BIOL 212 Genetics Lab
Spring 2007
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.
Materials:
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.
Procedure:
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:
http://biog-101-104.bio.cornell.edu/BioG101_104/tutorials/cell_division/CDCK/cdck.html

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
diploid=2N)?
 When does DNA replication take place?
 How much DNA is present at each stage in comparison to a nondividing 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 nondividing 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:
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

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
 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:
Station #2
Station #3
Station #4
Station #5
Station #6
Station #7

unfertilized primary oocytes (B),
sperm entrance (B),
oocyte with tetrads in meiosis I (C),
formation of first polar body (C),
secondary oocyte with dyads in meiosis II (D),
formation of second polar body (D&E),
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.
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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
http://biog-101-104.bio.cornell.edu/BioG101_104/tutorials/cell_division/CDCK/cdck.html 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 on-line.
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