life cycle of yeast????
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Answer:
This is the sexual form of the fungus. Under optimal conditions, yeast cells can double their population every 100 minutes. However, growth rates vary enormously both between strains and between environments. Mean replicative lifespan is about 26 cell divisions.
Eukaryotic • unicellular (some may seem multicellular due to formation of psuedohyphae) • 3-4 µm in diameter, 5-30 m in length • Chemoorganotrophs, aerobic • Grow mainly in neutral and slightly acidic conditions. • oval or round shape. • Yeast grow typically in moist environments where there is plentiful supply of simple , soluble nutrients such as sugar and amino acids. • commonly found on leaf and fruit surfaces , on roots and in various types of foods.
Reproduction Asexual Budding Fission Endospore Formation Sexual Conjugation
Budding *Most common method *Under favourable condition the yeast cell gives rise to a tiny outgrowth that gradually increase in size. The nucleus divides amitotically, one of which migrates into the newly formed outgrowth and forms a bud. The bud seperates from parent and becomes independent.
*Sometimes due to repeated budding yeast cell appears in one or more chains and called pseudomycelium.
Fission *Occurs only in some yeast cells *Yeast cell elongates and its nucleus divides into two. Two nucleus move apart and are seperated by a transverse wall. Cell wall is formed in the middle and the yeast cell divides into two cells each having a nucleus.
Endospore formation *Under unfavourable condition *Thick walled, can withstand adverse conditions. *The protoplast divides into four parts, each becomes surrounded by a thick wall.
Sexual Reproduction *By conjugation *Occurs at the end of growing season under unfavourable condition *Two haploid somatic cells(ascospores) fuse together to form a diploid zygote nucleus *During sexual union two cells fuse (plasmogamy) and this is followed by the fusion of the two nuclei (karyogamy).
On basis of sexual reproduction: 1. Haplobiontic life cycle 2. Diplobiontic life cycle 3. Haplo-diplobiontic life cycle
Haplobiontic Life Cycle *Dominant haploid phase *Diploid phage short and represented by zygote only. *Here each somatic cell acts as a potential gametangium. *The fusion or zygote nucleus divides thrice of which the first division is meiotic one. Now the zygote cell becomes ascus containing eight ascospores. After their liberation from the ascus the ascospores behave as vegetative cells.
Diplobiontic Life Cycle *Dominant diploid phase represented by somatic cell *Short haploid phase represented by ascospores. *The ascospores are not liberated from the ascus but they copulate within the ascus. The zygotes found within the ascus bud out diplobiontic vegetative cells.
Haplo-diplobiontic Life Cycle *Both haploid and diploid phase are equally represented. *Yeasts the vegetative cells consist of both haploid and diploid phases. The haploid cells copulate and form a diploid cell (zygote). The zygote nucleus divides meiotically producing 4 nuclei which metamorphose into 4 ascospores. Only the diploid cells produce the ascospores which give rise to haploid vegetative cells by budding.
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