Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 28

Most yeasts undergo vegetative reproduction by a process called budding. Some yeasts, such as species belonging to the genus Schizosaccharomyces, reproduce by binary fission.

Figure 1.11 (Tuite and Oliver (1991)) represents the life cycle of S. cerevisiae divided into four phases: M, G1, S, and G2. M corresponds to mitosis; G1 is the period preceding S, which is the synthesis of DNA; and G2 is the period before mitosis. As soon as the bud emerges, at the beginning of S, the splitting of the SPB can be observed in the nuclear membrane by electron microscopy, while the cytoplasmic microtubules orient themselves toward the emerging bud. These microtubules seem to guide numerous vesicles that appear in the budding zone and are involved in the reshaping of the cell wall. As the bud grows larger, discontinuous nuclear microtubules begin to appear. The longest microtubules form the mitotic spindle between the two SPBs. At the end of G2, the nucleus begins to push and pull apart in order to penetrate the bud. Some of the mitochondria also pass with some small vacuoles into the bud, whereas a large vacuole is formed at the other pole of the cell. The expansion of the latter seems to push the nucleus into the bud. During mitosis itself, the nucleus stretches to its maximum, and the mother cell separates from the daughter cell. This separation takes place only after the construction of the separation cell wall and the depositing of a ring of chitin on the bud scar of the mother cell. The movement of chromosomes during mitosis is difficult to observe in yeasts, but a microtubule–centromere link almost certainly guides the chromosomes. In grape must, the duration of budding is approximately one to two hours. As a result, the cell population doubles in two hours during the active yeast growth phase of fermentation.

FIGURE 1.11 Saccharomyces cerevisiae cell cycle (vegetative reproduction) (Tuite and Oliver, 1991). M, mitosis; G1, period preceding DNA synthesis; S, DNA synthesis; G2, period preceding mitosis.