• They are single celled fungi
• Size: generally larger than most bacteria; (1-5) um wide and (5-30)um length
• Shape: cell is egg shaped, some are elongated or spherical
• Size and shape varies among species
• Yeast cell lacks flagella and other organ of locomotion.

Morphology of yeast cell

• Cell wall: composed of thin chitinous cell wall
• The protoplasm is surrounded by cell membrane which contains all the usual cell organelles like ribosomes, mitochondria, ER, nucleus and other granules
• Vacuole is single, large and centrally located.

Reproduction in yeast cell

• Yeasts generally reproduce by Asexual method such as Budding or fission,
• Yeasts lacks sex organs ( anthridium and oogonium)
• Sexual reproduction in yeast is highly variable

1. Budding

• It Occurs during abundant supply of nutrition
• Parent nucleus divides and moves toward daughter cell
• Enzymatic activities increases,
• Increased turger pressure acts on weaker part of cell wall and bud erupts
• Septum formation and bud separates into individual cell

2. Sexual reproduction

• Sexual reproduction is highly variable in yeasts
• Three different pattern of life cycle found in different genus

i. Haplodiplobiontic life cycle

ii. Haplobiontic life cycle

iii. Diplobiontic life cycle

life cycle of Saccharomyces cerevisiae: Haplodiplobiontic life cycle

• This cycle occurs in Saccharomyces cerevisiae
• In this life cycle somatic cell of yeast exists in two form ( Haploid dwarf cell and Diploid large cell)
• Haploid cell contains two mating types “a” and “α”
• During favorable condition each of the haploid cell multiply by budding only
• If these mating types comes in contact with each other, they form gametangia and starts sexual reproduction
• Fusion of these two haploid cell form large fusion cell called Plasmogamy
• Nucleus also fused by karyogamy to form zygote
• Zygote multiply by budding and forms several diploid cells
• these diploid cells are larger than haploid cells
• Like haploid cell, these large diploid cells also live independent life and reproduce by budding
• Under unfavourable condition, diploid large cell become spherical and directly behaves as ascus mother cell
• Nucleus of ascus mother cell divides by meiosis to form 4- haploid nuclei
• Out of 4-nuclei, 2 belongs to mating type “a” and t2 belongs to “α”
• Each nuclei gather some cytoplasm and becomes ascospore
• Each ascospore is globular and thick walled structure and relased by rapture of ascus wall.
• Ascospore germinates to give haploid dwarf cell.

life cycle of Schizosaccharomyces: Haplobiontic life cycle

• This life cycle occur in Schizosaccharomyces.
• In haplobiontic cycle, somatic haploid cell multiply by fission producing number of haploid cell.
• During sexual reproduction, two sister haploid cell behaves as gamatangia and produce beak like structure
• The beaks of two gamatangia fused and wall at the point of contac dissolve to form canal called conjugation tube.
• Nucleus of two cell moves and fused to form diploid zygote
• The zygote directly function as ascus mother cell
• Nucleus of ascus mother cell divides by meiosis forming 4-haploid nuclei which themselves divides by mitosis to form 8- haploid cells
• Each 8- nuclei gather some cytoplasm and become ascospore
• Ascospore released by rapture of ascus
• Ascospore germinates to give haploid cell and continue life cycle

Life cycle of Saccharomyces ludwigii: Diplobiontic life cycle

• This life cycle is found in Saccharomyces ludwigii
• In diplobiontic life cyle, diploid somatic cell multiply by budding and fission
• Under certain condition, these somatic cell directly functions as ascus mother cell and nucleus of ascus mother cell divides by meiosis to form 4-haploid nuclei
• These haploid nulceii gather cytoplasm and transform into ascospore4 ascospore,
• Out of 4-ascospore, 2 are mating type “A1” and 2 mating type “A2”
• These two mating ascospore fused within ascus to form diploid zygote.
• The diploid zygote gerinates within ascus producing a germ tube
• The germ tube breaks the ascus wall and function as diploid sprout mycelium.
• The sprout mycelium on budding gives sprout diploid cell which give diploid somatic cells.

Yeast: morphology and life cycle

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