Barbara McClintock was a pioneering geneticist and botanist who made several important discoveries in genetics and cytogenetics. She was born in 1902 in Connecticut and studied at Cornell University, where she earned her PhD. In the 1920s and 1930s, she conducted early cytogenetic studies in maize and published the first genetic map of maize chromosomes. In 1948, she discovered transposable elements in maize and showed that genes can move within a genome, a finding that was not widely accepted until the 1970s. For this groundbreaking work, she received the Nobel Prize in Physiology or Medicine in 1983, becoming the first woman to receive an unshared Nobel in science.

2. BARBARA MCCLINTOCK
PRESENTED BY :
SANJEEV KUMAR

3. IMPORTANT EVENTS
• NAME: Barbara McClintock
• OCCUPATION: Botanist, Geneticist
• BIRTH DATE: June 16, 1902
• DEATH DATE: September 02, 1992
• EDUCATION: Cornell University
• PLACE OF BIRTH: Hartford, Connecticut
• PLACE OF DEATH: Huntington, New York

4. • She moved to Brooklyn, New York when she was eight and
attended Erasmus Hall School.
• Bachelor of Science. Master of Arts, Ph. D. Studied at the
Kaiser Willhelm Institute in Berlin in 1933, but returned the
following year because of the Nazis.
• Ms. McClintock went on to attend Cornell University, where
she found her passion for Genetics.
Education

5. Ms. McClintock was “America’s most distinguished
cytogeneticist”
In 1920’s she started cytogenetic studies in maize
In 1930, she was first to describe the crossing over in
maize
In 1931, She published the first genetic map for maize
Genetics study

6. In 1938, she produced a cytogenetic analysis of the
centromere.
She observed the breakage and fusion of
chromosomes in irradiated maize cells.

7. Awards Received by Ms. McClintock
• 15 different awards from various
places all across the country, and
from different parts of the world
• 12 honorary doctors of science from
various colleges and universities
• Finally, she received the NOBEL
PEACE PRIZE in 1983 for her
discovery of mobile genetic
elements.

8. Nobel Peace Prize
• Eighty-one years old.
• 32 years after the discovery she
was for was made.
• Third woman to be awarded an
individual prizawarded e in the
sciences.
• “The prize is such an extraordinary
honor. It might seem unfair,
however, to reward a person for
having so much pleasure, over the
years, asking the maize plant to
solve specific problems and then
watching its responses.”

9. 1920 1945
1963 1983

10. BREAKAGEFUSIONBRIDGECYCLE(BFB)
 Is a mechanism of chromosomal in stability, discovered by
Barbara MacClintock in1930.(in maize)
 This cycle involves breakage of chromosome followed by
fusion and bridge formation .
 A type of chromosomal behavior in which a broken
chromatid fuses to its sister chromatid thus forming a
“bridge” .
 When centromere separates at mitosis ,the chromosomes
break again (not necessarily at the bridge )there by
restarting the cycle.

11. STEPS IN BFB
• STEP 1 :BREAKAGE

12.  Telomeric region of chromosome breaks (breakage)
 This chromosome replicate(S phase) forming sister
chromatids that both lacks telomere at one end.
STEP 2 :FUSION :
• The lack of a telomere on these two sister chromatids
causes them to fuse with one another.

14. STEP3:BRIDGEFORMATION:

15. • STEP 3 : BRIDGE FORMATION:
• During anaphase ,the centromere in one of the sister
chromatids will be pulled in opposite direction forming a
bridge as the ends are fused ultimately.
• So two sister chromatids break apart at any point or
region.
STEP 4 : CYCLE REPEATS :
• Two daughter cells receives an un even chromosome
without a telomere.
• As the chromosome lacks telomere ,cycle continues
during next division.
• Therefore called as BFB cycle.

19. Transposable Elements
• Transposons are mobile DNA
Elements that move from one
Place to another place within
the same chromosome or different
chromosome in a genome.
• Discovered them in 1940s,
• but nobody paid attention
• Ideas finally accepted in 1970s
• Nobel prize in 1983
• She was 81 years old!!

20. Ac / Ds elements in maize
• Maize is proved to be perfect organism for the studyof
transposable elements(TEs),also known as jumping genes.
• McClintock discovered Ac/Ds system in maize.
• By conducting standard genetic breeding experiments with
unusual phenotype.
• During these experiments McClintock recognized that
breakage occurred in specific chromosomes ( that always
occurred at the same locus on maize chromosome no 9. )
•

21. • So first transposable element she discovered was site of
chromosome breakage ,aptly named “dissociation Ds”
• then she noted some TEs jump autonomously and
regulate the movement of Ds and called them as
activators (Ac),which can promote its own
transposition.
• before McClintock Rollins A Emerson AN ANOTHER
American maize geneticist and the rediscoverer of mendel
s laws of inheritence.

22. • McClintock discovered the unstable mutation emersons
puzzled over was actually a four gene system.
• C` DOMINANT( C-9 ) COLORLESS PHENOTYPE.
• c recessive (c-9) color development.
• Bz DOMINANT (C-9)PURPLE PHENOTYPE.
• bz RESESSIVE(C-9)DARK BROWN PHENOTYPE.
• Ds (c-9) chromosomal breakage.
• As (unknown location) impact the expression of Ds.

23. • Maize genes code for variegated kernels .
• Kernels where described as colorless (white or
yellow),except for spot or streakings of purple and brown .
• Emerson had proposed that the variegated streaking was
due to an “unstable mutations”
• He could not explained how and why this occurred.

25. • ccbzbz__ × C`C`BzBzDsDs
• colo
• C`ccBzbzbz__Ds
• (aleuron3n)colorless
• many of kernels where colorless. McClintock also
recovered many kernels with colorless background and
varying amount of dark brown streaks,
• She concluded that those kernel has lost C` AND Bz
allele because of a chromosomal break at Ds locus.

26. • Ds didn`t just break chromosome ,but actually move from
one chromosomal location to another.
• When Ds allele inserts itself into the Bz allele –causes
mutation of Bz gene(but only when Ac present.)----
• -
• destroying the ability of the Bz gene to produce any
pigment at al.
• Ds can also excise from Bz allele (in presence of Ac
only)------ causing revert back to purple or brown
phenotype.
• THE AMOUNT OF PURPLE OR BROWN DEPEND
UPON WHEN DURING DEVELOPMENT OF Ds
INSERTED OR EXISED.

27. • (Ds.Ac) +Bz gene =no pigments.(colourless).
• Bz –(Ds.Ac) =purple or brown pigments
• .
• Based on addition or deletion of Ds from Bz gene color of
the aleuron layer (purple or brown ) veries.

28. • In brief :
• Ac/Ds elements in maize discovered by Barbara
McClintock.studied on striping and spotting of maize
kernel caused due to chromosomal breakage.(c-9.)
• Called them as controlling elements
• Ac(activator)Ds(dissociation)elements structurely related
and move from one site and inserted into another site.
• When insertion takes place near a gene ,the gene
function is altered.
• Ds elements are non-autonomous elements they are
responsible for causing breakage In chromosome and act
only in the presence of Ac elements.
• Ac elements are autonomous elements thy encode for tr
ansposase enzyme essential for transposition and contain
11 nucliotide pair long inverted repeats.

29. THANK YOU