how big is budding yeast cell
TRANSCRIPT
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How big is a budding yeast cell?
The budding yeast Saccharomyces cerevisiae has served as the model
eukaryote in much the same way that E. coli has served as the
representative prokaryote. Due to its importance in making beer and
baking bread (thus also called Brewers or Bakers yeast) this easily
cultured and accessible organism was also an early favorite of scientists
as interestingly recalled by James A. Barnett in a set of papers on the
beginnings of microbiology and biochemistry: the contribution of yeast
research. These cells are significantly larger than common bacteria and
as such, are a convenient single-celled organism to study under themicroscope. In large part due to the ease with which its genome can be
manipulated, yeast has remained at the forefront of biological research
and in 1996, was the first eukaryotic organism to have its genome
completely sequenced. One feature that makes yeast handy for
geneticists is their dual life style as either haploids, having one copy of
each gene, or diploids which harbor two copies of each gene, as humans
do. This comes about as a result of the fact that haploid cells have only
one copy of each chromosome just like a human female ovum. By way
of contrast, diploid cells have two copies of each chromosome,analogous to the status of the egg cell after fertilization of the ovum by a
sperm cell from a male. The haploid/diploid coexistence enables
changing genes, merging gene sets and studying effects on mutations
much more easily.
We note that a simple rule of thumb for the dimensions of yeast cells is
to think of them as spheres with a diameter of roughly 4 m for haploids
and roughly 6 m for diploids
as shown in Figure 1 (BNID
101796). To put the relativesizes of yeast and bacteria in
everyday terms, if we think of a
world in which E. coli is the
size of a human, then yeast is
about the size of an elephant.
Prominent components of the
cell volume include the nucleus
which takes up about 10% of
the total cell volume (100491,
103952), the cell wall, often
Figure 1: Electron micrograph of budding yeast cells
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ignored but making up 10-25% (104593, 104592) of the total dry mass
and the endoplasmic reticulum and vacuole, which serve as the largest
organelles.
One of the ideas that we will emphasize in a quantitative way repeatedly
is the idea of cell-to-cell variability and its role in establishing the
different behaviors of cells in response to different environmental cues.
As yeast replicate by budding off small daughter cells from a larger
mother, any population has a large range of cell sizes spread around the
median as shown in Figure 2. The haploid strain shown has a median cell
volume of 422 m3 (BNID 100450). Another common metric is the 25th-
75thpercentile range which here is 30-60 fL. The median cell size itself
is highly dependent on genetic and environmental factors. A diploid cell
is almost twice as big as its haploid progenitors at ~82 m3 (BNID100490). This reflects the more general observation from cell biology
that DNA content is directly correlated with median cell size. Yeasts
where ploidy can be manipulated to higher than two serve as useful test
cases for illuminating this question.
Beyond the bulk DNA content, the median cell size can differ by more
than 2-fold in different strains of S. cerevisiae, that evolved in different
parts of the world, or more recently in different industries utilizing them.
Finally, like E. coli, median cell size in yeast is correlated with growth
rate the better the environmental conditions and growth rate, the
larger the cells (Tyson, J. Bact 1979). An intriguing open question is what
is the evolutionary advantage of shifting cell size in response to
environmental conditions. Recent measurements (Jorgensen et al.,
Science 2002) have probed how sensitive yeast cell size is to single gene
deletions. In some of these deletion mutants, the median volume was
only 40% of the wild type size whereas in others it was larger by >70%
than wild type
revealing
strongcoupling
between size
regulation
and the
expression of
critical genes
and also the
tight-coupling
of wild type
Figure 2: Histogram of distribution of cell sizes for wild type budding yeast
cells (from Jorgensen et al. Science 2002).
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cell size to the cell cycle itself. It still remains largely unknown how
genetic and environmental changes shift the median cell size in yeast.