How Big is Budding Yeast Cell

7/29/2019 How Big is Budding Yeast Cell

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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.

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How Big is Budding Yeast Cell