Editing Barbara McClintock (section)

==Education and research at Cornell==
{{Listen|type=speech|pos=right|filename=Barbara McClintock (As Told By Jo Handelsman).ogg|title=Barbara McClintock (As Told By Jo Handelsman)|description= }}
McClintock began her studies at Cornell's [[Cornell University College of Agriculture and Life Sciences|College of Agriculture]] in 1919. There, she participated in [[student government]] and was invited to join a [[sorority]], though she soon realized that she preferred not to join formal organizations. McClintock broke her sorority pledge because of their antisemetic policy (Kass 2024, p.&nbsp;18). Instead, McClintock took up music, specifically [[jazz]]. She studied [[botany]], receiving a [[Bachelor of Science|BSc]] in 1923.{{sfn|Comfort|2001|pp=23–27}} Her interest in genetics began when she took her first course in that field in 1921. The course was based on a similar one offered at Harvard University, and was taught by [[C. B. Hutchison]], a plant breeder and geneticist.{{sfn|Kass|Provine|1997|p=123}}{{sfn|Kass|2000|p=64}}{{sfn|Fedoroff|1995|p=216}} Hutchison was impressed by McClintock's interest, and telephoned to invite her to participate in the graduate genetics course at Cornell in 1922. McClintock pointed to Hutchison's invitation as a catalyst for her interest in genetics: "Obviously, this telephone call cast the die for my future. I remained with genetics thereafter."{{sfn|McClintock|1983}} Although it has been reported that women could not major in genetics at Cornell, and therefore her MS and PhD—earned in 1925 and 1927, respectively—were officially awarded in botany, recent research has revealed that women were permitted to earn graduate degrees in Cornell's Plant Breeding Department during the time that McClintock was a student at Cornell.{{sfn|Kass|2003|pp=1251–1260}}(see also Kass 2024, Chapter 2).

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. This group brought together plant breeders and cytologists, and included [[Marcus Rhoades]], future [[Nobel Prize|Nobel]] laureate [[George Beadle]], and [[Harriet Creighton]].{{sfn|Kass|2005a|pp=118–125}}{{sfn|Kass|2007}}{{sfn|The Barbara McClintock Papers – Cornell}} [[Rollins A. Emerson]], head of the Plant Breeding Department, supported these efforts, although he was not a cytologist himself.{{sfn|Kass|Bonneuil|2004|pp=91–118}}{{sfn|Kass|Bonneuil|Coe|2005}} (see also Kass 2024, chapter 4).

She also worked as a research assistant for [[Lowell Fitz Randolph]] and then for [[Lester W. Sharp]], both Cornell botanists.<ref>{{cite web|first=Federica Turriziani |last=Colonna |title=Barbara McClintock (1902–1992) |url=http://embryo.asu.edu/pages/barbara-mcclintock-1902-1992 |website=Embryo.asu.edu |access-date=November 27, 2014}}</ref> (see also Kass 2024, chapter 3).

McClintock's cytogenetic research focused on developing ways to visualize and characterize maize chromosomes. This particular part of her work influenced a generation of students, as it was included in most textbooks. She also developed a technique using [[carmine]] staining to visualize maize chromosomes, and showed for the first time the morphology of the 10 maize chromosomes. This discovery was made because she observed cells from the [[microspore]] as opposed to the [[root cap|root tip]].{{sfn|Kass|Bonneuil|2004|pp=91–118}}{{sfn|Fedoroff|1995|p=217}} (Kass 2024, Chapter  4) By studying the morphology of the chromosomes, McClintock was able to [[genetic linkage|link]] specific chromosome groups of traits that were inherited together.{{sfn|Fedoroff|1995|p=212}} Marcus Rhoades noted that McClintock's 1929 ''[[Genetics (journal)|Genetics]]'' paper on the characterization of [[polyploidy|triploid]] maize chromosomes triggered scientific interest in maize cytogenetics, and attributed to her 10 of the 17 significant advances in the field that were made by Cornell scientists between 1929 and 1935.{{sfn|Rhoades}}

In 1930, McClintock was the first person to describe the cross-shaped interaction of homologous chromosomes during [[meiosis]]. The following year, McClintock and Creighton proved the link between [[chromosomal crossover]] during meiosis and the recombination of genetic traits.{{sfn|Fedoroff|1995|p=212}}{{sfn|Coe|Kass|2005|pp=6641–6656}} (Kass 2024,pp.&nbsp;75–79). They observed how the recombination of chromosomes seen under a microscope correlated with new traits.{{sfn|The Barbara McClintock Papers – Cornell}}{{sfn|Creighton|McClintock|1931|pp=492–497}} Until this point, it had only been hypothesized that [[genetic recombination]] could occur during meiosis, although it had not been shown genetically.{{sfn|The Barbara McClintock Papers – Cornell}} It is often reported that McClintock published the first genetic map for maize in 1931, showing the order of three genes on maize chromosome 9.,{{sfn|McClintock|1931|pp=485–491}} however, it was her genetics professor C. B. Hutchison who has published the first genetic linkage maps for Chromosome 9 in 1921 and 1922 (See Kass 2024, p.&nbsp;50). Her chromosome map showed that the arrangement of genes was consistent with the linkage map published by Hutchison in 1921 (see Kass 2024, p.&nbsp;51). This information provided necessary data for the crossing-over study she published with Creighton;{{sfn|Coe|Kass|2005|pp=6641–6656}} they also showed that crossing-over occurs in [[sister chromatids]] as well as [[homologous chromosomes]].{{sfn|Fedoroff|1995|p=218}} In 1938, she produced a cytogenetic analysis of the [[centromere]], describing the organization and function of the centromere, as well as the fact that it can divide.{{sfn|Fedoroff|1995|p=212}}

McClintock's breakthrough publications, and support from her colleagues, led to her being awarded several postdoctoral fellowships from the [[United States National Research Council|National Research Council]]. This funding allowed her to continue to study genetics at Cornell, the [[University of Missouri]], and the [[California Institute of Technology]], where she worked with E. G. Anderson.{{sfn|Kass|2003|pp=1251–1260}}{{sfn|Fedoroff|1995|p=218}} During the summers of 1931 and 1932, she worked at the University of Missouri with geneticist [[Lewis Stadler]], who introduced her to the use of [[X-ray]]s as a [[mutagen]]. Exposure to X-rays can increase the rate of mutation above the natural background level, making it a powerful research tool for genetics. Through her work with X-ray-mutagenized maize, she identified [[ring chromosome]]s, which form when the ends of a single chromosome fuse together after radiation damage.{{sfn|Fedoroff|1995|p=219}}  McClintock found the first ring chromosome in maize in 1931, but the first ring chromosome was first reported by Michael Navashin, who she cited in her first study with Stadler (Kass 2024, p.&nbsp;79). From this evidence, McClintock hypothesized that there must be a structure on the chromosome tip that would normally ensure stability. She showed that the loss of ring-chromosomes at meiosis caused [[variegation]] in maize foliage in generations subsequent to irradiation resulting from chromosomal deletion.{{sfn|Fedoroff|1995|p=212}} During this period, she demonstrated the presence of the [[nucleolus organizer region]] on a region on maize chromosome 6, which is required for the assembly of the [[nucleolus]].{{sfn|Fedoroff|1995|p=212}}{{sfn|Fedoroff|1995|p=218}}{{sfn|McClintock|1934|p=294–328}} In 1933, she established that cells can be damaged when [[Non-homologous end joining|nonhomologous recombination]] occurs.{{sfn|Fedoroff|1995|p=212}}{{sfn|Green|1959|p=1243}} During this same period, McClintock hypothesized that the tips of chromosomes are protected by [[telomere]]s.{{sfn|CSHL Biography}}

McClintock received a fellowship from the [[John Simon Guggenheim Memorial Foundation|Guggenheim Foundation]] that made possible six months of training in Germany during 1933 and 1934.{{sfn|Fedoroff|1995|p=219}} She had planned to work with [[Curt Stern (geneticist)|Curt Stern]], who had demonstrated crossing-over in ''[[Drosophila]]'' just weeks after McClintock and Creighton had done so; however, Stern emigrated to the United States. Instead,  she worked with geneticist [[Richard B. Goldschmidt]], who was a director of the Kaiser Wilhelm Institute for Biology in Berlin.{{sfn|Keller|1983}}<ref>{{cite journal |last1=Dietrich |first1=M.R. |s2cid=13478874 |title=Richard Goldschmidt: hopeful monsters and other 'heresies' |journal=Nature Reviews Genetics |date=2003 |volume=4 |issue=1 |pages=68–74 |doi=10.1038/nrg979|pmid=12509755 }}</ref> She left Germany early amidst mounting political tension in Europe, returned to Cornell, but found that the university would not hire a woman professor.<ref>{{Cite magazine |last1=Magazine |first1=Smithsonian |last2=Doudna |first2=Jennifer |title=By Studying Corn, Barbara McClintock Unlocked the Secrets of Life |url=https://www.smithsonianmag.com/smithsonian-institution/by-studying-corn-barbara-mcclintock-unlocked-secrets-life-180981555/ |access-date=February 14, 2023 |magazine=Smithsonian Magazine |language=en}}</ref> Legends such as the one cited here are based on undocumented evidence. Kass 2024 has evidence that McClintock worked at Cornell on her return to Cornell after spending only 5 months in Germany.  Emerson hired her as his assistant in the Department of Plant Breeding where McClintock conducted independent research which led to a job offer as an assistant professor at the University of Missouri (Kass 2024, pp. pp.&nbsp;92–95). In 1936, she accepted an Assistant Professorship offered to her by Lewis Stadler in the Department of Botany at the University of Missouri in [[Columbia, Missouri|Columbia]].{{sfn|Kass|2005b|p=52–71}}{{sfn|Fedoroff|1995|p=220}} While still at Cornell, she was supported by a two-year [[Rockefeller Foundation]] grant obtained for her through Emerson's efforts.{{sfn|Fedoroff|1995|p=219}}