barbara mcclintock - university of...
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Barbara McClintock
Papers: A correlation of cytological and genetical crossing-over in Zea Mays (1931)
Some Parallels Between Gene Control Systems in Maize and in Bacteria (1961)
Eric Oberla
LPIB Jan 25
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Outline
● some historical context
– Cytology/genetics
● first paper
– Cross-over, recombination
● second paper
– Transposable elements, gene control systems
– A modern view
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Meiosis
● Process by which haploid gamete cells are produced for
sexual reproduction
● Discovered by German biologist Oscar Hertwig 1876
● Not until 1890 was meiosis associated with reproduction
and inheritance
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Meiosis
Meiosis I
Meiosis II
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Thomas Morgan 1900-1910s
● Observation of sex-linked
traits in Drosophila
● Non-Mendelian statistics
observed when crossing
specific genotypes
● Idea of genetic linkage
and hypothesized cross-over
sex linked inheritance of white-eye mutant
(only males in F2 have white eyes)
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Thomas Morgan 1900-1910s
● Chromosomal cross-over
hypothesized to explain results
● Frequency of inherited
characteristic mixing explained by
genetic linkage
● Using concept of linkage, first
genetic map produced by
Sturtevant in 1913
● Results deduced from phenotypes
of offspring (no direct observation)
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Barbara McClintock
● Born 1902
● Studied botany at Cornell University
● First genetics course in 1921, stuck with it
● Officially earned Ph.D in botany
● While a grad/post-doc, formed group of plant breeders and
cytologists to study maize
● Field of cytogenetics invented
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Barbara McClintock
● Barbara McClintock as a graduate student at Cornell, 1929. (L-R standing) Charles
Burnham, Marcus Rhoades, Rollins Emerson, and Barbara McClintock. George
Beadle (eventual Nobel Prize winner himself) is kneeling by the dog.
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1930 paper - McClintock
A Cytological Demonstration of the Location of An Interchange
Between Two Non-Homologous Chromosomes of Zea Mays
● Introduced novel experimental technique:
● Stained samples with carmine put on
slides and gently heated
● Chromosomes spread out horizontally,
nuclear membrane disappears ,
allowing easy observation
● Observed cells various stages
of prophase
● Laid groundwork for 1931 paper
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1930 paper
● Important results/notes:
● Smallest chromosome (#9) has
conspicuous knob on one end (easily
distinguished)
● This knob behaves like a gene through
successive generations
● Interchange between two non-homologous
chromosomes (#8 + #9) observed
● Small, large normal = (n,N)
● Small, large interchanged = (i,I)
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First paper -1931
McClintock and Creighton compared direct observation of
chromosomal cross-over to the phenotypes of offspring kernels
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Experiment
● Chromosome #9
● Cross knobbed I,
knobless N with 2
knobless normal or 2
knobbed normal
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Experiment
● Chromosome #9
● Cross knobbed I,
knobless N with 2
knobless normal or 2
knobbed normal
● Cross-over gametes
arise from knob-
interchange exchange.
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ExperimentResults:
Amount of crossing-over between knob and interchange found to be 39%
-Furthermore, it was shown that that knobbed chromosome carried the
genes for color (C) and waxy (wx) endosperm
-The knobless #9 carries colorless (c) and starchy (Wx) alleles
-These genes located on short arm of chromosome
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Experiment
Also shown was the close
association between the
„knob‟ and C (color)
Much like genetic map of
Drosophila
This was done by crossing
Knob (C-wx)/knobless(c-Wx)
with double knobless (c-wx)
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Experiment
No crossover
Region 1
Region 2
Color gene
Wax gene
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Experiment
crossover region 1
Region 1
Region 2
Color gene
Wax gene
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Experiment
crossover region 2
Region 1
Region 2
Color gene
Wax gene
Knob and C shown to have a fairly close association
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Experiment
Bringing it together
• have 2 data pieces of knowledge:
knob/interchange crossover
color gene and knob relation
• cross as shown
• Combination of phenotype
observation and microscopic
chromosomal observation
• Track knob+interchange
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Results
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Results Phenotype chromosome characteristics
No crossover
Crossover in region 2
No crossover
Crossover in region 2
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Conclusion of 1931 paper
“Pairing chromosomes, heteromorphic in two regions, have
been shown to exchange parts at the same time they
exchange genes associated to those regions”
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Back to Barbara
• Moved to University of Missouri
• Started research using X-rays as a mutagen
• Discovered ring chromosomes that form when ends of
a single chromosome fuse together after rad damage
• Observed cycle of breakage, fusion and bridging of
chromosomes as a source of large-scale mutation
• Still an active area of cancer research today
• Left Missouri when she felt unsatisfied with her position
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Cold Spring Harbor
• In 1944, McClintock began studying the mosaic color
patterns of maize seed and their apparent instability
(again with chromosome 9)
• Discovered 2 dominant „loci‟
• Dissociator (Ds)
• Activator (Ac)
• In 1948, she discovered that
these elements could „jump‟ to
different positions on the chromosome -- transposons
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Transposons (Ac/Ds)
• Classified these „mobile genetic elements‟ through
controlled crossing to influence colorization
• Found Ac controls transposition of Ds
• Chromosome breaks when Ds moves
• The movement of Ds initiates pigment
synthesis
10- no Ac
11-13 – 1Ac
14 – 2 Ac
15 – 3 Ac
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Some maize
phenotypes caused
by transposable
elements excising in
somatic tissues.
Parental plants are
mutants defective in
starch synthesis
(endosperm
phenotypes) or
anthocyanin
synthesis (aleurone
and pericarp
phenotypes).
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Transposons – animation
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Maized and Confused
• In late 40‟s McClintock developed a theory that these
mobile elements undertook „gene regulation‟
• Hypothesized that this gene regulation is why cells with
identical genomes have different function
• Published several papers of her findings…very critical
reception and hard for people to believe
• Stopped publishing on controlling
elements in 1953
• Started research on maize
in South America
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1960‟s
• Series of papers by Francois Jacob and Jacques Monod
described genetic regulation in bacteria
• McClintock responded to their 1961 paper Genetic
regulatory mechanisms in the synthesis of proteins with
comparisons to her own work
• McClintock‟s1961 paper: Some Parallels Between Gene
Control Systems in Maize and in Bacteria
They describe similar elements with similar functions!
operator = Ds, located adjacent to structural gene
regulator = Ac, located close or elsewhere on the
chromosome
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1961 paper - Bacteria
• Bacterial control systems made from 2 genetic elements
• Regulator – produces repressor substance in
cytoplasm
• Operator – responds to regulator, adjacent to structural
gene
• Structural gene – when activated, codes for a particular
sequence of amino acids
• When phage introduced to bacterial chromosome and
induced by UV or chemicals, inhibition of gene action in
phage AND surrounding neighborhood on chromosome.
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1961 paper - Maize
• Several different two-element control systems identified in
maize
• Discovered because the elements belonging to each group
could transpose without changing identity
• Transposition not always characteristic of controlling element
• „Operator‟ elements in Maize may transpose, or may turn on
and off – latter is similar to bacteria
• McClintock went on to used the complicated Supressor-
mutator element in her comparison – will not cover
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1961 paper
• Signaled „re-discovery‟ of McClintock‟s controlling elements
• “One must await the right time for conceptual change”
• New technologies in 60s and 70s led to further discoveries
• Molecular basis for transposition
• 1983 Nobel Prize for Physiology
or Medicine (unshared)
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Transposons today
• We now know 50%! of human genome is comprised of
transposable elements (TE)
• 2 main types (based on mechanism):
• Class I: Retrotransposons (“copy and paste”)
• Class II: DNA transposons (“cut and paste”)
humans
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Some Bio
• Chromosomes comprised of DNA (info) and protein (structure)
• DNA includes genes, regulatory elements, other sequences
• Genes comprised of exon and introns
• mRNA only transcribes exon portion proteins
• Large portions of DNA are non-coding
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Picture model
Copy and paste of TE
In general, transposons are
strands of DNA that
encodes a protein
(enzyme) to perform a
specific task.
In most organisms, TE DNA
code includes „address‟
and „script‟
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DNA transposons
• Encode the enzyme Protein Transposase
• This is required for excision (cut) and insertion (paste)
• Move on their own (no intermediaries)
• Include “terminal inverted repeats,” series of base pairs to be
recognized by the transposase enzyme
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Retrotransposons
• Require RNA intermediate to move
• Produce RNA transcripts (copy) and rely on reverse
transcriptase enzymes to convert back to DNA to be inserted
(paste) at new site
• Both class I and II have a „flanking direct repeat‟ series of
base pairs that are not part of the TE, but act as a marker
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Mechanism (“pasting”):
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DNA Transposon: Ac
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Retrotransposons: example
• Alu element most common TE in human genome
• 300 base pairs (occurs 300K-3M times, ~10%)
Green marker indicates Alu
element
• Alu insertions implicated in some inherited diseases
• Studied extensively in population genetics
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Autonomous/Non-autonomous TEs
• Autonomous TEs can move on their own
• Example: Ac (the „regulator‟) in Maize
• Non-Autonomous TEs need other TEs to jump
• Lack gene for transposase or reverse transciptase
• Example: Ds (the „operator‟) in Maize
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Significance
• Most TEs are silent – no phenotypic effect or jumping around
• Some are silent due to mutations
• Others are silent due to epigenetic (inherited gene
expression) defense
• Example: methylation – (O-H O-CH3)
• Effects of Non-silent TEs depend on „landing‟ spot
• Landing within a functional gene will likely disable that
gene
• Diseases/mutations may result
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Not always destructive
• Can be useful for repairing broken DNA strands
• In fact, transposons can facilitate genome evolution via the
translocation of genomic sequences
• The ability of transposons to increase genetic
diversity, in combination with the ability of the genome
to inhibit most TE activity plays an imporant role in
regulation and evolution
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Other uses
• Transposons useful as a genetic tool
• Characterize gene/protein function
• Biologists insert transposons into model
organism genome (mutagenesis)
• E. Coli and Drosophila studied extensively
Medaka fish embryos:
Top: specific transposon within pigmentation
gene
bottom: transposon jumps to different location,
causing instability in pigmentation
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