CROSSING OVER IN Sordaria
By Mrs. Jones
MAIN MENU
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Intro to the fungi
Sordaria Life Cycle
Structures of Sordaria
Formation of Asci
Lab
Common Characteristics of
Fungi
• eukaryotic
• most are multicellular
• Reproduce by spores
– Sexual or asexual
• heterotrophic
– absorptive nutrition
Structure of a Fungus
• hyphae
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mycelium (mass of hyphae)
mycelium
Septa (cross walls)
chitin cell wall
haploid except during sexual reproduction
hyphae
septa
CLASSIFICATION OF THE FUNGI
Eubacteria
Archae
Eukaryotes
Chytridiomycota
Animals
Zygomycota
Fungi
Basidiomycota
Plants
Viruses ?
Algae
Protists
Ascomycota
Ecological Impact of Fungi
• decompose wastes and dead organisms
– return nutrients to the ecosystem
• Saprophytic
– decomposers in a woodland
– One year over a ton of debris falls onto an acre of woodland
– return nutrients to the soil.
Pathogenic Fungi
valley fever
Valley Fever
disease of the
common in the southwestern United States and northwestern Mexico.
caused by the fungus Coccidioides immitis,
which grows in soils
fungal spores become airborne when the soil is disturbed
infection occurs when a spore is inhaled
Within the lung, the spore changes into a larger, multicellular structure
called a spherule. The spherule grows and bursts, releasing endospores
which develop into spherules..
athletes foot
a fungal infection.
lives on the skin and breeds under warm,
moist conditions.
more common during hot weather
sweaty footwear is usually the culprit.
Adult males suffer most often from this
condition.
Pathogenic Fungi Cont.
– Powdery mildews that attack ornamental and food plants
– The chestnut blight, which in a few decades killed almost all of
the mature American chestnut trees in the Appalachians of North
America.
– The Dutch elm disease, which has killed many of the American
elms in the United States.
Food Source
• The truffle and the morel are both highly-prized food delicacies.
Truffles establish a symbiotic relationship with the roots of such trees
as oaks.
morel
Oregon White Truffle
Tuber
• Saccharomyces cerevisiae or budding yeast
– Ferments sugar to ethanol and carbon dioxide and thus is used to make
alcoholic beverages like beer and wine
– to make ethanol for industrial use
– in baking (it is often called baker's yeast). Here, it is the carbon dioxide
that is wanted (to make bread and cakes "rise" and have a spongy
texture).
Sordaria Life Cycle
Life Cycle of Sordaria
• The ascospore is haploid.
• It divides by mitosis producing haploid
filaments
• As it continues to divide by mitosis the
haploid mycelium grow inside the
organism the fungus is digesting
Life Cycle Cont.
• The ends of two mycelia fuse.
• The nuclei of two haploid cells join
(fertilization) forming a diploid zygote.
• The zygote then divide by meiosis
producing 4 haploid cells.
• The four cells divide by mitosis producing
8 haploid nuclei.
Life Cycle Cont.
• The 8 nuclei develop into ascospores.
• The spores are discharged from the
perithecium and the cycle begins again.
Wild and tan strains being crossed on agar plate.
Where the mycelia of the two strains meet fruiting
bodies called perithecia develop.
Perithecium containing asci.
When the perithecia are crushed the
asci can be seen.
Sordaria ascus (non hybrid)
Fertilization did
not take place
between different
strains of fungi.
If genes crossed
over we cannot
tell because all of
the spores are the
same color.
Asci containing black and tan
ascospores.
Formation of asci
During prophase I of meiosis homologous chromosomes
pair and form tetrads through the process of synapsis.
While the four chromosomes are close together DNA can
be exchanged between homologous chromatids.
Chiasmata (crossing over)
A Closer Look at Chiasmata
The + designate wild type genes (black)
tn designates the mutant with tan spores.
When no crossing over occurs the tan asci
will all be together and the black asci will all be together.
Asci showing no crossing over.
Meiosis with Crossing Over
When crossing over occurs the ascospores will form
one of the patterns above .
Notice that only half of the chromosomes crossed over .
This means that half of the spores in the ascus are the
result of crossover.
Ascospore showing crossover.
Mapping the Genes of Sordaria
• OBJECTIVES
– Explain how meiosis and crossing-over result
in different arrangements of ascospores within
the asci
– Calculate the map distance between a gene for
ascospore color and the centromere of the
chromosome on which the gene is found.
Background
The frequency of crossing over appears to
be governed largely by the distance
between genes, or in this case, between the
gene for spore coat color and the
centromere. The probability of a crossover
occurring between two particular genes on
the same chromosome (linked genes)
increases as the distance between those
genes becomes larger. The frequency of
crossover, therefore, appears to be directly
proportional to the distance between genes.
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
and the centromere is equal to the percentage
of the recombinants. Customary units cannot
be used because we cannot directly visualize
genes with the light microscope. However,
due to the relationship between distance and
crossover frequency, we may use the map
unit.
Materials
• perithecia (paper bag) from a black X tan
cross
• asci (paper strips) with eight dots
representing ascospores
Procedure
• remove the asci form the bag and separate
into hybrid and nonhybrid stacks.
• separate and count the MI and MII asci
• do NOT count the nonhybrid asci
• record your personal data in your lab book
and in the class data in the computer.
Calculate Map Distance
(use the class data)
• divide the number of cross-over asci by the
total number of asci and multiply by 100 to
calculate the percent of cross-over
• divide the percent of cross-over by 2 to
calculate the map distance
Questions
• Why are the nonhybrid asci not counted?
• Why is the percent of cross-over divided
by 2?
• Draw a pair of chromosomes in MI and in
MII, and show how you would get a 2:4:2
arrangement of ascospores by crossing
over.
GLOSSARY
Absorptive nutrition - obtaining
nourishment from the surroundings
(often having first digested it by
secreting enzymes)
Dikaryotic – possess two haploid nuclei
Diploid – the presence of pairs of homologous chromosomes
Haploid – single chromosomes NOT pairs
Heterotrophic –obtaining nutrients by eating other
organisms or their byproducts.
Pathogenic – disease causing