Transcript BARBARA McCLINTOCK-Biography
Graduate student in Biology, Memorial University
PhD in botany from Cornell University in 1927
Leader in the development of maize cytogenetics
She developed the technique for visualizing maize chromosomes
and used microscopic analysis to demonstrate many fundamental
She was recognized among the best in the field, awarded prestigious
fellowships. Member of National Academy of sciences and presided
the Genetics society of America.
Awards and recognition for her contributions to the field followed,
including the Nobel Prize in Physiology or Medicine, awarded to her
in 1983 for the discovery of genetic transposition; she is the only
woman to receive an unshared Nobel Prize in that category.
One of those ideas was the notion of genetic
recombination by crossing-over during meiosis—a
mechanism by which chromosomes exchange information.
She produced the first genetic map for maize, linking regions
of the chromosome to physical traits.
She demonstrated the role of the telomere and centromere,
regions of the chromosome that are important in the
conservation of genetic information
She made an extensive study of the cytogenetics
and ethnobotany of maize races from South America
Studies of the origin and expression of gene instability at a
number of known loci in the maize chromosomes were
summarized by 1951-52 studies by Barbara McClintock .
It was concluded that changes in genie expression result from
chromosome alterations at the locus of a gene and these are
initiated by units other than those composing the gene itself.
These mutations are therefore considered as chromosomal
modifications effecting gene expressions Each exerts a specific
type of control of the action of the gene with which it becomes
These units may be transposed from one location to another
within the chromosome complement.
When incorporated at a new location, each expresses its mode of
control of the action of the associated gene, and in a manner
similar to that which occurred at the former location.
Called Dissociation Activator ( Ds-AC) two-unit system
Transpositions of Ds or AC, or both, may occur in a few
gametic cells. Consequently, a few gametes ma)- be formed
with Ds or Ac or both, located at new positions.
Following such transposition, each remains at the new
location until, in a subsequent cell or plant generation,
transposition to another location again occurs
Dicentric chromatid formation deficiency,
Duplication of segments,
Reciprocal translocations between
In all of these cases, the chromosome breaks
are produced as the consequence of some
initial change involving the Ds unit
When Ds is transposed to the locus of a known gene, it may
immediately- or subsequently affect its action.
This is expressed either by partial or complete inhibition, or by a
previously unrecognized type of altered gene expression.
As long as Ds remains in this position, gene action is subject to
further change. It has been possible to determine that the
subsequent changes are reflections of alterations occurring at
the locus, and these are initiated by Ds.
No Ds-initiated changes will occur, however, unless Ac is also
present in the nucleus.
If Ac is absent, no further modification of gene action occurs.
The mutation present at the time of removal of A will be stable
in expression in subsequent generations until Ac is again
introduced into a zygote. In some cells of the plant arising from
this zygote, alterations at the locus of the gene, initiated by Ds,
again will occur.
Some of these may result in further modifications of gene
action-that is, new mutations.
Both the initial mutation and subsequent mutations are
expressions, therefore, of interaction between Ds and AC.
Thus, these two chromosomal units comprise a nuclear
system capable of controlling gene expression ; Ds initiates
changes in gene expression and .--Ac controls when they will
development of endosperm
Sh1- shrunken endosperm, located
a quarter of a crossover unit distal
Bm1- colourless secondary cell
walls in plant tissues
bm1-brown mid-rib, brown colour
in secondary cell walls;
A1 locus of chromosome 3.a1 is
associated with Ac elements
Culture 6424 is a particular culture
of previously crossed plants
Shows Ac control of mutation
AC is responsible for producing
breaks at Ds, wherever it may be located, and for
controlling mutability at known locus with specific
Ds elements may transpose from one part to
another by transposase enzyme.
Viruses discovered later use transposition to
infect mammalian cells.eg: HPV which causes
In reverse genetics transposition can be used
to study functions of genomes.
Most elements in mammalian genes are
McClintock, Barbara. "Induction of instability
at selected loci in maize."Genetics 38.6