barbara mcclintock

WELCOME

BARBARA MCCLINTOCK

PRESENTED BY :SANJEEV KUMAR

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

• 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

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

In 1938, she produced a cytogenetic analysis of the centromere.

She observed the breakage and fusion of chromosomes in irradiated maize cells.

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.

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.”

1920 1945

1963 1983

BREAKAGE FUSION BRIDGE CYCLE(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.

STEPS IN BFB• STEP 1 :BREAKAGE

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.

STEP 3 : BRIDGE FORMATION:

• 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.

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!!

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. )

•

• 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.

• 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.

• 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.

• 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.

• 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.

• (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.

• 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.

THANK YOU