Meiosis and crossing over in sordaria answers

Mitosis and Meiosis

Experiment 4: Crossing Over

In this lab, pictures of a prepared slide of the fungus Sordaria fimicola will be used to examine the ef- fects of crossing over. The life cycle of this fungus begins in the haploid state. However, after the com- bination of two different types of strains, they devel- op a diploid nucleus. As the life cycle continues, the diploid nucleus undergoes meiosis, along with mito- sis, and produces eight haploid ascospores which are stored in a sac called an ascus (Figure 6). Once maturation has been reached, the sac will burst, allowing the ascospores to be released. These spores are haploid and thus begin the life cycle again.

Sordaria fimicola is often used to observe crossing over because the wild type strain has black ascospores and the mutant type has tan ascospores. When the two strains go through meiosis, the location of the asco- spores will directly show if crossing over has occurred.

If crossover does not occur, the ascospores will be arranged like Figure 7.

If crossover does occur, the ascospores will be arranged like Figure 8.

The measurement to describe the difference between genes is referred to as map unit. As the distance in- creases between genes, the likelihood of crossing over becomes greater, therefore demonstrating that the

Figure 6

Figure 7 Figure 8

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Mitosis and Meiosis

proportion of crossovers corresponds with the distance between genes. As a rule, the percentage of recombi- nations is equal to the number of map units between two genes or a gene and the centromere.

Procedure

1. Examine the three fields of view of the Sordaria fimicola.

2. There are at least 10 different hybrids (crossovers) in the following images. Count the total number of clearly visible crossovers, as well as the number of clearly visible non-crossovers. Record your data for each image in Table 4.

Materials

Sordaria fimicola Digital Slide Images

Sordaria fimicola 400X (Image 1)

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Mitosis and Meiosis

Sordaria fimicola 400X (Image 2)

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Mitosis and Meiosis

Sordaria fimicola 400X (Image 3)