Cell Division: Meiosis and Tetrad Analysis
Objectives:
• To understand how meiosis and crossing over leads to increased genetic diversity,
which is necessary for evolution.
• Explain how meiosis and crossing over result in the different arrangements of
ascospores within asci.
• Learn how to calculate the map distance between a gene and the centromere of the
same chromosome.
Introduction:
All new cells come from previously existing cells. The process of cell division, which
involves both division of the nucleus and division of the cytoplasm, forms new cells. There are
two types of nuclear division: mitosis and meiosis. Mitosis typically results in new somatic
(body) cells. Formation of an adult organism from a fertilized egg, asexual reproduction,
regeneration, and maintenance or repair of body parts are accomplished through mitotic cell
division.
Meiosis involves two successive nuclear divisions that produce four haploid (monoploid)
cells. Meiosis I is also known as reduction division. It is the first division, the chromosome
number is reduced from diploid with 2 sets of chromosomes (2n) and separates the homologous
pairs to haploid each cell with 1 set of chromosomes (n). Meiosis II, the second division,
separates the sister chromatids. The result is four haploid gametes. In humans, the diploid
number is 46 and the haploid number is 23. Meiosis results in the formation of either gametes
(in animals- eggs and sperm) or spores (in plants- pollen, in fungus- ascus). These cells have
half the chromosome number of the parent cell.
Three other important characteristics of meiosis are the exchange of genetic material
(crossing over) between homologous chromosomes, the independent assortment of the
chromosomes and the separation of alleles of the same gene. These characteristics, nalong with
random fertilization, increase the genetic variability in the offspring. There are more than eight
million combinations possible! The relative distance between two genes on a given chromosome
can be estimated by calculating the percentage of crossing over that takes place between them.
Answer the following pre-lab questions after talking to your group. Place your own responses
in your lab notebook using complete sentences.
1. How do sexually reproducing organisms produce gametes from diploid progenitors?
2. How does the process increase gamete diversity?
3. How does the distance between two genes or a gene and a centromere affect
crossover frequencies?
Background: Sordaria fimicola is an ascomycete fungus that can be used to demonstrate the
results of crossing over during Meiosis. This is the life cycle of Sordaria fimicola.
General- Sordaria exchanges genetic
material when two mycelia meet and
fuse. The resulting zygote undergoes
meiosis to produce asci; each ascus
contains eight haploid spores. A single
gene determines the spore color.
Detail- Sordaria is haploid for most of its
life cycle
1. When ascospores (haploid) are
mature the ascus ruptures,
releasing the ascospores. Each
ascospore can develop into a
new haploid fungus.
2. It becomes diploid only when
the mycelia (filament-like
groups of cells that contain
haploid nuclei) fuse
3. The diploid nucleus must then
undergo Meiosis to resume its
normal, haploid state. This
results in 4 haploid cells.
4. One Mitotic division results in the formation of eight haploid ascospores.
(Ascospores are contained within a sac called an ascus (plural, asci). Many asci are contained
within a fruiting body called a perithecium). The picture below shows a mature Perithecium
and several asci.
Normally, the asci would break open and release the
haploid spores into the air where they would germinate
and divide by mitosis to produce a new organism.
In the classroom, we can view the asci by taking a
perithecium from a culture and place it in a drop of
water on a slide. If you press gently on a coverslip, the
perithecium will burst open and release the asci. Too
much pressure however will burst the asci as well.
Because almost everyone short of lab technicians burst
the asci, we will use printed pictures! Woohoo! Trust
me, your life (and mine!) is easier this way.
Hypothesis: Generate a hypothesis about the outcome of a cross between wild type (+; black)
and tan (tn) strains. You will be looking for a certain pattern of ascospores within each sleeve of
the offspring (black and/or tan combination). Write you hypothesis in your journal now.
To the right is a photo of asci that resulted from a cross between
two black strains. All of the spores are black. (FYI, spore color of the
normal (wild type) Sordaria, is black because of the production of the
pigment melanin which is deposited in the cell walls...cool, huh!)
To the right is a photo of asci that resulted from a cross between
two tan strains. All of the spores are tan.
To the right is a photo of asci that resulted from a cross between
black and tan strains. Look at a single ascus and note that it
contains both black and tan spores.
- Those on which the pattern of spore distribution in the ascus is 4
tan to 4 black were produced from cells in which no crossing over
occurred.
These are called non-recombinants.
Other asci contain black and tan spores that are distributed in 2:4:2 patterns or 2:2:2:2
patterns. These asci only result from cells in which crossing over has occurred and are called
recombinants. Because the recombinant patterns result only from crossing over, the
frequency of occurrence of recombinants is a measure of how often crossing over occurs.
Formation of crossover/recombinant asci
Formation of Recombinant Asci
Answer the following pre-lab questions after discussing with you group. Place your own
responses in your lab notebook using complete sentences.
4. How do you explain the differences between the recombinant asci and the parental
types?
5. What meiotic event can account for this difference?
Procedure:
1. Using the photos provided, determine if crossing over has occurred in the hybrid asci. Keep
track of the tally of non-recombinant and recombinant asci on a separate sheet of paper (hint: it
is easiest to start at one point and work clock-wise). Count as least 50 asci, you may need more
than one card.
2. Based on your counts, fill in the first four columns of your data table.
3. One map unit equals one recombinant per 100 total events. The percentage of asci showing
crossover divided by 2 equals the map units in this activity. This is done because each spore
produced by meiosis undergoes a mitotic division.
4. Fill out the table for the class averages.
The frequency of crossing over appears to be governed largely by the distance between the
gene for spore color and the centromere. The frequency of crossover, therefore, appears to be
directly proportional to the distance between genes. (The farther away it is, the more likely it is
to be part of crossing over)
A map unit is an arbitrary unit of measure used to describe relative distances between a gene
and the centromere (or between two linked genes which will be discussed later). The number of
map units between a gene and the centromere is equal to the percentage of recombinants.
Individual Data:
Number of Nonrecombinant
Asci
Number of
Recombinant
Asci
Total
Asci
% Showing
Crossover
Gene to
Centromere
Distance
Number of
Recombinant
Asci
Total
Asci
% Showing
Crossover
Gene to
Centromere
Distance
Class Data:
Number of Nonrecombinant
Asci
Analysis Questions: Answer the analysis questions on your own in your notebook using
complete sentences.
1. Why did you divide the percentage of asci showing crossover (recombinant) by 2?
2. Published results indicate that the map distance from the centromere of the gene for
spore color in S. fimicola is 26 map units (approx. 52% crossover frequency).
How close are your results to the published results?
3. How close are the class result to the published results?
4. Calculate your percent error.
5. How can you account for any disparities between the class data and the published data?
6. Illustrate what happened during meiosis to produce the results you found.
7. Sordaria could reproduce asexually by the mitosis of cells to form a new organism. Why
go through all of the extra steps to reproduce sexually?
Individual lab: should include in this order: Pre lab Qs 1-3, Hypothesis, Pre lab Qs 4-5, Data
tables, Analysis Questions.
Group lab: Choose one of the following to investigate and discuss. During your investigation,
answer the following MEIOSIS questions:
a.How are mitosis and meiosis fundamentally different? Similar?
b.What happens if a homologous pair of chromosomes fails to separate, and how might
this contribute to genetic disorders such as Down Syndrome and Cri Du Chat syndrome?
Include the following format:
Title, Research Question, Variable Table, Meiosis Questions (answered in your project or
separately.
OPTION 1: Can any environmental factors affect the amount of crossing over that occurs in
Sordaria? How would you set up an experiment to test this? For example, how does humidity or
pH affect the crossover frequency? You need to include a research question, hypothesis,
variable table, procedure (with details), and explanations for predicted outcomes.
OPTION 2: How do the mechanisms of cell replication affect genetic diversity and evolution?
Consider the mechanisms such as crossing over, independent assortment, segregation,
nondisjunction, and random fertilization.
OPTION 3: Prepare a video or write and produce a play about the process of chromosome
movement during meiosis.