains long strands of DNA.   Normally, the DNA is in a stringy mess, like this.  (Hold up the mass of yarn.)  When a cell divides, each cell must get a complete copy of the DNA or  the cell will not work correctly.   Can anyone think of a way to make the DNA less messy before it is divided?" (Organize the DNA  in some fashion.)  Ask for two student volunteers.  Tape one end of the yarn and securely tape it to the chuck of the electric drill.  Give this assembly to the first volunteer.  Give the free end of the yarn to the other student.  Direct the students to stand about ten feet apart.  Turn on the drill.  The yarn will begin to coil and become shorter.  After a short amount of time, this coil will begin to coil on to itself.  Explain to the students that this twisting of DNA is known as supercoiling.  It is one way that the large amount of DNA in a cell is packaged, so that it may be divided up more easily.

FOCUS FOR VIEWING 
Distribute the handout "Steps of Mitosis" to each student.  Tell your students, "Now that you have seen how DNA is packaged into a manageable size, you are going to watch a video that shows how a cell divides its DNA into two identical portions."  To give students a specific responsibility while viewing, say, "On your handout, there are four steps diagrammed.  As you watch the video, place these steps in correct sequence by numbering them from one to four.  At the end of the lesson, I will be asking you to describe these four stages, and explain the importance of the organized way in which the cell divides."

VIEWING ACTIVITIES 
Begin tape where the video shows a time-lapse image of bacteria dividing.  Audio is: "...plants can reproduce; cells can do the same thing."   To allow students an opportunity to record the sequence of events in mitosis, pause the tape at the end of the computer animation sequence of  a cell dividing.  Video is  an image of the words "Are cells in your body dividing right now?"  Audio is  the sound of a computer printer.   To check comprehension of the sequence of events that occurs during a mitosis, and to allow students time to record their answers,  rewind the tape to the beginning of this animation sequence.  Video is a black screen with a simple cell containing red chromosomes.  Audio is: "Dogs have seventy-eight, and cabbage have eighteen."  Mute the sound.  Use the frame advance  (or slow) option to move through the tape slowly.  Say, "Explain, step by step, the process occurring in the animation sequence."   (1. The DNA coils up.  2. The DNA lines up in the center of the cell.  3.  The DNA is pulled into two groups, toward opposite ends of the cell.  4. The cell begins to divide.)   Pause the tape.  Video is  an image of the words: "Are cells in your body dividing right now?" 

Audio is the sound of a computer printer.

Distribute the handout "Mysteries of Mitosis".  Say,  "We have examined how a cell organizes and divides its DNA into two sets, so that each cell gets a complete set of instructions. This process is called mitosis. Now lets name the events that happen during mitosis."  Draw a circle on the chalkboard to represent the cell membrane.  Draw a smaller circle within to represent the nuclear membrane.  Uncoil the yarn used earlier, and hold it up.  Ask the students,  "If I am going to divide a cell into two new cells, what do I do to make sure each cell will have a complete set of instructions?"  (Duplicate the DNA.)  Explain to the class that before mitosis begins,  DNA is already duplicated.  Next, the DNA becomes visible, as it is supercoiled into chromosomes.  Attach the three paper chromosome pairs to the board.  Say,  "On your lab paper, find the first diagram that shows DNA beginning to supercoil.  This first phase of mitosis is known as Prophase."  Label the first diagram on the transparency "Phases of Mitosis".

Tell the  class, "Now that the DNA has condensed into chromosomes, you can see that each chain of DNA is attached to its copy."  Point out the X-shape of the chromosomes on the chalkboard.  Say,  "Before division can happen, a few things need to happen.  First of all, the chromosomes need room to move.  Because of this, the membrane that usually surrounds the nucleus disappears."  Erase the chalk line which represents the nuclear membrane.  Say,  "Each human cell contains 46 chromosomes.  How can I make sure that every new cell will also contain 46 chromosomes?" (The DNA needs to be organized or sorted.)  Say,  "To make sure that the chromosomes will be sorted out correctly, the chromosomes are lined up along the very center of the cell.  This phase is called metaphase." Line the chromosomes up in the center of the circle drawn on the chalkboard, and label the second diagram in the transparency "Phases of Mitosis".

Say,  "Now that the chromosomes are lined up and organized, what happens next?"  (Now the DNA can be divided up into two sets.)  Say, "Each X contains two complete sets of DNA.  If I pull them apart, I will still have a complete set of information.  In anaphase, the two portions of the chromosome are pulled apart, and tugged to opposite sides of the cell."  Take each chromosome model and separate it from its matching, sister chromatid.  Place one member of each pair on opposite sides of the cell model. Label the third diagram in the transparency "Phases of Mitosis".

Say,  "Now the DNA has been separated into two portions."  What happens next?  Elicit the response that now the cell can go back to the way it was.  Say,  " The DNA does not need to be in packages anymore, so it begins to uncoil (uncoil the yarn).  The DNA is kept separate from the rest of the cell, so the nuclear membrane must reform." Redraw the circle representing the nuclear membrane.  Label the last diagram in the transparency "Phases of Mitosis".

Say, "Now that the nucleus has been divided, mitosis is over.  What will happen next?" (The rest of the cell can divide.)   Say, "Now that we have examined what is occurring during mitosis, let us go back and look at the process in a living cell."  Rewind tape to the image of a living cell undergoing mitosis.  Video is a blue screen containing an image of a living, dividing cell with chromosomes and organelles visible through digital interference contrast technique.  Audio is: "So that each cell functions correctly."  Mute the sound.  Use the frame advance or slow option to move slowly through this sequence.  To show more clearly the living chromosomes, use the overhead transparency marker to draw directly on the television screen.  Outline one chromosome, and ask the students to point out its match.  Have the students follow the chromosome as the video sequence plays.  Ask, "Explain, step by step, what is occurring in this sequence, referring to their handout "Mysteries of Mitosis" for assistance." (Discussion.) Stop the tape.  Video is an image of a living cell which is in the process of cytokinesis.  Audio is: "At the time of reproduction..."

POSTVIEWING ACTIVITIES 
Ask students to look at the handout "Mysteries of Mitosis".  Say, "We have just looked at mitosis in a living cell, and we have seen how the cell uses a system to package and organize the DNA before separating it into two groups.  You might have noticed that the chromosomes were hard to see, and the picture was a little blurry."  Ask, "Can anyone think of something we could do to improve the picture?  (Stain the cells.)  Unfortunately, you cannot stain the chromosomes in living cells without damaging the DNA.  Ask, "What would damaged DNA do to a cell?" (Disrupt cell functions.)  The other day we harvested and killed onion cells that were going through mitosis.  Ask, "How were these cells killed?"  (By pouring a mixture of ethanol and acetic acid over the root tips.) When the alcohol entered each cell, it killed it immediately.  Explain, "When we  look at these fixed cells, it is like looking at a photograph of the time when they were killed.  Whatever the cell was doing at that time is still visible.  If a cell spends most of its time dividing, the chance of dividing at the time it is killed will be great.   If we stain these cell and look at them, we 
can tell what phase of the cell cycle each cell was in." 

Say,  "In the first part of this lab, we will collect data to find out how many cells in the root tip are dividing, or at rest."  Project the transparency "Cell Division in Onion Root Tips".  Say,  "In plants, one part of the root, called the zone of cell division, produces all of the cells for the entire root.  (Point out this portion of the root tip.)  If we examine this part of the root we should be able to see cells going through mitosis." Direct students into pairs and have them examine the onion root tips prepared previously following the procedure detailed in the handout "Mysteries of Mitosis". 

If necessary, after students have collected data for fifty cells,  review how to calculate the percent in each phase.  Ask one group to give the number of cells they found in prophase.  Record this number on the chalkboard.  Say,   "If  ten cells out of fifty were in prophase, how many cells out of 100 should be in prophase?"  (Twenty.) This number equals the percentage of cells in prophase.   This means that out of every one hundred cells, twenty would be expected to be in prophase.  Ask, "How could you show this in another way?  If this pie chart equals a total of 100 cells, and each wedge equals five cells, how many wedges should be taken up by prophase?"  (Four.)

After all data has been manipulated, compare data as a class. Ask, "Which phase of mitosis takes the most time?"  (Prophase.)  Also ask, "Which takes the least amount of time?" (Metaphase.)  For the entire cell cycle, interphase should take the largest amount of time.

ACTION PLAN 
1. Invite a plant geneticist or horticulturist from the county or state agricultural extension service to speak about mitotic (somatic) mutations and their importance in producing new varieties of plants. 

EXTENSIONS 
Computer Science
Use multimedia software such as Hyperstudio or Digital Chisel to create an interactive tutorial on mitosis and meiosis.

Agriculture
Research the effect of drugs such as colchicine on cell division.  How has colchicine been used in plant breeding to produce stronger plants?

Medicine
Research how the cells of the human body "know" what to become during development.  What will happen if one cell of an embryo does not divide correctly?
 
 

Teacher Set-up of Chromosomes for Mitosis Demo

Step1 -- Prophase: Draw a circle to represent the nucleus.  Align 2 chromosome pairs within.

Step2 -- Metaphase:  Line up chromosomes in a straight vertical line.  Use chalk to draw spindle fibers.

Step3--Anaphase:  Separate matching halves of each chromosome and slide along the board.

Step4 -- Telophase: Redraw nuclei and show cytoplasm beginning to divide.  Hold up yarn chromosome; unwind to show how DNA uncoils.
 
 

CELL DIVISION IN ONION ROOT TIPS

The Cell Cycle: Relative amount of time spent in interphase and mitosis.

Phases of Mitosis

Chromosomes for Mitosis Demo.  Print one copy using blue paper, laminate, and attach magnets or double sided tape to back of centromere

Chromosomes for Mitosis Demo.  Print one copy using yellow paper, laminate, and attach magnets or double sided tape to back of centromere

Chromosomes for Mitosis Demo.  Print one copy using pink paper, laminate, and attach magnets or double sided tape to back of centromere

Steps of Mitosis:  Place the following steps of mitosis in order by numbering them from 1 to 4.

Biology One Lab--Mysteries of Mitosis.

Name________________

Label the following stages of mitosis:

1.________  2.________  3._________ 4.________

Procedure for visualizing the stages of mitosis:

1.  Open the film canister containing the root tips you prepared previously.

2.  Using the forceps, remove on root tip.

3.  Use the eyedropper to rinse the root tip in water.

4.  Next, place the root tip on a clean glass slide.

5.  Cover with a plastic cover slip.

6.  Examine the slide to find the location of the very tip of the root.

7.  With the tip of an eraser, carefully flatten the cover slip against the slide.

8.   Add one drop of the stain toluidine blue to slide, wicking the stain under by touching the opposite side of the cover slip with a paper towel.

9.  Wait 3 minutes.  Then add one drop of water to the edge of the slide, and wick the water under with a paper towel.

10.  Locate the very tip of the root under the microscope.  Close to this region is the root apical meristem, the part of the root where most cell division occurs.  Locate this region.

11.  Count 50 cells.  Observe whether each cell has a normal, dark nucleus (interphase) or a nucleus undergoing mitosis.  Keep track of the number of cells in each phase of the cell cycle by filling in chart A.

12.  Calculate the percent of cells in each phase of the cell cycle.

13.  Use your calculations to complete the pie chart labeled "the cell cycle."

14.  On your own paper, answer the following;  "Why is it important to divide the nucleus into identical,  organized sets?  What is the major event that happens in each stage of mitosis?

Results:   Use the following chart to record your data on the number of cells in each phase of the cell cycle.  Then calculate the percent of cells found in each phase.

Example:   8 cells in prophase = 16%
 
 

Phase	Interphase	Prophase	Metaphase	Anaphase	Telophase
Number of times seen
Percent

The following pie is divided in 20 wedges. Each wedge equals 5 % of the total pie.  Using the data in chart A, fill in and label the relative amount of time a cell spends in each phase of the cell cycle.

Updated:  April 01, 2008