Lab 4: Mitosis and Meiosis by W656u9JN


									                                                                  BIOL 212 Genetics Lab
                                                                  Fall 2006

Lab 4: Mitosis and Meiosis

Purpose: To review the stages of mitosis (somatic cell division) and meiosis (cell
division in gametes). An understanding of these basic processes is fundamental to the
study of Mendelian genetics.


       Laptop computers/internet access
       Slides, compound microscopes, lens paper
       Mitosis slides: whitefish blastula (Leucichthys blastodisc)
       Meiosis slides: Ascaris or Parascaris (nematode) oogenesis (slides #M1194,
       M1195, M1196)
       Colored pencils
       Index cards

Background: Hartl and Jones, Essential Genetics, 4th ed. Chap. 3 pp. 76-88.

1. A. Practice working through the processes of mitosis and meiosis for a diploid
cell containing 2 or 3 pairs of homologous chromosomes.
     You can do this by color-coding the drawings of meiosis and mitosis on the last
        pages of the handout using color pencils, or by working through examples from
        the following web site:

      In genetics, we will focus on what the chromosomes are doing during cell
       division, rather than on the changes in cell structure that are also taking place.
       Ask yourself the following questions as you work through mitosis and meiosis.
            What is the number of chromosome sets at each phase (haploid=1N or
            When does DNA replication take place?
            How much DNA is present at each stage in comparison to a non-
               dividing cell of the same type?
                    I prefer to use the symbol “C” to refer to DNA content separately
                      from chromosome content (i. e. haploid or diploid). Thus a non-
                      dividing diploid cell would have 2C DNA, a diploid cell which has
                      undergone DNA replication prior to cell division has 4C DNA (yet
                      is still 2N). Gametes are haploid (1N) and contain 1C DNA.

B. You will be assigned one of the following stages of mitosis or meiosis:

       Interphase, Prophase, Metaphase, Anaphase, or Telophase of mitosis
       Interphase, Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II,
        Metaphase II, Anaphase II, Telophase II of meiosis.
Prepare to present in class a cell with a given chromosome set that is in that stage of
cell division.

2. Review the process of mitosis using the diagrams in Fig. 2 of the handout.

      Observe mitosis on prepared slides of whitefish blastula under the light
       microscope. Identify cells in the stages of prophase, metaphase, anaphase,
       telophase and interphase. See instructor for help with setting up microscopes or
       locating structures.

3. Review the process of meiosis in Ascaris using diagrams B through E in Fig. 4.
In Ascaris, penetration by the sperm initiates meiosis in the oocyte. Note that in
oogenesis, only one of four gametes produced in meiosis survives to be fertilized. Other
genetic material is expelled as polar bodies. Once meiosis is complete in the oocyte, the
gametes (male and female pronuclei) fuse.

      Observe oogenesis in Ascaris. Examine a slide containing sections of the Ascaris
       uterus with oocytes in various stages of maturation. Ascaris contains four large
       chromosomes (2 homologous pairs).

Some terms used in diagrams/descriptions of Ascaris meiosis:
tetrads: 4 duplicated chromosomes (the two homologous pairs after replication)
dyads: 2 duplicated chromosomes (one from each homologous pair)
polar body: genetic material that is discarded after each meiotic division in the oocyte
pronucleus: male and female haploid nuclei prior to fertilization
     Groups of three or four students will be assigned to identify a particular stage
       of Ascaris meiosis and to set up one of the microscope stations for the class.
       Using Fig. 4 as a reference, try to identify the following stages on the prepared
       slides set up on the demonstration microscopes (B-E refer to the labels in
       Figure 4). Please consult the instructor for help in identifying stages.

       Station #1:    unfertilized primary oocytes (B),
       Station #2     sperm entrance (B),
       Station #3     oocyte with tetrads in meiosis I (C),
       Station #4     formation of first polar body (C),
       Station #5     secondary oocyte with dyads in meiosis II (D),
       Station #6     formation of second polar body (D&E),
       Station #7     male and female pronuclei (E).

      When all the stations are set up, try to visit them in sequence to better
       understand the process of meiosis in Ascaris.

Quiz: There will be a quiz next week in lab (20 pts.) following a short discussion. The
quiz will emphasize what happens to the chromosomes during mitosis and meiosis
and some of the major distinctions between the two processes. It would be good
practice to use the “random” feature of the Cell Division Construction Kit for practice
in visualizing the number and arrangement of chromosomes in cells in different stages of
cell division. Some questions may also be drawn from the slides which you observed. A
sample quiz will be posted in the lab.


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