NAME:_________________________ DATE:_____________
MITOSIS AND MEIOSIS
PERIOD:_________
1. Recognize the processes of meiosis and how it differs from mitosis.
2. Identify support cells from human spermatogenesis and oogenesis.
Topic Introduction:
If I asked you “Where do cells come from?” what would you answer? In modern biology our understanding
of a the cell as the basic building block of life is codified in a set of principles called the Cell Theory which was first
codified by Schleiden and Schwann in 1838-39. The cell theory is second only to the theory of evolution by natural
selection in understanding the relatedness of life. Cell Theory says the following:
1. All living organisms are composed of one or more cells.
2. Cells are the basic building blocks of all life.
3. All cells are descended from a preexisting cell.
While these may seem like relative simple points it took scientists several centuries to produce the cell theory.
The development of the cell theory directly follows the development of the microscope. The name “cell”
was coined by Robert Hooke6 in 1665. While observing a piece of cork under his microscope he thought that the
microscopic units that made up the cork looked like the rooms, or “cella” in Latin, that monks lived in. This was
closely followed by the discovery of single celled organisms by Antoni van Leeuwenhoek3-5 in 1676. Leeuwenhoek
discovered motile microscopic particles by examining scrapings from his teeth under his microscope. In 1838
Metthias Schleiden7 and Theodor Schwann8 presented evidence that all plants and animals are composed of cells.
However, there were still some questions as to where cells came from, as Schleiden believed cells formed through a
process of crystallization. This theory was simply a variant on the belief of Aristotle that life could come into
existence by spontaneous generation.
It was not until the 1850s that a group of scientist was able to show that new cells were produced from
preexisting cells9. However, most scientists believe the definitive test disproving spontaneous creation of microbial
life was conducted by Louis Pasteur in 186210. In Pasteur’s experiment two flasks were each set-up with bacterial
growing broth (a liquid that is conducive to the growth of bacteria) and sterilized. Both flasks were left open to the
air but in such a way that dust could enter only flask 1 not flask 2. After a period of time bacteria growth was seen
in only flask 1 and not in flask 2. This showed that dust (bacteria) had to be added to the broth in order for
bacteria to grow.
There are two processes involving the production on new cells, the first process, mitosis, is used for growth
and to replace old or dead cells. The second process, meiosis, is used to produce gametes (egg and sperm) cells
that are used for sexual reproduction. An important point to keep in mind is that we name the type of cell cycle
based on what is happening to the nucleus and genetic material.
Meiosis is used in sexual reproduction to produce gametes (sperm and egg in most animals and plants). In
plants and animals each organism contains two copies of each chromosome; this is called diploid. In order for
sexual reproduction to occur properly the number of chromosomes need to be reduced by half; which is called
haploid. If the chromosome number was not reduced by half then each new generation would have twice the
number of chromosomes as the previous organism; which is called polyploidy. In many organisms a state of
polyploidy causes biological defects.
Mechanistically meiosis differs from mitosis in that two rounds of cell division occur, referred to as meiosis
I and meiosis II, with only one round of DNA synthesis. Figure #3 shows the stages of meiosis were there are
differences between the corresponding mitotic and meiotic stages. This produces 4 haploid cells; the number of
mature gametes varies depending on whether the final mature cell is a sperm cell or egg cell (Figure 3).
In meiosis I S stage occurs as normal.
The first difference between mitosis and meiosis
I occurs in Prophase I, during Prophase I the
homologous chromosomes pair up and exchange
genetic material by crossover (Figure 3). This
exchange of genetic material increases the
genetic variation in the offspring. The next
difference occurs in anaphase I, during anaphase
I instead of the centromere dividing it stays
connected and the homologous chromosomes
are segregated to the opposite poles (Figure 3).
During Cytokinesis I we see the first difference
between spermatogenesis (sperm formation)
and oogenesis (egg formation).
During
cytokinesis of the egg the cytoplasm divides
unequally with one of the daughter cells getting
most of the cytoplasm, the smaller cell is called a
polar body (Figure 3B). The presperm cells
undergo an equal cytokinesis (Figure 3A). The cells will then enter a second cell cycle, meiosis II, without
replicating DNA. The length of interphase between meiosis I and meiosis II varies from nonexistent too years
depending on the organism. In meiosis II during Anaphase II the kinetochore divides and the sister chromatids are
pulled to opposite poles of the cell. Again the in oogenesis the cell undergoes unequal cytokinesis producing an
oogonia and another polar body, while the sperm cells divide equally. This produces four haploid spermatocytes in
the male line and one haploid oogonia and 2 or 3 haploid polar bodies in the female line. The spermatocytes and
oogonia go on to mature in to sperm and egg cells, which will give rise to a new generation.
Now that we have and understanding of the mechanisms of mitosis and meiosis it is clear how each
separately links to the third principle of the cell theory: all cells descend from preexisting cells. For instance we
know that new somatic cells arise from mitosis, when an older cell divides. Additionally, we know that the
development of a new multi cellular organism starts with the fusion of two gametes (fertilization) which produces
a zygote. The remaining question though is how does mitosis and meiosis relate to each other. The answer to this
question depends on whether we are talking about a multi cellular animal or plant. In multicellar animals the
zygote divides a few times mitotically then the cells are separated into two populations one population will
continue to divide mitotically and will go on to form all the somatic cells. The second population will form the
germline (Figure 4A). In plants the first few stages are the same, the difference occurs in that a population of cells
is not set aside to form a germline (Figure 4B). Instead germline cells are recruited from the somatic cells when
they are needed.
EQUIPMENT



Paper
Pencil/pen
Slides
o Onion Root Tip-Won’t use, part of set
o Whitefish Blastula-Won’t use, part of set
o Lilium c.s. Anthers (plant)
o Ascaris lumbricoides
EXERCISE 1: STAGES OF MEIOSIS
Pre-lab:
Observing the different stages of meiosis is often difficult do to the structure of the organs in which meiosis and
fertilization occur. One way scientist gets around this type of problem is through the use of model organisms. A
model organism is an organism in which a particular biological process is easily observed or manipulated. Two
examples of model organisms used in the study of meiosis are the grasshopper testis and the Ascaris lumbricoides
ovary. The reason that these are good model organisms for the process of meiosis is that meiotic cells travel down
the organ in a liner path. Later stages of meiosis are farther along in the organ than earlier. For example in a
grasshopper testis it is often possible to observe all stages of both meiosis I and meiosis II. The ovary of the Ascaris
lumbricoides (a nematode worm) is similarly arranged. However, in the case of the Ascaris lumbricoides ovary you
can see the polar bodies produced during oogenesis as their life time is long enough that they are preserved in the
fixed tissue. Additionally, fertilization also occurs in the ovary allowing for the observation of the pronuclei in
early fertilization. In this lab we are going to use these two model organisms to observe the processes of meiosis
and fertilization.
In humans meiosis occurs in special tissues in specialized organs, the ovary in females and the testes in
males. The biological function of these organs is to isolate, protect, support, and deliver the gametes. Early in the
process of development the cells that will become the gametes temporally exit the cell cycle and are segregated to a
region of the embryo that will become the testes or ovaries. This process of segregation helps protect the DNA of
germline cells from damage in two ways. The first is that these cells will undergo fewer rounds of division and
therefore DNA synthesis then the other cells in the body. This is important because DNA synthesis is one of the
most common ways DNA modification can occur. Second these cells live inside the structure of the testes or ovary
and get some protection from the outside world. In this exercise we will observe the cells needed to support the
development of the sperm and eggs in humans in this exercise. In addition to identifying fully developed sperm and
eggs.
Pre-lab Questions:
1. Why are we not using human ovaries and testis to observe meiosis and pronuclei?
Data Collection/Experiment:
2. Select the Ascaris lumbricoides Female slide and place on microscope, cycle through objectives starting with
low, medium and finally high power.
3. Locate the image of a developing oocyte with a polar body attached. (Draw and label on separate sheet of
paper) making sure to title ALL drawings.
4. Draw and label two cells in meiosis I and two cells in meiosis II.
5. Draw an image of a fertilized egg (pg 1013, figure 46.15-AP Bio Book/Pg 1000, figure 9 in Reg. Bio) and
label on a separate sheet of paper.
EXERCISE 2: MEIOSIS IN PLANTS
Pre-Lab:
Meiosis is a crucial step in the life cycle of almost all multicellular organisms. It produces gametes
(eggs and sperm) in animals, but forms spores in plants. The haploid male and female spores
develop into tiny organisms that then produce gametes. Meiosis is easier to view in plants, so we will
use a plant example to visualize the meiotic process.
Many flowering plants
have both male and
female organs. This
photograph of a lily
flower illustrates the
male and female
parts.
The anthers produce
male spores (called
microspores) by
meiosis. They will
develop into pollen
grains. The pistil
contains an ovary at
its base in which
female spores (called
megaspores) will be
produced by meiosis.
The ovary is usually
deep within the flower.
It is shown in this cutaway view of a flower
model. Within the
ovary, many tiny
ovules are present
which produce female
megaspores by
meiosis. If the
resulting gametes are
fertilized by gametes
from the pollen, the
ovules will develop
into diploid seeds.
Materials
Lilium anther cross section slides (1st and 2nd divisions) and compound microscope.
Procedures
Using the slides of Lilium anthers. observe and draw the phases of meiosis 1. Identify and draw as many
as you can. Sketch and label each phase below, from Lilium anther slides. Note the total magnification
and approximate the cell size.
Prophase I
Metaphase I
Anaphase I
Telophase I
Questions
1. Define ``crossing over''.
2. Name the phase in which crossing over occurs.
3. Name two ways in which gene traits are ``mixed'' through the process of meiosis.
4. List 4 ways that meiosis differs from mitosis.
5. What function does meiosis serve in Lilium anthers?
6. In what types of animal tissues would you expect to observe meiosis occurring?
Attach drawings and other questions.