FUNGAL GROWTH 77
chitin, between the mother and daughter cells. This
plate of chitin is then overlaid by β-glucan and man-
nan (the major wall components) to form the secondary
septum, and the cells finally separate, leaving the
chitin plate on the mother cell.
The yeast cell cycle has been studied intensively as
a model of the regulation of cell growth and division.
Four stages in this cycle are recognized: G1(first gap),
S(DNA synthesis), G2(second gap), and M(mitosis).
In each turn of the cycle a bud emerges, grows to nearly
full size, receives one of the daughter nuclei from
nuclear division and then separates from the parent
cell. At its fastest the cycle takes about 1.5 hour in
S. cerevisiae, but the time can vary within wide limits,
depending on the availability of nutrients. Almost all
of this variation occurs in G1, because S, G2, and M
together occupy a more or less constant time.
The most important checkpoint in this cycle is
termed start. It occurs during G1 in S. cerevisiaeand it
is the stage where the cell integrates all the informa-
tion from intracellular and environmental signals to
determine whether the cell cycle will continue, or
enter stationary phase, or the cell will undergo sexual
reproduction (Chapter 5). Many cell division cycle(CDC) geneshave been identified in the budding
yeast S. cerevisiae, and homologous genes that regulate
development have also been found in the distantly
related fission yeast, Schizosaccharomyces pombe.
Analysis of the genes and gene products of these
two organisms has helped us to understand how
cells establish their polarity of growth (Fig. 4.12). In
budding yeast (sequence shown in Fig. 4.12a) the first
bud develops at one of the poles of the cell. When this
bud has developed and separated from the mother cell,
the next bud arises at a point adjacent to the bud scar.
It grows initially by polar growth (stage 4) but then
by wall growth over most of the bud surface (stage 5).
At the time of bud emergence, a “tag” or “landmark”
is laid down at the site where a new bud will form,
and a ring of septin proteins is deposited at this point.
At a later stage of development (stage 6) the cellular
machinery, including actin microfilaments, will direct
vesicles and wall precursors to this site, resulting in
the localized development of a septum to separate the
daughter cell. The fission yeast, Schizosaccharomyces
pombe(Fig. 4.12b), grows in a different way from
Saccharomyces, because it produces cylindrical cells
that extend at both ends, with tags at the poles of the(a)(b)(c)(d)Fig. 4.12Comparison of cell division in the budding yeast Saccharomyces cerevisiae(a,c) and the fission yeast
Schizosaccharomyces pombe(b,d), showing how an axis of polarity is established at different points during cell devel-
opment. (Reprinted from Mata, J. & Nurse, P. (1998) Discovering the poles in yeast. Trends in Cell Biology 8 , 163–167,
copyright 1998, with permission from Elsevier.)