Question

(20 pts) Write about Barbara McClintock. Half of it should be about her life and her educational background (whatever you find interesting) and the other half should be specifically about her work with transposons.

Answer 1

Barbara McClintock was an American scientist and cytogeneticist who got nobel prize  in Physiology or Medicine. McClintock did her PhD in botany from Cornell University in 1927.  

Born on June 16, 1902 at Hartford, Connecticut, U.S.

Died on September 2, 1992 (aged 90) at Huntington, New York, U.S

Her father was a homeopathic physician Thomas Henry McClintock and mothers name is Sara Handy McClintock. When she was a young girl, her parents changed her name from Eleanor to Barbara. From the age of three until she began school, McClintock lived with an aunt and uncle in Brooklyn, New York in order to reduce the financial burden on her parents while her father established his medical practice. She was close to her father, but not that much attached to her mother.

McClintock completed her secondary education there at Erasmus Hall High School in Brookyln. she graduated early in 1919. She wanted to continue her studies at Cornell University's College of Agriculture. Her mother resisted, but her father and she matriculated at Cornell in 1919.

McClintock began her studies at Cornell's College of Agriculture in 1919. She studied botany, receiving a B.Sc in 1923. Her interest in genetics began when she took her first course in that field in 1921.

During her graduate studies and postgraduate appointment as a botany instructor, McClintock was instrumental in assembling a group that studied the new field of cytogenetics in maize. McClintock's cytogenetic research focused on developing ways to visualize and characterize maize chromosomes. In 1930, McClintock was the first person to describe the cross-shaped interaction of homologous chromosomes during meiosis. During her time at Missouri, McClintock expanded her research on the effect of X-rays on maize cytogenetics. McClintock observed the breakage and fusion of chromosomes in irradiated maize cells. In the summer of 1944 at Cold Spring Harbor Laboratory, McClintock began systematic studies on the mechanisms of the mosaic color patterns of maize seed and the unstable inheritance of this mosaicism.

McClintock spent her later years, post Nobel Prize, as a key leader and researcher in the field at Cold Spring Harbor Laboratory on Long Island, New York. McClintock died of natural causes in Huntington.

McClintock is best known for her discovery of transposable elements through experimentation with maize. Through these experiments, McClintock recognized that breakage occurred at specific sites on maize chromosomes.The  first transposable element she discovered was a site of chromosome breakage, aptly named "dissociation" (Ds). She initially noted that the movements of Ds are regulated by an autonomous element called "activator" (Ac), which can also promote its own transposition but later identified that TEs can shift position autonomously.

In her experiments, McClintock bred females that were homozygous for C and bz and that lacked Ds (denoted CCbzbz--, where the dashes indicate the absence of Ds alleles) with males that were homozygous for C', Bz, and Ds (denoted C'C'BzBzDsDs) to yield heterozygotes with an aleurone layer that had the genotype C'CCBzbzbz--Ds. Because of the presence of the dominant inhibitor allele C', the offspring kernels were expected to be colorless, no matter what their genetic makeup at the Bz/bz locus. In fact, upon crossbreeding, many of these kernels were indeed colorless. But she  also observed many kernels with colorless backgrounds and varying amounts of dark brown spots or streaks, and she concluded that individual cells in those kernels had lost their C' and Bz alleles because of a chromosomal break at the Ds locus. Without either the C' allele (to prevent color expression) or the Bz (purple) allele, the cells that had experienced a breakage at the Ds locus ended up with some brown coloring.

Within the affected seeds, the amount of colored streaking or spotting depended upon when during seed development the somatic cell mutation at Ds occurred. If this mutation occurred early in development, then, as the one mutant cell continued to divide, more cells in the mature kernel would have the brownish phenotype, and the spot or streak of color on the kernel would be larger. On the other hand, if the mutation occurred later in development, the spotting would be smaller, because the kernel would undergo less cell division prior to maturity.

McClintock also performed additional experiments to demonstrate that the phenotypic effect of Ds depended upon the presence of another element, which she called Ac. Further experiments showed that Ds didn't just break chromosomes, but it could actually move from one chromosomal location to another. When Ds inserts itself into the Bz allele, it causes a mutation in the Bz gene but only when Ac is present, thereby destroying the ability of the Bz gene to produce any pigment at all. Ds can also jump out from the Bz allele again, only in the presence of Ac, causing Bz to revert back to its purple or brown phenotype. The amount of purple or brown depends upon when during development Ds is inserted or excised. If jumping out happens prior to fertilization, then the affected kernel will be entirely purple or brown.