Introduction to Genetic Analysis 102

Introduction to Genetic Analysis 102 - Neurospora is an...

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101 3.4 Chromosome behavior and inheritance patterns in eukaryotes 2 n meiocyte First meiotic division Second meiotic division Postmeiotic mitotic division Development of sexual spores (ascospores) around nuclei Octad of four spore pairs Four meiotic product nuclei (tetrad) 1 2 3 4 1 2 3 4 (a) Meiocyte after chromatid formation First meiotic division (b) Second meiotic division Mitosis Tetrad Octad A a Sexual spores grow to adults Asci Meiosis Synchronous division and fusion to form diploid meiocytes Cross- fertilization Maternal nucleus Mating type Maternal nucleus Mating type Figure 3-38 The life cycle of Neurospora crassa, the orange bread mold. Self-fertilization is not possible in this species: there are two mating types, determined by the alleles A and a of one gene. A cross will succeed only if it is A 3 a . An asexual spore from the opposite mating type fuses with a receptive hair, and a nucleus travels down the hair to pair with a nucleus in the knot of cells. The A and a pair then undergo synchronous mitoses, Fnally fusing to form diploid meiocytes. Figure 3-37
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Unformatted text preview: Neurospora is an ideal model system for studying allelic segregation at meiosis. (a) The four products of meiosis (tetrad) undergo mitosis to produce an octad. The products are contained within an ascus. (b) An A / a meiocyte undergoes meiosis, then mitosis, resulting in equal numbers of A and a products, demonstrating the principle of equal segregation. al 1 represents the wild-type allele coding for pigment production, and al the albino-causing allele: Pink wild type albino al 1 3 al In the haploid sexual cycle, we have seen that haploid parental cells of each type fuse to form transient diploid meiocytes, each one of which produces an eight-spore ascus. Every ascus from the above cross will have four pink ( al 1 ) and four white ( al ) ascospores, directly demonstrating the Mendelian law of equal segregation at the level of a single meiocyte (Figure 3-37b). 44200_03_p73-114 3/3/04 1:48 PM Page 101...
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.

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