VHS tapes:  Mitosis / Meiosis

LAB 5

MITOSIS AND MEIOSIS

All living things must reproduce in order to propagate their species. In unicellular organisms, this may be accomplished asexually: one parent cell divides to produce two identical daughter cells. This is reproduction since a new individual is produced but no diversity or genetic variation has been introduced through asexual reproduction. A multicellular organism will produce new cells for growth or to replace damaged or worn cells through the process of mitosis. In this laboratory exercise, you will become familiar with the events that take place in the cell cycle and during the phases of mitosis.  You will use your microscope to find examples of each stage of mitosis in a plant cell (Allium root tip) and in an animal cell (whitefish blastula).

Meiosis is division for sexual reproduction and results in gametes, either egg or sperm in mammals. Meiosis is also called reduction division since the number of chromosomes is reduced by half. For example, human somatic cells each contain 46 chromosomes or 23 homologous pairs but human gametes will contain only 23 chromosomes. The reduction division occurs when the homologous pairs are separated during meiosis.

LAB OBJECTIVES:

1.      Identify the stages and events of plant and animal mitosis.

2.      Contrast plant and animal mitosis.

3.      Identify major differences and similarities between the processes and results of mitosis and meiosis.

PROCEDURE:

View the videos on "Mitosis” and “Meiosis" as directed by your instructor. 

Observe the process of mitosis microscopically.

Manipulate colored pipe cleaners to illustrate the events of mitosis and meiosis.

I.        Mitosis

In order for a multicellular organism to grow and survive, the cells of which it is composed must be able to reproduce.  Cells reproduce by the process of mitosis.  The entire process of mitosis is one of constant, integrated change, with one stage leading into another.  The end result of mitosis is two cells with the same chromosome number as the mother cell.  Through mitosis organisms grow and mature and replace dead cells and damaged tissue.

A.      Mitosis In Plant Cells

You will observe the stages and processes of mitosis in Allium (onion) root-tips.  Obtain a prepared slide of the onion root tip.  Examine one of the root tips under the 4X power of your microscope.   Locate the rounded end where most of the dividing cells will be found.  For detailed examination, 40X must be used.  NOTE:  Since each section is very thin, not all of them will be equally good for studying cell division.

Be prepared to examine other sections on the slide or even change slides in order to locate and study the series of events described here.  Refer to "Plant Mitosis" for clarification of the stages and events.  Many of the cells in your preparation will be in interphase, once known as "the resting stage" between divisions.  Locate these cells on your slide.  Look for cells with the nuclear membrane in tact containing nuclei with granular chromatin.  During this stage the mother cell is not dividing, but duplication of chromatin material (DNA), synthesis of cell organelles, and growth of the cell occurs.

Question 1:         What are the functions of mitosis?

Question 2:         Where in the human body would you expect to find large numbers of cells dividing by

mitosis? 

Question 3:         Why is the term "resting stage" a poor term to describe cells in interphase? 

1.      Prophase
During prophase the chromosomes coil and become distinguishable in the nucleus.
The nuclear membrane then breaks down and the chromosomes become distributed randomly
throughout the cytoplasm.  Each chromosome has now doubled and is now composed of two
chromosomes.

Question 4:         Why is it necessary that the chromosomes double? 

2.      Metaphase
During metaphase the duplicated chromosomes become arranged near the center of the cell at a region known as the equatorial plate.  This arrangement is in preparation for the separation of duplicate chromatids which occur later.  Spindle fibers, forming the spindle, become more apparent.  Some of these fibers are attached to the chromosomal centromeres and will pull chromosomes to opposite poles of the cell during anaphase.

3.      Anaphase
At the beginning of anaphase the two members of the previously doubled chromosomes (chromatids) separate and move toward opposite poles (ends) of the cell. This can be recognized by two groups of roughly V-shaped chromosomes on opposite sides of the cell.  Since the onion has sixteen chromosomes, it is seldom possible to see all of them at one time.  Reduce the light penetrating through the objective of your microscope and try to find the spindle fibers near the center of the cell.  (They are often not visible in a study of this kind.)

4.      Telophase
During telophase the chromatids arrive at each pole, and a cell plate forms across the center of the plant cell, called cytokinesis.  When complete the cell plate will divide the original cell into two daughter cells.  As telophase progresses the nuclei begin to reorganize and the chromosomes again become distinct.  In late telophase the spindle disappears and the nuclear membrane begins to reappear.  

Question 5:         The two resulting daughter cells have the same number and kind of chromosomes
 as the mother cell from which they came.  Why? 

B.      Mitosis In Animal Cells

You can readily observe mitosis in animal cells by studying a prepared slide of a whitefish blastula, an early developmental stage formed by successive cell divisions following fertilization of the egg by the sperm.  The behavior of the chromosomes in animal cells is essentially the same as that observed in plant cells.  In whitefish blastula cells, however, the chromosomes are much smaller and more numerous than in the onion root-tip cells.  You will note other more important differences:

1.      In animals there is a pair of centrioles located at each pole of the spindle. 

2.      In addition to the fibers of the spindle there are other fibers called aster rays which radiate away from the centrioles.

3.      The mother cell is apparently "pinched" in half at the equator as the two daughter cells are separating during telophase.  This forms a cleavage furrow.  No cell plate is formed in mitosis of animal cells. 

For clarification of the stages and events of animal mitosis, refer to "Animal Mitosis".

       II.          Meiosis

Meiosis is a unique biological event that not only maintains the chromosome number constant for a species of plants or animals, but provides a means of genetic variability because of "crossing over", an event which allows the exchange of genetic material.  In animal meiosis, immature germ cells undergo a "reduction" from the diploid number of chromosomes characteristic for the species and become mature haploid gametes.  Mature gametes are sperm and eggs in humans and occur in males in the testes and in females in the ovaries or oviducts.  In plants, meiosis may result in the formation of spores.

Meiosis involves two cellular divisions instead of one, resulting in the formation of four gametes, each with half the original chromosome number.  Refer to "Meiosis".  The two cellular divisions are designated as Meiosis I and Meiosis II.  Major events are as follows:

A.      Meiosis I

1.       Chromosomes appear as double structures consisting of two chromatids.

2.       Chromosomes carrying similar (but not identical) traits pair up.  This pairing is called synapsis and may result in an exchange of genetic material between chromosomes called crossing over.  Thus, tetrads of chromatids are formed.

3.       Tetrads gather on the equatorial plane.

4.       Whole chromosomes (consisting of two chromatids each) move to the opposite poles.  At this point, homologous (like) chromosomes are essentially separated into two different new cells.  The chromosome number has been halved.

Question 6:         Which chromosomes go to which pole is entirely by chance.  What is the significance of this? 

B.      Meiosis II

1.       Chromatids, still joined, move to the equatorial plane.

2.       Chromatids separate and move to opposite poles.

3.       We now have four (4) cells each with half the normal number of chromosomes.

Question 7:         What does homologous mean?  What are homologous chromosomes? 

Question 8:         Where in the human body would you expect meiosis to occur?

Question 9:         How many chromosomes are in a human somatic cell?  How

many chromosomes are in a sperm or egg?

Question 10:       Are the end results of meiosis always gametes?

Explain.

      III.          An exercise to illustrate what happens to chromosomes during Mitosis and Meiosis

A.      Obtain twelve pipe cleaners from your instructor, four each of three different colors.  Two of each color should be plain and two of each color should have black dots.

B.      To illustrate metaphase of mitosis twist the two identical pipe cleaners together at one point.  You now have six chromosomes, each made up of two chromatids.  The two chromosomes (pipecleaners) of the same color make up a pair.  The dotted pipecleaner is the homolog of the plain pipecleaner of the same color.  The point of contact represents the centromere.  Line the six chromosomes up in a single line as in metaphase.  The six chromosomes make up two sets (one maternal and one paternal) or 3 pairs.

To illustrate anaphase of mitosis separate the chromatids and move them apart.  You now have two cells with six chromosomes each.  Each chromatid is now a chromosome.

C.      You will now demonstrate what happens during Meiosis I and Meiosis II.

1.       Meiosis I

Twist identical pipe cleaners together to represent chromosomes consisting of two chromatids.  You now have two sets of chromosomes.  Now, also, twist together all four pipe cleaners of the same color.  Synapsis of chromosome pairs has occurred.  Line the three resulting tetrads up together as occurs in metaphase I of Meiosis I.

To illustrate anaphase I of Meiosis I now separate the solid color pipe cleaners from the dotted ones and move the joined chromatids toward opposite poles.  (It makes no difference if some dotted "chromosomes" and some plain ones move toward the same pole.) You have now reduced the chromosome number in the resulting cells by one half.  Note that the chromosomes in the new cells do not have both members of a pair.

2.       Meiosis II

Continuing from above, line up the pipe cleaners (still joined as chromatids) along their respective equatorial planes.  In Meiosis II, chromatids separate and move to opposite poles.  To illustrate this, now separate the chromatids and move them to opposite poles.  The end result is four groups of three chromosomes.  This represents four   gametes each containing three chromosomes.

Question 11:       Disregarding crossing over, how many different gametes are possible from these six different chromosomes?