by light microscopy. Most yeasts divide by budding from
one or more locations on the cell surface. During this
process a small outgrowth appears at the bud site,
then the bud progressively elongates before expanding
into a rounded form, by synthesis of new wall com-
ponents over the whole of the cell surface. When the
bud has nearly reached its final size, the nucleus of the
mother cell migrates towards the bud site and divides,
so that one nucleus remains in the mother cell and a
second nucleus enters the daughter cell.
The final separation of the two cells is achieved by
the development of a septum. In Saccharomycesthis
occurs when a ring of chitin is produced at the “neck”
site, and this ring of chitin expands inwards until it
becomes a complete chitin plate between the mother
and daughter cell. Then other wall materials are
deposited on each side of this chitin plate, and the cells
separate by enzymic cleavage of the wall between the
chitin plate and the daughter cell. This process leaves
a bud scaron the mother cell and a birth scaron the
daughter cell. These scars are inconspicuous by normal
light microscopy, but the chitin plate on the mother
cell can be seen clearly if yeasts are treated with a fluores-
cent brightener such as Cellufluor (calcofluor), which
binds to chitin and fluoresces blue when the cell is
observed under a fluorescence microscope.
By using fluorescent dyes that bind to chitin, we
can count the number of bud scars on the cell surface.
This reveals that Saccharomyces cerevisiaeis a multipolar
buddingyeast – it always buds from a different point
on the cell surface, never from a previous bud site,
whereas some other yeasts exhibit bipolar budding–
the buds always arise at the same positions, often at
the poles of the cell. In theory, the cells of bipolar
species are immortal, with no limit to the number of
times they can bud, whereas the cells of multipolar
species would eventually run out of potential bud sites- calculated to be up to 100, although only about
40 bud scars have been seen on a single cell. However,
in practise this distinction is unimportant because in
54 CHAPTER 3
Fig. 3.7(a) The common budding yeast, Saccharomyces cerevisiae, viewed by phase-contrast microscopy. (b) Crypto-
coccus albidus, a yeast that produces a rigid polysaccharide capsule over the cell surface, seen by mounting the cells in
a suspension of India ink particles. Different stages in the budding process are labeled from 1 (nonbudding cell) to 5
(cell separation). Note the presence of bright lipid bodies in the plane of focus of some cells (e.g. 5) and the presence
of a large central vacuole in the plane of focus of some other cells (e.g. 2, 3, 4).
(a) (b)Fig. 3.8Diagrammatic representation of
a budding yeast, Saccharomyces cere-
visiae, about 5μm diameter. BS =bud
scar; ER =endoplasmic reticulum; G =
Golgi; L =lipid body; M =mitochon-
drion; N =nucleus; SPB =spindle-pole
body; V =vesicle; Vac =large central
vacuole; W =wall.