yeast cell (Pov-Ray model)
ascus containing four ascospores
yeast cell showing two bud scars
yeast cell budding
yeast cell showing three bud scars
yeast cell with bud detaching
schmoos meeting
schmoos touching
schmoos fused
fusion of nuclei in schmoos
Yeast
Above: a yeast cell (computer model) showing the large central vacuole, smaller nucleus and the
birth scar, where the cell budded off and detached from the mother cell. In turn, new cells will bud
from this cell too, completing the cycle. Yeast typically reproduce asexually by
budding, in which
a smaller daughter cell buds from a larger mother cell, enlarge and then produce their own buds.
Some yeast undergo the more conventional binary fission, in which a mother cell splits into two
equal-sized daughter cells.
Left: a computer model of a yeast cell bearing a birth
scar and two smaller
bud scars where new cells have
budded from it. The sequence shows a third bud being
produced, which detaches to leave a third bud scar. The
detached bud bears its own birth scar.

Click images to enlarge.
In addition to asexual budding (or sometimes asexual
binary fission) some yeast are known to reproduce
sexually. Those yeasts in which sexual reproduction is
unknown are grouped in the deuteromycetes - those fungi
which can not be classified as ascomycetes or
basidiomycetes as no sexual process is known. The mode
of production of sexual spores characterises fungi.

The sequence of images on the right illustrate sexual
reproduction in an
ascomycete yeast. Two compatible
haploid cells (hapolid: containing just one set of
chromosomes) develop elongations that grow towards
one-another. Such elongated cells are called
shmoos
(presumably after the cartoon character).

The processes meet and the intervening cross-walls break
down, as the protoplasts fuse and become continuous,
forming a protoplasmic bridge between the two partners.

The nuclei move toward each other, along the protoplasmic
bridge and fuse to form a zygote nucleus, which is
diploid
(having two sets of chromosomes) and sits in a spore cell
which grows (buds) on a short stalk from the bridge.

Finally, the diploid nucleus undergoes
meiosis. Meiosis is
a form of cell division characteristic of sexual reproduction
and which produces four daughter cells from one parent
cell. It is a
reduction division, meaning that the number
of chromosomes is halved. Since the diploid nucleus,
containing two sets of chromosomes, duplicates its DNA
prior to meiosis (resulting in four sets of chromosomes)
each of the four daughter cells is haploid (containing one
set of chromosomes). The four daughter cells develop
inside the wall of the parent cell, which becomes the
sporing body or
ascocarp. The daughter cells are thus
ascospores.

The purpose of sexual reproduction is to bring two different
sets of chromosomes together and to rearrange them, so
that the set received by each daughter cell is a new
combination of chromosomes, selected from the two
parental sets. E.g. chromosome 1 may come from the
left-hand shmoo, chromosome 3 from the right-hand
schmoo, etc. at random.
Basidiomycete yeasts. Some yeast are basidiomycetes and produce basidiospores. In one example,
which grows on the surface of leaves, certain cells develop a tiny stalk, which stands upright above
the surface, and on the tip of this stalk a single basidiospore is developed. This basidiospore is
forcefully discharged, as is typical of basidiospores, being fired into the air.
yeast life cycle
Above: the life-cycle of the ascomycete yeast, Saccharomyces cerevisiae. The cells alternate
between haploid and diploid stages, both of which can persist indefinitely and reproduce asexually by
budding. The patterns of budding differ. In the haploid cells each bud forms on the same pole of the
mother cell, with each bud appearing adjacent to a previous bud scar. This is called
axial budding.
In the diploid cells, the mother forms successive buds at either end (
bipolar budding), whilst the first
generation daughter buds at the pole opposite its birth scar.

The diploid cells are more resilient to harsh conditions and reproduce asexually by bipolar budding,
but when conditions become too unfavourable the diploid cell develops into an ascus. Meiosis occurs
within the ascus, producing four haploid ascospores. These ascopores germinate when conditions
become more favourable, producing haploid cells that continue to bud (by axial budding). When a
haploid cell encounters another haploid cell of compatible mating type, then instead of budding, the
cells grow protrusions towards one-another, becoming shmoos, mate and bud to produce a diploid
cell.

Yeasts undergo a typical eukaryotic cell cycle when budding, similar to that of animal cells, with G1, S,
M and G2 phases (see
cell cycle). Budding is a form of mitosis - a type of cell division in which a
eukaryotic cell duplicates or clones itself. However, unlike mitosis in animal cells, mitosis in yeast does
not involve dissolution of the nuclear envelope. As in other eukaryotes, the nuclear envelope of yeast
cells is a double membrane, each membrane consisting of a typical phospholipid bilayer
membrane.
Instead of a centrosome consisting of a pair of centrioles, yeast cells develop a pair of plaques
(
spindle plaques or spindle pole bodies), each about 150 nanometres in diameter and lying
between the two nuclear membranes, one plaque at each end/pole of the nucleus. The two plaques
are connected by a protein bridge that span the nucleus from one plaque to another. Like centrioles
these plaques are microtubule-organising centres and microtubules grow from them, some projecting
into the cytoplasm (where they determine the site of bud formation) and some projecting into the
nucleus as a
mitotic spindle (where they attach to and move the separating chromosome pairs to
their respective poles). The nucleus becomes hourglass-shaped and eventually separates in two
(nuclear binary fission) and subsequently the bud, with one daughter nucleus inside it, separates
from the mother cell. The
actin cytoskeleton is also involved in polarising the cells, that is in breaking
symmetry when a more-or-less spherical cell decides to bud from one pole, or develop into a shmoo.

Yeasts can respire aerobically, for example utilising glucose and respiring it with oxygen to produce
water and carbon dioxide, with the liberation of energy to power the yeast cell. In a closed vessel, the
oxygen soon runs out. When this happens growth of the yeast ceases, but it continues to respire by
anaerobic respiration (
fermentation) converting sugars to ethanol (ethyl alcohol) and carbon
dioxide, with the liberation of less energy than in aerobic respiration, but enough to maintain the yeast
cell and keep it alive. This is exploited in the
brewing of beer and wine (ethanol) and in baking
bread (which relies on the carbon dioxide gas produced to rise the bread). Beer was very important in
medieval and post-medieval Europe, since the alcohol and other chemicals in the beer (derived from
the hops) suppress bacterial growth and a weak form of beer was used in place of water for drinking,
a dilute form being also given to children. This was important as the water was often just too
contaminated for drinking, especially in urban areas!