Model Organisms

Model Organism 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 Cellular function Development Disease

Model Organisms

E. coli Drosophila Xenopus Zebrafish Mouse C. elegans Yeast Arabidopsis In vitro Cell Culture

The Nematode Worm Caenorhabditis elegans

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

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 life cycle of the worm, C.elegans

C. elegans’s cell lineages

C. elegans has a simple body plan. Its cell lineages are relatively few and well studied.

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 tumor-supressor genes that when altered can lead to cancer.

What is an oncogene? Tumor supressor gene?

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.

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

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

E. coli (Example) relatively simple cells and can be grown and

manipulated with comparative ease Molecular biology owes its origin to experiments

with bacterial model systems

Assays of bacterial growth

Bacterial cells (2µm, in length) Scatter light, allowing

the growth of a bacterial culture to be measured conveniently in liquid culture by the change in optical density.

Also, can be cultured and plated on solid (agar) in a petri dish. Knowing how many colonies and how much the culture was diluted makes it possible to calculate the concentration of cells in the original culture.

Bacteria harbor self replicating DNA plasmids.

An adaptation to resist bacteriophages restriction enzymes

Circular DNA elements serve as vectors for bacterial DNA as well as foreign DNA.

Antimicrobial resistance

PLASMIDS

BAKER’S YEAST Saccharomyces cerevisiae

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.

Can precisely and rapidly modify individual genes.

Generating precise mutations in yeast is easy

Prion disorders

Very detailed questions concerning the function of particular genes or their regulatory sequences to be pursued with relative ease.

First eukaryotic organism to have its genome entirely sequenced. This landmark was accomplished in 1996.

S. cerevisiae cells change shape as they grow

Simple microscopic observation 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 chromosome segregation.

The Fruit Fly Drosophila melanogaster

Drosophila has a rapid life cycle

Very rapid period of embryogenesis Less than two weeks per generation

The Drosophila life cycle

One of the key processes that occurs during larval development is the growth of the imaginal disks

Imaginal disks differentiate into their appropriate adult structures during metamorphosis (or putation).

Figure 21-16 Imaginal disks in Drosophila

The first genome maps were produced for Drosophila

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.

Hermann J. Muller -first evidence that environmental factors can cause chromosome rearrangements and genetic mutations.

Bridges first gene map for any organism).

A variety of additional genetic mutants were created.

Figure 21-18 Balancer chromosome

The House Mouse, Mus musculus The mouse enjoys a special status due to its

exalted position on the evolutionary tree: it is a mammal and, therefore, most closely related to humans.

The mouse provides the link between the basic principles, discovered in simpler creatures like worms and flies, and human disease.

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

Figure 21-24 Creation of transgenic mice by microinjection of DNA into the egg pronucleus

Homologous Recombination Permits the Selective Ablation of Individual

Genes The single most 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

Mice Exhibit Epigenetic Inheritance

Studies on manipulated mouse embryos led to the discovery of a very peculiar mechanism of non-Mendelian, or epigenetic, inheritance.

What is epigenentics?