AP Lab 3: Mitosis and Meiosis Crossover in Sordaria

Review of Meiosis I

Review of Meiosis II

Review of Crossing Over

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.

Growth of Fungi http://www.wisc- online.com/objects/ViewObject.aspx?ID=BIO 304

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.

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. 1. 2. 3. 4.

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.

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.

Life Cycle of Sordaria

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.

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

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.

Crossover Lab Practice http://www.phschool.com/science/biology_pl 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.

Pattern WITHOUT crossing over

Pattern with crossing over

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

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

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