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Cell Division

Page history last edited by Derek Weber 11 years, 9 months ago

A.  Learning Objectives

In this lab, students will:

• distinguish between the different phases of the cell cycle, mitosis, and meiosis.

• prepare and view a slide of chromosomes from HeLa cells.

• identify the different stages of the cell cycle in both plant and animal cells.

• recognize the events distinguishing Meiosis I and Meiosis II and identify cells in the different phases.


B. Textbook Correlation: 

Please review Chapter 15: The Eukaryotic Cell Cycle, Mitosis, and Meiosis when preparing for the lab.


C.  Introduction:  Mitosis 

One of the tenets of the cell theory is that all cells arise from preexisting cells. It is easy to see how this works when you watch an Amoeba grow larger and then divide into two new organisms. It is a little harder to comprehend when one thinks that each of us began life as a single cell that developed into a multicellular organism as each cell divided into two new cells.

There are two different types of cell division, mitosis and meiosis. The purpose of mitosis is growth and to increase the number of cells. As a cell grows larger, it synthesizes all the components necessary for two cells and then divides to form two identical new cells. Figure 1 illustrates this growth and division cycle called the cell cycle. The majority of the cell’s life is spent in interphase. This phase is subdivided into three periods – G1, S, and G2.


• During G1 the cell grows larger

• During S the cell replicates its DNA

• During G2 the cell prepares to divide


The division phase lasts a much shorter length of time. It begins with mitosis which is division of the nuclear material. The replicated chromosomes are evenly divided so that each daughter cell receives a complete set. The entire cell cycle is completed when the cytoplasm divides, called cytokinesis.


On the other hand, the purpose of meiosis is sexual reproduction. As a result of this division process, the number of sets of chromosomes in each daughter cell, called a gamete, is halved. At fertilization, when a male and female gametes fuse, the chromosome number of the resulting zygote is restored to the normal number.


D.  The Cell Cycle in Allium Root Tip

Plants grow longer from the tips of the roots and shoots. This region is called the apical meristem. The stained root tips of onion, Allium, plants are particularly good for learning to recognize the different stages of the cell cycle.


Use the images you inserted in the introduction to help identify the following phases of mitosis in a plant cell: prophase, metaphase, anaphase, telophase.


Procedure:  Identify the phases of the cell cycle 

1. Place your Allium root tip slide on your microscope.

2. First focus on a single root tip with the scanning lens to orient yourself to the structure of the root tip.

3. Observe the area that is a short distance above the very tip of the root. This is the apical meristem region.

4. Use the low and then high power objectives to locate cells in each stage of the cell cycle.    

5.  Take images of each stage.  Include in you presentation. 


E. Introduction: Meiosis

On the surface, the processes of mitosis and meiosis appear very similar. Each process uses the same strategy to equally divide the chromosomes between two new cells. However, to complete meiosis and halve the chromosome number requires two rounds of division and a different initial organization of the chromosomes.


To begin with, each cell of your body has 46 chromosomes. But there are only 23 different kinds of chromosomes. Therefore, you have two of each kind of chromosome – one from your mother and one from your father. These pairs of chromosomes are called homologues or a homologous pair. The number of different kinds of chromosomes, 23 in humans, is referred to as the haploid (n) number of chromosomes. And the total number of chromosomes, 46 in humans, is the diploid (2n) number. Therefore, in mitosis a diploid parent cell produces two diploid daughter cells and in meiosis a diploid parent cell produces four haploid daughter cells.


These haploid daughter cells are the gametes, sperm in males and ova (ovum is singular) in females. This cycle of meiosis halving the number chromosomes and fertilization restoring the full set of chromosomes is the basis of sexual reproduction. In addition, any organism that uses this process is said to have sex, in the biological sense.


H.  Following Chromosomes through Mitosis and Meiosis

In the process of meiosis, diploid (2n) germ cells produce haploid (n) gametes.  The flow of chromosomes is as follows:



In order to simulate the flow of chromosomes in each stage of mitosis and meiosis, we will use pop beads.  We will build a cell with a diploid (2n) number of 4.  This means that there are two pairs of homologous chromosomes in our cell.  They are also color coded (red and yellow) to differentiate between paternal and maternal chromosomes.  




In this exercise you will follow the flow of chromosomes through mitosis and meiosis.  For meiosis, take into account independent assortment.  Show the production of all the different gamete combinations obtained in this process.  Take a picture at each step (metaphase, anaphase, telophase and cytokinesis).   Questions of ponder for your presentation.



Question 1:  How many chromosomes are in your cell at the start?  Is this cell diploid or haploid?



Question 2:  How many chromosomes are in your cell at the completion of mitosis? Meiosis I?  Meiosis II? Is this cell diploid or haploid?



Question 3:  How many chromosomes are in your cell at the completion of meiosis?  Is this cell diploid or haploid?



 Question 4:  How many unique gametes are prodcued by meiosis in the absence of crossing over due to independent assortment?






In this exercise there is a single crossover event in homologous pair #1.  Take pcitures of the chromosomes through meiosis and explain what is happening at each step.  Use the flow chart below as a guide.




 Question to Ponder:  How many unique gametes are prodcued by meiosis in the presence of crossing over with a diploid number (2n) = 4?




I.  Phases of meiosis

Identify as many of the different stages of meiosis as possible.  Place the letter associated with each picture by the appropriate stage.




J.  Case Study in Cell Division

The following case study is titled, "Cross Dressing or Crossing Over?  Sex Testing of Woman Athletes".  This is a PowerPoint case study.  Please answer the questions in a separate presentation.

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