the genetics of caenorthabditis elegans brenner, 1974 lecture by assaf tal
TRANSCRIPT
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The Genetics of Caenorthabditis Elegans
Brenner, 1974
Lecture by Assaf Tal
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Talk Outline
Background for Paper Concept of a Genetic Map Present Experimental System Present & Discuss Results Further Work
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From Genes to Structure
“How do genes specify the structure of an organism?”
- Brenner, 1974
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Sydney Brenner (1927 - )
Discoverer of frameshift mutations (1962)
Believed “classical” molecular biology was “solved”
Letter to Max Perutz (1963): “ ... Attempt to define the unitary steps of development in terms of genetic analysis … “
1963 Research Proposal: “The New Major problem in molecular biology is the genetics … of control mechanisms”
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The Goal:
To study the connection between micro (genes) and macro (development) on a multi-cellular “simple” organism
A Genetic Map
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Genetics Refresher
Gene:
Set of bases in DNA
Paradigm: 1 Gene per Protein
Allele: A particular Gene Realization
Homologous Chromosomes
Gene
Alleles(recessive/dominant)
Genotype: Aa
Phenotype: “what we see”
Wild Type: “common” phenotype
Mutant: “non-natural” phenotype
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How to Make a Genetic Map
Step I Find out how many genes are responsible for each phenotype
( “Complementary Analysis”)
Step II Place them on Chromosomes( “Linkage Mapping”)
Step III Determine their distances & refine (“Linkage, 3-point Mapping”)
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Complementary Groups
Gene A
Blue Eyes
Induce Mutations in Population
No blue eyes
Gene B
Protein A Protein B
How to Find Complementary
Groups?
Same Complementary
Group
Some disrupt Gene A
Some disrupt Gene B
No blue eyes
Breed & Observe Phenotypes
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Complementation Analysis
Mutation I Mutation II
Case I: Allelic Mutations Case II: Non-Allelic Mutations
Mutation I Mutation II
Recessive Mutations
Mutant
Phenotype
Wild Type
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Mapping “Linkage Groups”“Unlinked” Reproduction
A B a b
a b a bA a
B b bb
a a
25% aB
25% ab
25% AB
25% Ab
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Mapping “Linkage Groups”“Linked” Reproduction with Recombination (w/ probability p)
A a
B b bb
a a
ab
AB
A a
Bb bb
a a
aB
p/2
Cis
1
2
p
Recombination
w/
Prob. p
1
2
p
Ab
p/2
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Mapping “Linkage Groups”
Placing Genes on Chromosomes (to 0th order):
Recombination Frequency (p) ~ Distance of Genes
Part of X-chromosome of Drosophila
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C. Elegans – “Nature’s Gift”Small ( ~ 1 mm).
Fast life cycle ~ adulthood in less than 24 hrs, live 3 weeks
959 somatic cells.
Most adults are hermaphrodites.
Genetic composition:
5AA + XX
Rare males:
5AA + XO
Easy to handle in large quantities.
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Popularity of C. Elegans
First organism to have its DNA sequenced (1998).
Only organism to survive Columbia shuttle crash (2003)
Exhibit same symptoms as humans when quit smoking.
www.wormbase.org
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Popularity of C. Elegans
Search Term Google Entries
C. Elegans 2,550,000
Drosophila 1,040,000
Depeche Mode 4,120,000
God 421,000,000
Sex 460,000,000
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Isolating Mutants
For recessive mutations:
The F2 generation of an infected parent will be 25% homozygote mutants
Continue for another generation to ensure mutants can reproduce
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Classifying Mutants
Recessive vs. Dominant Need to maintain wild-type male population
Male population 0.02% in nature Keep male population by crossing w/
hermaphrodites In real life, we need to take into account co-
dominant mutations, sex-linked mutations, etc …
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Some Mutants . . .
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Gene Mapping in C. Elegans
Mutant 1 Mutant 2
Allelic Mutations Non-Allelic Mutations
Example: Complementation test for Recessive Mutations
Mutant
Phenotype
Wild Type
Mutant 1 Mutant 2
Allelic Mutations
Mutant
Hermaphrodite
WT
Male
WT Male
Mutant
Hermaphrodite
WT
Male
WT MaleMut. Herm, Mut. Herm,
Non- Allelic Mutations
50% Male
WT
50% Male
Mutants
100% Male WT
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Brenner’s Map
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Brenner’s Map
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Conclusions (cont.)
Map 258 Autosomal Mutations into 77 Complementation Groups
Accompanying paper: ~ 6.7x107 base pairs Naïve reasoning: ~ 6.7x104 proteins (genes) EMS induces mutations at rate 5x10-4/gene Mutations per worm: ~ 34
In practice: induced lethal frequency is 0.15 per X chromosome
Conclusion: 300 “essential” X-Chrom. Genes Scaled estimate: 2000 “essential” genes
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Further Work
John White
Mapping the Nervous system
John Sulston
Tracing cell lineage, apoptosis
Bob Horvitz
Connection between worms & humans
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Tracing the Cell Lineage
• Somatic cells vs. Germ cells
• John Sulston: the first to observe cell differentiation in a multicellular organism in real time
• Always the same
• Cells die of themselves
• C. Elegans is ideal:
• Simple (<1000 somatic cells)
• Transparent
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Complete Lineage Map:
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Cell Differentiation is “Rigid”
Nature or nurture? Development of
Reproduction Organs
It seems that nature!
Do genes really specify the development?
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Genes dictate development
• 302 Neurons for adult hermaphrodite (382 for male)• To map: 20,000 slices, 0.05μm thick• Lineage of neurons can be traced• Re-enter: Brenner
Induce movement-
related Mutation
Examine Nervous System
Check if it is
Hereditary
Genes indeed dictate development!!!
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Cell Death (Apoptosis)
• Cell death:• Injury, infection, . . . • By their own volition:
• By external signals• By internal “pre-programming”
• Purposes: structure formation, removing aged cells, …• In C. Elegans, 131 cells are “programmed to die”• Internal preprogramming ↔ genetic origin?
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Evidence of Cell “Fate” J. E. Sulston, Philos. Trans. R. Soc. London Ser.B 1976, 275, 287-298: The
nuc-1, controlling the activity of DNA endonuclease E.M.Hedgecock,J.E.Sulston,J.N.Thomson, Science 1983, 220, 1277-1279:
ced-1, ced-2 genes responsible for cell engulfment (phagocytosis) “Jackpot”: ced-3
H. R. Horvitz et al. ,Neurosci. Comment. 1982, 1,56-65. H. M. Ellis, H. R. Horvitz, Cell 1986, 44, 817-829.
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Visual Evidence of Cell Death
Taken from Horvitz (1986)
(bar = 10 micron)
Programmed cell death does not occur in a ced-3 mutant.
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Mapping the Genetic Pathway
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From Worms to Humans
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Biological Universality
C. Elegans shares about 35% of its genome with humans
Horvitz Nobel lecture, 2002:
“One point that emerges . . . is the striking similarity of genes . . .
among organisms . . . I like to refer to this theme as “the principle of
biological universality” . . . and it underlies my conviction strong
conviction that the . . . study of the biology of any organism is likely
to lead to findings of importance in the understanding of other
organisms, including ourselves.”
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The End
Thank you!
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