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The ascomycete
yeast Saccharomyces
cerevisiae that is so beloved of bakers, biotechnologists
and brewers has proved to be a good model of how
eukaryotic cells behave.
This means that the principles underpinning the secret life of yeast
may well underpin the functioning of of other eukaryotic cells –
such as human cells. Much work in understanding how eukaryotic cells
regulate their division processes has been conducted using yeast
as a model. This work has greatly helped our understanding of how
rogue cell division occurs – cancer, for example.
Yeast cells growing in liquid culture
On a more fundamental level, Saccharomyces
has chromosomes much like ours, that is, the DNA is packaged tightly
with histone proteins.
The only difference is that there are only three yeast histone proteins,
compared with the four human histones. However, the mechanics of
cell division are similar, and the
structure of the cells is comparable to that of animal and plant
cells. Unlike bacteria (prokaryotes) which lack
true membrane-enclosed organelles, fungi have nuclei,
mitochondria, Golgi bodies, Endoplasmic
Reticuli, and other organelles. Unlike green plants, fungi
don’t photosynthesise, and so don’t have chloroplasts.
Saccharomyces, being eukaryotic, also transcribes
and translates its genes in the same way as plants and animals.
Protein synthesis is said to be “uncoupled”, with transcription
taking place in the nucleus and translation in the cytoplasm. The
signals used are the same too, and mature mRNAs are “capped
and tailed”, a process which does not occur in prokaryotes.
Yeast also have a chemically modified guanine at the point where
transcription starts.
One feature yeast cells have which is rare for
eukaryotes is a plasmid. This plasmid, called the
2 µm plasmid, works in much the same way
as bacterial plasmids (we came across these in the section on molecular
cloning). This oddity has made molecular cloning easy in yeast,
and we can produce human
proteins cheaply in large quantities.
Saccharomyces has also been used as a
tool in genomics. Using segments of DNA from the
2 µm plasmid as well as other genetic elements, molecular
biologists have created a Yeast Artificial Chromosome (YAC)
to allow the wholesale cloning of DNA. Bacterial plasmids are limited
in the quantity of DNA that they can carry, usually less than 5
thousand base pairs (kbp) of inserted DNA, which would make cloning
the entire human genome in a library extremely
tedious. YACs, however, can carry as much as 2 million
base pairs! It could be said that if plasmids are molecular “taxis”
then YACs would be the molecular jumbo-jets of the DNA cloning world!
However, Saccharomyces isn’t only
used as a tool in genomics – it has been sequenced too (see
http://www.yeastgenome.org/)
and it is thought that Saccharomyces cerevisiae lives happily
with about 6,300 genes. Molecular mycologists have painstakingly
“knocked out” each gene by modifying the sequence. This
is allowing researchers to infer the function of each gene (for
example, if yeast strain X has been KO’d in gene Y, and it
fails to divide in culture, then gene Y probably has something to
do with cell division.)
Considering that Buchner discovered enzymes
in 1896 by the process of yeast invertase on sugar, it is apt that
yeast metabolomics is being pieced together nicely,
and we can map the sum of yeast
metabolic pathways.
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