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
Outline
● some historical context
– Cytology/genetics
● first paper
– Cross-over, recombination
● second paper
– Transposable elements, gene control systems
– A modern view
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
Meiosis
Meiosis I
Meiosis II
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)
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)
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
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.
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
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)
First paper -1931
McClintock and Creighton compared direct observation of
chromosomal cross-over to the phenotypes of offspring kernels
Experiment
● Chromosome #9
● Cross knobbed I,
knobless N with 2
knobless normal or 2
knobbed normal
Experiment
● Chromosome #9
● Cross knobbed I,
knobless N with 2
knobless normal or 2
knobbed normal
● Cross-over gametes
arise from knob-
interchange exchange.
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
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)
Experiment
No crossover
Region 1
Region 2
Color gene
Wax gene
Experiment
crossover region 1
Region 1
Region 2
Color gene
Wax gene
Experiment
crossover region 2
Region 1
Region 2
Color gene
Wax gene
Knob and C shown to have a fairly close association
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
Results
Results Phenotype chromosome characteristics
No crossover
Crossover in region 2
No crossover
Crossover in region 2
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”
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
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
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
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).
Transposons – animation
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
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
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.
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
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)
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
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
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‟
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
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
Mechanism (“pasting”):
DNA Transposon: Ac
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
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
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
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
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
