Yeast: morphology and life cycle

Yeast: morphology and life cycle

  • 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