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Mutation

Mutation

FIGURE 19.18 Variation of VSG by Gene Conversion

(A) Early in infection, the entire VSG gene undergoes gene conversion. The gene at the expressed site (dark orange) is entirely replaced by a non-expressed copy of a VSG gene (light orange). The two VSG genes encode the same protein, but the shape of the variable domain is altered slightly between the two. (B) Later in infection, the expressed gene undergoes mutations and partial gene conversion events. These events amplify the amount of genetic diversity available for this one protein.

FIGURE 19.19 Alternating Haploid and Diploid Phases of Yeast

Haploid cells come in two mating types, a and a (top of figure). When an a haploid cell and an a yeast come in contact, they fuse to form a diploid yeast cell. During specific conditions, the diploid yeast cell sporulates and the nucleus undergoes meiosis, forming four haploid cells surrounded by an ascus. The ascus ruptures to release four haploid cells, starting the cycle over again.

similar and grow by budding. In yeast and some other primitive eukaryotes, the cells of the haploid phase may continue to divide or may act as gametes, depending on the circumstances. Although haploid yeast cells are all structurally identical they belong to one of two mating types, known as a or a. During mating, two haploid cells of different mating types fuse to give a diploid cell (Fig. 19.19). Such diploid cells grow and divide until poor environmental conditions trigger spore formation. Then the diploid cell is converted into an ascus, inside which meosis occurs. Four haploid spores (ascospores) are produced inside each ascus. After release from the ascus, the ascospores germinate so re-establishing the haploid phase of the life cycle. Although diploid cells alternate with haploid cells, in practice most natural cultures of yeast are diploid. This is because the haploid cells released by meiosis are normally in close proximity and soon mate again. In the laboratory, asci with the spores still inside can be isolated from yeast cultures and the spores may be examined individually. This allows genetic analysis of the inheritance patterns of specific genes.

Mating type is controlled by the MAT locus, which may exist in two alternative states, MAT a or MATa. Both states contain two genes that are transcribed divergently (either MATal MATa2 or MATal MATa2). These gene products (designated MATa1p, MATa2p, MATa1p and MATa2p) activate synthesis of specific peptide ascospore Type of spore made inside an ascus by fungi of the ascomycete group, including yeasts and molds ascus Specialized spore forming structure of ascomycete fungus mating types Equivalent of different sexes found in lower eukaryotes. They are structurally identical but biochemically distinct MAT locus Chromosomal locus in yeast that controls the mating type and exists as two alternative forms, MATa or MATa

©DIVISION DIVISION

Mating

Cell division Sporulation

Mating

FIGURE 19.19 Alternating Haploid and Diploid Phases of Yeast

Haploid cells come in two mating types, a and a (top of figure). When an a haploid cell and an a yeast come in contact, they fuse to form a diploid yeast cell. During specific conditions, the diploid yeast cell sporulates and the nucleus undergoes meiosis, forming four haploid cells surrounded by an ascus. The ascus ruptures to release four haploid cells, starting the cycle over again.

Ascus rELEASE OF

ascospores a a

Haploid cells

Ascus rELEASE OF

ascospores a a

Haploid cells

Yeasts alternate between haploid and diploid cells. Formation of diploids involves fusion between haploid cells of different mating types.

Yeast cells of different mating types signal each other by releasing distinct pheromones from the cell.

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