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Model Organisms
Model Organism
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Important features of all model organisms
Short lifespan
Small, easy and inexpensive to maintain
Produce large numbers of offspring
Development external as well as internal
Availability of mutants
History/previous experiments and discoveries
Genome is sequenced
Homologues for large % of human disease genes
Exhibit complex behaviors
Few ethical concerns
The choice of a model organism depends
on what question is being asked.
Specific species
 Uniform from research lab to research lab
 Ability to apply new knowledge to other
organisms
 Advance our understanding of
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Cellular function
Development
Disease
Model Organisms
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E. coli
Drosophila
Xenopus
Zebrafish
Mouse
C. elegans
Yeast
Arabidopsis
The Nematode Worm
Caenorhabditis elegans
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In 1965, Sydney Brenner settled on the small nematode worm
Caenorhabditis elegans to study the important questions of development
and the molecular basis of behavior, because of their suitable
characteristics.
Due to its simplicity and experimental accessibility, it is now one of the
most completely understood metazoans.
What is unique to this organism is that wild-type individuals contain a
constant 959 cells. The position of cells is constant as is the cell number.
If the 6th chromosome pair is XX, then C. elegans will be a
hermaphrodite. A XO combination in the 6th chromosome pair will
produce a male. Hermaphrodites can self-fertilize or mate with males but
cannot fertilize each other. In nature, hermaphrodites are the most
common sex.
C. elegans has a very rapid life cycle
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C.elegans is transparent. It is easy to track cells and follow
cell lineages.
The genome size of C. elegans is about a hundred million
base pairs. This is approximately 20X bigger than that of E.
coli and about 1/30 of that of human.
At 25℃, fertilized embryos of C. elegans complete
development in 12 hours and hatch into free-living animals
capable of complex behaviors.
The first stage juvenile(L1) passes through four juvenile
stages(L1-L4) over the course of 40 hours to become a
sexually mature adult.
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The life cycle of the worm, C.elegans
Under stressful conditions, the L1 stage
animal can enter an alternative
developmental stage in which it forms what
is called a dauer.
 Dauers are resistant to environmental
stresses and can live many months while
waiting for environmental conditions to
improve.
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C. elegans’s cell lineages
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C. elegans has a simple body plan. Its cell
lineages are relatively few and well studied.
Among C.elegans genes are components
of highly conserved receptor tyrosine
kinase signaling pathways that control cell
proliferation.
 Many of the mammalian homologs of
these genes are oncogenes and tumorsupressor genes that when altered can
lead to cancer.
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The cell death pathway was
discovered in C. elegans
The most notable achievement to date in
C. elegans research has been the
elucidation of the molecular pathway that
regulates apoptosis or cell death.
 Analysis of the ced mutants showed that,
in all but one case, developmentally
programmed cell death is cell autonomous,
that is, the cell commits suicide.
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Cell death is as important as cell
proliferation in development and disease
and is the focus of intense research to
develop therapeutics for the control of
cancer and neurodegenerative diseases.
RNAi was discovered in C. elegans
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In 1998 a remarkable discovery was
announced. The introduction of dsRNA into
C. elegans silenced the gene homologous to
the dsRNA. It is significant in two respects.
One is that RNAi appears to be universal
since introduction of dsRNA into nearly all
animal, fungal, or plant cells leads to
homology-directed mRNA degradation.
The second was the rapidity with which
experimental investigation of this
mysterious process revealed the
molecular mechanisms.
Bacteria
The attraction of bacteria such as E. coli or
B. subtilis as experimental systems is that
they are relatively simple cells and can be
grown and manipulated with comparative
ease.
 Molecular biology owes its origin to
experiments with bacterial model systems.
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Assays of bacterial growth
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Bacterial cells are
large enough, about
2µm, in length to
scatter light, allowing
the growth of a
bacterial culture to be
measured
conveniently in liquid
culture by the change
in optical density.
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The number of bacteria
can be determined by
diluting the culture and
plating the cells on solid
(agar) medium in a petri
dish. Knowing how many
colonies are on the plate
and how much the culture
was diluted makes it
possible to calculate the
concentration of cells in
the original culture.
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Bacterial often harbor
autonomously
replicating DNA
elements known as
plasmids.
Evolved as an
adaptation to protect
from bacteriophages
These circular DNA
elements can serve as
convenient vectors for
bacterial DNA as well
as foreign DNA.
BAKER’S YEAST
Saccharomyces cerevisiae
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Unicellular eukaryotes offer many
advantages as experimental model systems.
The best studied unicellular eukaryote is the
budding yeast S. cerevisiae.
These cell types can be manipulated to
perform a variety of genetic assays.
 The genetic analysis of S. cerevisiae is
further enhanced by the availability of
techniques used to precisely and rapidly
modify individual genes.
 Generating precise mutations in yeast is
easy
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recombinational transformation in yeast
The ability to make such precise changes in
the genome allows very detailed questions
concerning the function of particular genes
or their regulatory sequences to be pursued
with relative ease.
 Because of its rich history of genetic studies
and its relatively small genome, S.
cerevisiae was chosen as the first
eukaryotic organism to have its genome
entirely sequenced. This landmark was
accomplished in 1996.
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S. cerevisiae cells change shape as they grow
Simple microscopic observation of S.
cerevisiae cell shape can provide
information about the events occurring
inside the cell.
 A cell that lacks a bud has yet to start
replicating its genome. A cell with a very
large bud is almost always in the process
of executing chromosome segregation.
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The Fruit Fly
Drosophila melanogaster
Drosophila has a rapid life cycle
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The salient features of the Drosophila life
cycle are a very rapid period of
embryogenesis, followed by periods of larval
growth prior to metamorphosis.
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The Drosophila life cycle
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One of the key processes that occurs
during larval development is the growth of
the imaginal disks, which arise from
invaginations of the epidermis in mid-stage
embryos.
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Imaginal disks differentiate into their
appropriate adult structures during
metamorphosis (or putation).
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Figure 21-16 Imaginal disks in Drosophila
The first genome maps were
produced for Drosophila
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Morgan’s lab studies on Drosophila (1910)
led to two major discoveries:
genes are located on chromosomes, and each
gene is composed of two alleles that assort
independently during meiosis;
genes located on separate chromosomes
segregate independently, whereas those linked
on the same chromosome do not.
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Hermann J. Muller provided the first
evidence that environmental factors can
cause chromosome rearrangements and
genetic mutations.
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Bridges used the polytene chromosomes
to determine a physical map of the
Drosophila genome (the first produced for
any organism).
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Figure 21-17 Genetic maps, polytene chromosome, and
deficiency mapping
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A variety of additional genetic methods
were create to establish the fruit fly as the
premiere model organism for studies in
animal inheritance.
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For example, balancer chromosomes were
created that contain a series of inversions
relative to the organization of the native
chromosome.
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Figure 21-18 Balancer chromosome
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Embryos that contain two copies of the
balancer chromosome die because some
of the inversions produce recessive
disruptions in critical genes.
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In addition, embryos that contain two
copies of the normal chromosome die
because they are homozygous for the eve
null mutation.
Genetic mosaics permit the analysis
of lethal genes in adult flies
Mosaics are animals that contain small
patches of mutant tissue in a generally
“normal” genetic background.
 The analysis of genetic mosaics provided
the first evidence that Engrailed is required
for subdividing the appendages and
segments of flies into anterior and
posterior compartments.
 The most spectacular genetic mosaics
are gynandromorphs.
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Figure 21-19 Gyandromorphs
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These are flies that are literally half male
female. The X instability occurs only at the
first division.
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And the “line” separating the male and
female tissues is random. Its exact
position depends on the orientation of the
two daughter nuclei after the first cleavage.
The yeast FLP recombinase permits
the efficient production of genetic
mosaics
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Drosophila possesses several favorable
attributes for molecular studies and wholegenome analysis. Most notably, the
genome is relatively small.
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The frequency of mitotic recombination
was greatly enhanced by the use of the
FLP recombinase from yeast.
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Figure 21-20 FLP-FRT
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This method is quite efficient. In fact, short
pulse of heat shock are often sufficient to
produce enough FLP recombinase to
produce large patches of zˉ/zˉ tissue in
different regions of an adult fly.
It is easy to create transgenic fruit
flies that carry foreign DNA
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P-elements are transposable DNA
segments that are the causal agent of a
genetic phenomenon called hybrid
dysgenesis.
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Figure 21-21 Hybrid dysgenesis
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P-element excision and insertion is limited
to the pole cells, the progenitors of the
gametes (sperm in males and eggs in
females).
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P-elements are used as transformation
vectors to introduce recombinant DNAs
into otherwise normal strains of flies.
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Figure 21-22 P-element transformation
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This method of P-element transformation
is routinely uses to identify regulatory
sequences such as those governing eve
stripe 2 expression.
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In addition, this strategy is used to
examine protein coding genes in various
genetic backgrounds.
The House Mouse, Mus musculus
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The mouse enjoys a special status due to its
exalted position on the evolutionary tree: it is a
mammal and, therefore, related to humans.
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The mouse provides the link between the basic
principles, discovered in simpler creatures like
worms and flies, and human disease.
Mouse Embryonic Development
Depends on Stem Cells
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Their small size prohibits grafting experiments of
the sort done in zebrafish and frogs, but
microinjection methods have been developed for
introducing.
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Figure 21-23 shows an overview of mouse
embryogenesis.
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Figure 21-23 Overview of mouse embryogenesis
It Is Easy to Introduce Foreign DNA
into the Mouse Embryo
DNA is injected into the egg pronucleus, and
the embryos are places into the oviduct of a
female mouse and allowed to implant and
develop.
 The injected DNA integrates at random
positions in the genome
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Figure 21-24 Creation of transgenic mice by
microinjection of DNA into the egg pronucleus
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Figure 21-25 In situ expression patterns of embryos obtained from
transgenic mice
Homologous Recombination Permits
the Selective Ablation of Individual
 The single most Genes
powerful method of
mouse transgenesis is the ability to disrupt,
or “knock out，” single genetic loci. This
permits the creation of mouse models for
human disease.
 Gene disruption experiments are done
with embryonic stem (ES) cells
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Figure 21-26 Gene knockout via homologous
recombination
Mice Exhibit Epigenetic Inheritance
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Studies on manipulated mouse embryos led
to the discovery of a very peculiar
mechanism of non-Mendelian, or epigenetic,
inheritance.
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This phenomenon is known as parental
imprinting.
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Figure 21-27 Imprinting in the mouse
The basic idea is that only one of the two
alleles for certain genes is active.
 It has been suggested that imprinting has
evolved to protect the mother from her own
fetus.
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