Class notes
LT20 Model Organisms
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Lecture covering each model organism and the techniques used to study them - pros and cons to each organism
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Model OrganismsNeeded because:
Unethical to experiment on humans
System/organism of interest might be
too complicated/expensive to rear in
lab/ unavailable
Needs to be:
Relevant to study and also well studied
Cheap, plentiful, short generation time, simple, easy to manipulate
1. Escherichia coli
- Enteric bacterium – concentrated in mammalian large intestine (10 11-1013 excreted
daily by humans
- Some strains are opportunistic pathogens
- Size: 0.5 x 5 microns, Doubling time: ~40 minutes
- Genome size: ~5 Mb (5416 genes)
Used to study:
- Mainly molecular biology (rather than cell biology)
- Transcription machinery
- DNA replication
- Recombinant protein production (via plasmids – particularly useful)
- Environmental sensing (engineer strains to sense toxins etc.)
- Enteropathogenic E.coli (EPEC) hijack host cell cytoskeleton to move around cell
(Intracellular “surfing” pathogens – paradigm or actin-based motility) CHECK
NOTES and source!
2. Yeast
Budding yeast (Saccharomyces cerevisiae)
- Size: 2 microns, cell cycle: 90 minutes
- Diet: fermentable carbon source and nitrogen source
- Reproduction: sexual (schmoos) and asexual (budding)
- Genome size: 12Mb (~6000 genes, 16 chromosomes)
Fission yeast (Schizosaccharomyces pombe)
,- Properties similar to S. cerevisiae, only that they divide by fission, not budding
(textbook)
- Genome size: 13.8 Mb (4900 genes on 3 chromosomes)
- Relatively simpler
Used to study:
- Cell cycle (Paul Nurse, Leland Hartwell, Tim Hunt prize on discovery of cdc
proteins)
- Two-hybrid protein-protein interaction system (Y2H system – S. cerevisiae only)
(search)
- Gene regulation and chromosome structure
- Cytoskeleton
3. Dictyostelium discoideum (slime mould) (Microbiology notes)
- Genome size: 34 Mb (12,000 genes, 6 chromosomes), due to complex life cycle
Used to study
- Chemotaxis
- Life cycle
- Cell-cell interaction
- Cell differentiation
4. Caenorhabiditis elegans (Nematode)
- Size: 1mm
- Diet: bacteria
- Lifespan: 2-3 weeks, Reproduction: male and self-fertilising hermaphrodite
- Development: 3 days from egg to maturity
- Genome size: 97 Mb (~20000 genes, 5 autosomes + X)
Used to study:
- Development and body plan (Adult hermaphrodite makes 1090 cells, of which 121
(mainly neurons) are lost by apoptosis, leaving 959 cells at the end)
- Cell lineage and aging
- Apoptosis
- Behaviour, nervous system
- RNAi (search) (Fire and Mello, 1998) – can downregulate genes, don’t need
transfection
5. Fruit fly (Drosophila melanogaster)
- Size: adults 3mm; eggs 0.5mm
- Diet: rotten fruit
,- Life cycle: 2 weeks, Reproduction: females can lay up to 100 eggs in 1 day
- Development: 9 days from fertilisation to adult
- Genome size: 180 Mb (15000 genes, 3 autosomes +X/Y)
Used to study:
- Genetics of early embryonic development (Hox genes, spatial distribution of gene
expression, imaginal discs etc.)
- Apoptosis
- Genetic control of behaviour
- Circadian rhythms
- Drosophila: salivary gland – giant polytene chromosomes – can be stained with DAPI
and antibodies to locate genes/mutation/recombination and see what proteins are
transcribed
- Cheap date – mutation makes Drosophila very sensitive to alcohol
6. Sea Urchin
- Genome size: 800 Mb (~24000 genes)
- Echinoderm – deuterostomes closely related to chordata
Used to study:
- Developmental biology – embryo is transparent, good for microscopy
- Genetics
- Cell cycle regulation (Tim Hunt, cyclins)
- Fertilisation studies: single female can produce 20x10 6 easily collectable eggs
7. Zebra fish (Danio rerio)
- Size: 3 cm long
- Diet: brine shrimp and algae
- Life span: ~5 years
- Genome size: 1700 Mb (25 diploid – unlike many other fish, ~24000 genes)
Used to study:
- Visible internal development
- Cardiovascular and neurological studies
- Eye development and eye diseases
- Gene knockdown (morpholino injection in embryo)
- Study wound healing and inflammation – visualising neutrophil migration in zebrafish
larvae (Nature, 2009)
8. Xenopus laevis (African clawed frog)
- Diet: Insects, crustaceans
- Life span: 5-15 years, Maturity: 1-2 years
- Genome size: 1500 Mb (36 tetraploid, ~8000 genes)
, - Oocytes: 1mm in diameter – relatively easy to inject (DNA, RNA) and exchange
nuclei
Used to study:
- Embryology
- Induction studies
- Nuclear transfer
- Chromosome studies
- Biochemistry of the cytoskeleton
9. Thale Cress (Arabidopsis thaliana)
- Height: 15-20 cm
- Life cycle: 6 weeks from germination to production of mature seeds, Fertilisation:
readily self-fertilises
- Seed production: 5000 per plant
- Genome size: 157 Mb (5 chromosomes, 25498 genes)
Used to study:
- Stress-inducible genes
- Circadian rhythm
- Chromatin structure
- Transgenics
10. Mus Musculus
- Weight: 20g, body temperature: 36.9oC
- Life span:1.3- years, Sexual maturity: after 4 weeks of age
- Estrus cycle: every 4-5 days
- Gestation: averages 19-21 days; 1-10 pups
- Genome size ~2600 Mb (19 autosomes +X/Y, ~30000 genes)
Used to study:
- Development of body tissues
- Mammalian immune system
- Formation of nervous system
- Models of human disease (mutations and knockouts)
11. Homo sapiens
- Weight: 50-100kg
- Life span: 70-90 years, sexual maturity: 12-16 years
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