WORLD OF MICROBIOLOGY AND IMMUNOLOGY Yeast609•
remarked that the “tragedy” of winning the Nobel Prize “is
that Dr. Berson did not live to share it.” Earlier Yalow had paid
tribute to her collaborator by asking the VA to name the labo-
ratory, in which the two had worked, the Solomon A. Berson
Research Laboratory. She made the request, as quoted in Les
Prix Nobel 1977,“so that his name will continue to be on my
papers as long as I publish and so that his contributions to our
Service will be memorialized.”
Yalow has received many other awards, honorary
degrees, and lectureships, including the Georg Charles de
Henesy Nuclear Medicine Pioneer Award in 1986 and the
Scientific Achievement Award of the American Medical
Society. In 1978, she hosted a five-part dramatic series on the
life of French physical chemist Marie Curie, aired by the
Public Broadcasting Service (PBS). In 1980, she became a
distinguished professor at the Albert Einstein College of
Medicine at Yeshiva University, leaving to become the
Solomon A. Berson Distinguished Professor at Large at Mt.
Sinai in 1986. She also chaired the Department of Clinical
Science at Montefiore Hospital and Medical Center in the
early- to mid-1980s.
The fact that Yalow was a trailblazer for women scien-
tists was not lost on her. At a lecture before the Association of
American Medical Colleges, as quoted in Lady Laureates,
Yalow opined: “We cannot expect that in the foreseeable
future women will achieve status in academic medicine in pro-
portion to their numbers. But if we are to start working
towards that goal we must believe in ourselves or no one else
will believe in us; we must match our aspirations with the guts
and determination to succeed; and for those of us who have
had the good fortune to move upward, we must feel a personal
responsibility to serve as role models and advisors to ease the
path for those who come afterwards.”See alsoLaboratory techniques in immunology; Radioisotopes
and their uses in microbiology and immunologyYYeastEAST
Yeasts are single-celled fungi. Yeast species inhabit diverse
habitats, including skin, marine water, leaves, and flowers.
Some yeast are beneficial, being used to produce bread
or allow the fermentationof sugars to ethanol that occurs dur-
ing beer and wine production (e.g., Saccharomyces cere-
visiae). Other species of yeasts are detrimental to human
health. An example is Candida albicans, the cause of vaginal
infections, diaper rash in infants, and thrushin the mouth and
throat. The latter infection is fairly common in those whose
immune systemis compromised by another infection such as
acquired immunodeficiencysyndrome.
The economic benefits of yeast have been known for
centuries. Saccharomyces carlsbergensis, the yeast used in the
production of various types of beer that result from “bottom
fermentation,” was isolated in 1888 by Dr. Christian Hansen at
the Carlsberg Brewery in Copenhagen. During fermentation,
some species of yeast are active at the top of the brew while
others sink to the bottom. In contrast to Saccharomyces carls-bergensis, Saccharomyces cerevisiaeproduces ales by “top
fermentation.” In many cases, the genetic manipulation of
yeast has eliminated the need for the different yeast strains to
produce beer or ale. In baking, the fermentation of sugars by
the bread yeast Ascomycetesproduces bubbles in the dough
that makes the bread dough rise.
Yeasts are a source of B vitamins. This can be advanta-
geous in diets that are low in meat. In the era of molecular biol-
ogy, yeasts have proved to be extremely useful research tools.
In particular, Saccharomyces cerevisiaehas been a model sys-
tem for studies of genetic regulation of cell division, metabo-
lism, and the incorporation of genetic material between
organisms. This is because the underlying molecular mecha-
nisms are preserved in more complicated eukaryotes, includ-
ing humans, and because the yeast cells are so easy to grow
and manipulate. As well, Ascomycetesare popular for genetics
research because the genetic information contained in the
spores they produce result from meiosis. Thus, the four spores
that are produced can contain different combinations of
genetic material. This makes the study of genetic inheritance
easy to do.
Another feature of yeast that makes them attractive as
models of study is the ease by which their genetic state can be
manipulated. At different times in the cell cycleyeast cells
will contain one copy of the genetic material, while at other
times two copies will be present. Conditions can be selected
that maintain either the single or double-copy state.
Furthermore, a myriad of yeast mutantshave been isolated or
created that are defective in various aspects of the cell divi-
sion cycle. These mutants have allowed the division cycle to
be deduced in great detail.
The division process in yeast occurs in several different
ways, depending upon the species. Some yeast cells multiply
by the formation of a small bud that grows to be the size of the
parent cell. This process is referred to as budding.
Saccharomycesreproduces by budding. The budding process
is a sexual process, meaning that the genetic material of two
yeast cells is combined in the offspring. The division process
involves the formation of spores.
Other yeasts divide by duplicating all the cellular com-
ponents and then splitting into two new daughter cells. This
process, called binary fission, is akin to the division process in
bacteria. The yeast genus Schizosaccharomycesreplicates in
this manner. This strain of yeast is used as a teaching tool
because the division process is so easy to observe using an
inexpensive light microscope.
The growth behavior of yeast is also similar to bacteria.
Yeast cells display a lag phase prior to an explosive period of
division. As some nutrient becomes depleted, the increase in
cell number slows and then stops. If refrigerated in this sta-
tionary phase, cells can remain alive for months. Also like bac-
teria, yeast are capable of growth in the presence and the
absence of oxygen.
The life cycle of yeast includes a step called meiosis. In
meiosis pairs of chromosomesseparate and the new combina-
tions that form can give rise to new genetic traits in the daugh-
ter yeast cells. Meiosis is also a sexual feature of genetic
replication that is common to all higher eukaryotes as well.womi_Y 5/7/03 9:15 AM Page 609