Download Modeling Meiosis

Name:_________________________
Modeling Meiosis
Meiosis resembles mitosis but serves a very different purpose. Meiosis is a cell division resulting in the
halving, or reduction, of chromosome number in each cell. A diploid organism has two sets of chromosomes
(2n), while a haploid cell or organism has one set (1n). Meiosis produces gametes (ova and sperm) in animals and
spores in fungi, plants, and protists. 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, along
with random fertilization, increase the genetic variability in the offspring.
These mechanisms are essential to our understanding of genetics and
evolution in sexually reproducing organisms.
The hallmark of sexual reproduction is the great
diversity seen in the gametes and in the resulting
offspring produced by fertilization. Meiosis is
integral to this process because this type of cell division
produces the sex cells, gametes. Before you begin the lab, answer the
following questions.
1. How do diploid organisms produce haploid gametes?
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2. How does the process increase gamete diversity? ________________________________________
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3. When is the DNA replicated during meiosis? _____________________________________________
4. Are homologous pairs of chromosomes exact copies of each other? _____________________________
5.How does the distance between two genes or a gene and a centromere affect crossover frequencies?
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6. What is meant by independent assortment?
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7. How can you calculate the possible number of different kinds of gametes? ______________________
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8. What happens if a homologous pair of chromosomes fails to separate?______________________________
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Background:The fungus Sordaria fimicola 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.
Sordaria fimicola is an ascomycete fungus that can be used to demonstrate the results of crossing over
during meiosis. Sordaria is a haploid organism for most of its life cycle. It becomes diploid only when the
fusion of the mycelia of two different strains results in the fusion of the two different types of haploid
nuclei to form a diploid nucleus. The diploid nucleus must then undergo meiosis to resume its haploid state.
Name:_________________________
Meiosis, followed by mitosis, in Sordaria results in the
formation of eight haploid ascospores contained within a
sac called an ascus (plural, asci). Many asci are contained
within a fruiting body called a perithecium. When
ascospores are mature the ascus ruptures, releasing the
ascospores. Each ascospore can develop into a new haploid
fungus. The life cycle of Sordaria fimicola is in Figure 7.
To observe crossing over in Sordaria, one must make
hybrids between wild-type and mutant strains of Sordaria.
Wild-type (+) Sordaria have black ascospores. One mutant
strain has tan spores (tn). When mycelia of these two
different strains come together and undergo meiosis and
then mitosis, the asci that develop will contain four black
ascospores and four tan ascospores. The arrangement of
the spores directly reflects whether or not crossing over
has occurred.
Figure 8
In Figure 8, no crossing over has occurred. Two
homologous chromosomes line up at metaphase I of
meiosis. The two chromatids of one chromosome each
carry the gene for tan spore color (tn) and the two
chromatids of the other chromosome carry the gene
for wild-type spore color (+). The first meiotic
division (MI) results in two cells each containing just
one type of spore color gene (either tan or wild-type).
Therefore, segregation of these genes has occurred
at the first meiotic division (MI). The second meiotic division (MII) results in four cells, each with the
haploid number of chromosomes (lN).
A mitotic division simply duplicates these cells, resulting in 8 spores. They are arranged in the 4:4 pattern.
Figure 9
In Figure 9, crossing over
has occurred in the region
between the gene for
spore color and the
centromere. The
homologous chromosomes
separate during meiosis I.
This time, the MI results
in two cells, each containing
both genes (1 tan, 1 wild-type); therefore, the genes for spore color have not yet segregated. Meiosis II
(MII) results in segregation of the two types of genes for spore color. A mitotic division results in 8 spores
arranged in the 2:2:2:2 or 2:4:2 pattern. Any one of these spore arrangements would indicate that crossing
over has occurred between the gene for spore coat color and the centromere.
Name:_________________________
Procedure:
1. Use the pictures provided by your teacher to determine the % crossing over. Do this by counting the
number of 4:4 asci and the number of recombinant asci and record your data in the table. You must
count at least 50 asci. 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.
4:4 pattern =
parental
2:2:2:2 pattern =
crossing over
occurred
Number of 4:4
Number of Asci showing
crossingover
Total
Asci
counted
% Asci
showing
crossing
over
Gene to
centromere
distance
(map units)
Class data
for gene to
centromere
distance
(map units)
Class
average
Answer-
A map unit is an arbitrary unit of measure used to describe relative distances between linked genes. The
number of map units between two genes or between a gene and the centromere is equal to the percentage of
recombinants.
Evaluating Results
9. Why did you divide the percentage of asci showing crossover (recombinant) by 2?
Name:_________________________
10. The published map distance between the spore color gene and the centromere is 26 map units. How did
the class data compare with this distance? Answer this question by completing a chi-square calculation to see
if the class data is “close enough” and just due to chance……if there is something majorly wrong with our data.
(hint: there are 2 options, normal vs crossing over)
Data Chart
Normal – 4:4 Asci
Crossed over – non
4:4 Asci
Observed = class
average expressed
as # of Asci out of
50 (o)
Expected (e)
Difference (o-e)
Difference squared
(o-e)2
(o-e)2/e
Sum
Accept or reject data?
11. How can you account for any disparities between the class data and the published data?
12. Draw a pair of Sordaria chromosomes in meiosis I and meiosis II and show how you would get a 2:4:2
arrangement of ascospores.
13. What is karyokinesis? Compare and contrast karyokinesis in mitosis and meiosis.
14. The cell cycle in a certain cell type has a duration of 16 hours. The nuclei of 660 cells showed 13 cells in
anaphase. What is the approximate duration of anaphase in these cells (in minutes)?
Research: What is a Philadelphia Chromosome? What is different in the karyotype of someone with a
Philadelphia chromosome versus a “normal” karyotype? What cancer results from a Philadelphia Chromosome?