1. AP Lab 3: Mitosis and Meiosis
Crossover in Sordaria

2. Review of Meiosis I

3. Review of Meiosis II

4. Review of Crossing Over

5. Review of Fungi
Fungi grow as filaments called hyphae. A mass of hyphae is called a mycelium.
Fungi are either parasites or saprobes, absorbing the breakdown products from the action of digestive enzymes they secrete.

6. Growth of Fungi
online.com/objects/ViewObject.aspx?ID=BIO 304

7. Reproduction in Fungi
Fungi live their lives primarily as haploid organisms, but they form diploid structures for sexual reproduction in 3 stages:
1. Plasmogamy—the fusing of cells from two different fungal strains to produce a single cell with nuclei from both strains.
A pair of haploid nuclei,
one from each strain is
called a dikaryon.

8. Reproduction in Fungi, continued
2. Karyogamy is the fusing of the two haploid nuclei of a dikaryon to form a single diploid nuclei.
3. Meiosis of the diploid nucleus restores the haploid condition.
Hyphae from two strains fuse
Nuclei from one strain pairs with nuclei from second strain
The diploid nucleus undergoes meiosis, producing 4 haploid cells
Each of the 4 haploid cells undergoes mitosis to produce 8 spores.

9. Meiosis in Sordaria
During meiosis I, homologous chromosomes pair and separate.
During meiosis II, each chromosome separates into two chromatids.
At the end of meiosis, there are 4 daughter cells, each possessing one chromatid from each tetrad of homologous chromosomes.

10. Meiosis in Sordaria
Each of the 4 daughter cells divides by mitosis to produce 2 ascospores. The order of ascospores corresponds to the chromatids.
If no crossing over occurs, then each set of four adjacent ascospores represents a single parent strain and will possess the same traits.

11. Life Cycle of Sordaria

12. Seeing Crossing Over in a Microscope
The results of crossing over during meiosis can be readily visualized under the microscope in the asci of many ascomycete fungi.
Asci are hyphae
that contain eight
haploid ascospores.

13. Fruiting Body of Sordaria Fungi
In Sordaria fungi, the asci are embedded in other hyphae that form a fruiting body.

14. Results of Crosses in Sordaria
Below: Asci that resulted from a cross between one black and one tan strain. Note that each contains some tan and some black spores. 4:4 arrangement=no crossing over.
Asci that resulted from a cross between two black strains. All of the spores are black.
Asci that resulted from a cross between two tan strains. All of the spores are tan.

15. Crossover Lab Practice
ace/labbench/lab3/intro.html
To the right is an ascus
which has released its
ascospores. Starting at
12 o’clock, count the
number of ascospores
that are recombinants.

16. Pattern WITHOUT crossing over

17. Pattern with crossing over

18. http://histoiresnaturelles. net/vegetaux/microscopie/sordaria_bouquet

19. # of crossover asci TOTAL # of asci
= % Asci showing Crossover
÷ 2 = Gene to centromere distance (map units)

20. DRAW THE EVENTS LEADING UP TO A 2:4:2 ascospore pattern (HINT: Work backwards from the final spore pattern)
MITOSIS
MEIOSIS II
MEIOSIS I