vvvvSordaria Genetics Background
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Sexual reproduction provides a mechanism to produce genetic variation, as the genes of two different individuals are arranged in various ways. This requires a reduction in the chromosome number of the parent cell, normally diploid, to half that, or haploid, in somatic cells. The type of cell division resulting in half the chromosome number of the parent cell is called meiosis.
In meiosis, a germ cell divides into four haploid gametes. When two gametes, typically an egg and sperm for most animals, combine during fertilization to form a zygote, the diploid chromosome number is restored. Meiosis consists of one DNA replication and two nuclear divisions, meiosis I and II. This results in the formation of four daughter cells, each with only half the number of chromosomes of the
parent.
Genetic variability is further increased by a process called crossing over. In the early stages of meiosis, the homologous pairs of chromosomes move close together in such a way that all four chromatids are entwined, forming a tetrad. This process, known as synapsis, allows for the exchange of chromosome sections between the homologous pairs.
The example that will be used in the investigation is Sordaria fimicola. S. fimicola is an ascomycete fungus that is haploid for the bulk of its life cycle; the haploids comprise the individual fungal filaments, called hyphae, which normally exist in a mass called a mycelium representing the “body” of the fungus, and the ascospores, from which mycelia develop. The only diploid portion of the life cycle of S. fimicola occurs when the nuclei of specialized hyphae come together.
These hyphae, which belong to different strains of the species, fuse to form a zygote. This zygote then undergoes meiosis to produce the haploid ascospores, yielding four haploid nuclei contained in a sac called an ascus. After meiosis, the four nuclei undergo mitosis, resulting in an ascus...