INS 2007-08 Pre-lab III Questions 1. The cell cycle in a certain cell type has the duration of 16 hours. The nuclei of 660 cells showed 13 cells in anaphase. What is the approximate duration of anaphase in these cells? 2. Look at the table below. How many of these asci contain a spore arrangement that resulted from crossing over? Number of Asci Counted Spore Arrangement 7 4 light/4 dark spores 8 4 dark/4 light spores 3 2 light/2 dark/2 light/2 dark spores 4 2 dark/2 light/2 dark/2 light spores 1 2 dark/4 light/2 dark spores 2 2 light/4 dark/2 light spores 3. A group of asci formed from crossing light-spored Sordaria with dark-spored produced the following results: Number of Asci Counted Spore Arrangement 7 4 light/4 dark spores 8 4 dark/4 light spores 3 2 light/2 dark/2 light/2 dark spores 4 2 dark/2 light/2 dark/2 light spores 1 2 dark/4 light/2 dark spores 2 2 light/4 dark/2 light spores From this small sample, calculate the map distance between the gene and centromere. 2. If the end result of meiosis from a single diploid cell is four haploid cells (gametes), why are there eight haploid ascospores in the ascus of Sordaria?2 INS 2007-08 Lab III – Mitosis and Meiosis This lab exercise courtesy of Mike Clayton, University of Wisconsin Adapted from a lab written by Jon Glase, Cornell University I. Following up on the first lab, today we will spend some time looking at cells in mitosis. II. Analysis of the Process of Meiosis Using Asci of Sordaria Meiosis is a process of two sequenced nuclear divisions neither of which is identical to mitosis. Meiosis always begins with a nucleus with two sets of chromosomes (a diploid nucleus) where each chromosome has two chromatids. Meiosis produces four nuclei each with one set of chromosomes (haploid nuclei) in which each chromosome has one chromatid. Meiosis together with syngamy (gametic fusion) constitutes sexuality in eukaryotes. Sexual reproduction generates individuals with new and unique genotypic combinations each generation. The consequences of independent assortment and crossing over are generally only apparent through analyzing patterns of inheritance over a number of generations. In the case of the fungus, Sordaria, however, the products of meiosis, the spores, are aligned in the narrow mother cell wall. Hence, for the trait “spore color”, the consequences of crossing over and of independent assortment are immediately apparent. The following activity was adapted from one written by Jon Glase at Cornell University. The life cycle of the fungus Sordaria fimicola (modified from Rushforth, 1976). ________________________________________________________________ Tetrad Analysis and Gene Mapping in the Fungus Sordaria fimicola Jon C. Glase, Cornell University, Ithaca, New York Introduction: The Genetics of Fungi: Because of the existence of dominance-recessive relationships among alleles, one of the problems inherent in genetic studies of diploid organisms, like Mendel’s pea plants, is the difficulty of inferring3 genotypes from phenotypes. For example, in peas, homozygous dominant individuals (TT) and heterozygous individuals (Tt) are phenotypically the same, both tall. In contrast, haploid organisms have only one chromosome for each of the chromosome types, so they have only one allele for each gene. This simplifies the interpretation of genetic crosses considerably because each allele is expressed in the phenotype. For this reason, and another discussed shortly, geneticists have long favored haploid organisms like the fungi as subjects for their studies. Early genetic studies with the fruit fly, Drosophila melanogaster, suggested that a mechanism must exist to allow exchange of genetic material between homologous chromosomes. Microscopic studies of meiosis show that this exchange, called crossing over, takes place during prophase I when homologous chromosomes are in synapsis. During crossing over, breakage-refusion points called chiasmata develop between synapsed chromosomes. These chiasmata result from pieces of the two homologues being switched in an equal and reciprocal fashion. Crossing over combines genetic material that had previously been on separate homologues and produces individuals with increased genetic variation. Geneticists also came to realize that crossing over could be used as an important tool for learning more about the location of genes on chromosomes. They reasoned that if chiasmata can form at any point between two homologous chromosomes, then the frequency of crossing over in the region between two different genes on a chromosome should vary directly with the physical distance between the genes. When this hypothesis was confirmed it was possible to begin mapping the positions of genes on chromosomes. In most genetic studies, a cross, or mating, is made between parents whose genotypes may be partially known. These parents contain gametes that have resulted from many meiotic divisions within their gamete-producing structures. Each meiotic division produces four haploid nuclei collectively called a tetrad. In most organisms the products of each meiotic division are not kept separate but become part of a “pool” of meiotic products (gametes). The mating activities of the parents combine these meiotic products in a random fashion to produce the next generation. Thus, in most organisms, it is impossible to examine the assortment of alleles in an individual meiotic division. However, such a genetic description of an individual meiotic division would be particularly advantageous in studying the occurrence and frequency of crossing over, and for demonstrating the random assortment of the chromosomes to the daughter nuclei during meiosis I. In certain fungi such as the pink bread mold, Neurospora crassa, and Sordaria fimicola (the organism you will study during this lab), meiosis occurs within a structure called an ascus, which isolates each tetrad. The four products of meiosis occur within the ascus in the order in which they arose during meiosis. With these organisms a special type of genetic analysis called tetrad analysis can be used. In tetrad analysis, the genetic make-up of each cell of a tetrad can be studied with respect to a particular trait, and this information can be related to the meiotic division that produced the tetrad. Tetrad analysis makes it possible to determine the alternate directions in which homologous chromosomes move during