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Unformatted text preview: BIOL 2040 Fungal Lab 14 WET LAB FUNGAL GENETICS Expectations Upon completion of this lab, you should be able to: Explain the features of the Neurospora life cycle that are useful to genetic mapping studies. Differentiate between first- and second-division segregation patterns. Be able to map the genetic distance between a gene and its centromere using tetrad identification. Reading Hartwell et al., (2008) p142-151 (Tetrad Analysis in Fungi), p193-194 (Extreme Magnificationthis is on gene conversion please see Fig. 6.21; you will need this to answer one of the questions.) Due Hand-in assignment due at end of lab. Please bring paper to the lab. INTRODUCTION The bread mold (it is a fungus), Neurospora crassa is used extensively in genetic research studies for its ease of manipulation and life cycle properties. Neurospora generally exists in haploid form. It multiplies by forming a network of haploid strands or hyphae (through mitosis) (see Fig. 6.1). The haploid hyphae produce haploid spores (conidia) by budding. These spores are asexual, and have a strong protective coating allowing them to travel long distances and remain dormant for long periods of time, without dessication. Alternative mating types of Neurospora have a special filament, called the trichogyne, that extends from the fruiting body (protoperithecia). The haploid conidia are capable of fertilizing the trichogyne. Cells from different mating strains ( A or a ) of Neurospora fuse and form a diploid (2n) zygote. This means that either an A conidia will fure with an a trichogyne, or the spore will be A and the trichogyne will be a (Figure 6.1). The zygote will soon afterward undergo meiosis. (The diploid phase in Neurospora is incredibly short). Because the products of meiosis in yeast and fungi result in spore, (rather than gamete) production, this process is referred to as sporulation for these groups; these spores are are referred to as ascospores, and are contained within a sac-like structure called the ascus. These haploid ascospores are capable of germinating into new organisms that produce colonies by mitosis. One of the most interesting features of Neurospora s meiotic process is that the four (4) original products of meiosis (the diploid nuclei that are the precursors to ascospores) must separate in a linear fashion (see Fig. 6.1). This is due to space constraints within the ascus. The linear separation can be taken advantage of for genetic analysis. The genetic significance of this system is that, because the products of meiosis separate linearly, the order of the ascopores directly reflects the order of chromatid segregation during meiosis . After meiosis is complete each of the four nuclei divides mitotically, creating two identical nuclei (twin nuclei) for each nucleus. In total, there will be four pairs of spores; if there is no mutation or gene conversion, the pairs are always identical. Because the two genetically identical cells resulting from each mitosis will lie adjacent conversion, the pairs are always identical....
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