What is Yeast?
Yeast (Saccharomyces cerevisiae) called as baker’s yeast is a fungus used by humans for thousands of years in for baking and brewing.
Yeast is a model organism
Yeast is a unicellular Eukaryote belons to the
subdivision Ascomycotina. The study of yeast laboratory strains has enabled
outstanding scientific discoveries including a comprehensive understanding of
the life cycles of eukaryotic cells, cell structure, DNA replication and cell
function.
We are still missing some of the basics in
terms of our understanding of their life cycles, especially in the case of wild yeast strains.
Let’s divide the life cycle into simple steps
for better understanding.
You can watch the video here
Steps in Life cycle of Yeast
The two mating types in yeast are denoted as a & α
Step 1:
Mating of somatic
cells (1n) of opposite strains ‘a’ and ‘alpha’ under the influence of pheromones
or chemical attractants.
Step
2: The diploid cell formed undergoes budding
forming more and more diploid (2n) cells. Thus, live as diploid yeast
Step
3: Starvation causes diploid cells to function as ascus.
Step 4:
The diploid nucleus
undergoes meiosis and form 4 ascospores within ascus.; two ascospores of ‘a’ strain
and 2 of ‘α’ strain.
Step 5:
Ascospores are
released from ascus forming haploid cells; a and α that
multiply by budding.
Thus, live as haploid yeast.
Step
6: At a later stage, these haploid cells ‘a’
strain and ‘alpha’ strain secretes pheromones; come together and fuse to form
diploid cell.
Why
Yeast life cycle is a haplodiplobiontic life cycle?
In this life cycle, haploid phase is
represented by haploid yeast cell and the diploid phase by diploid yeast cell. Both
phases are equally dominant. That’s why called as a haplodiplobiontic life cycle.