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Unformatted text preview: GENETICS: B. McCLINTOCK 7 Istrati,Gh.,Compt.Rend.Soc.Biol.,129,1010-1013 (1938). 8 Wagner-Jauregg,Th., Centralbl.f.Bakt.,144, I.Orig., 31-32 (1939). 9 Okell,C. C.,and Blake,A. V.,Jour.Path. & Bact.,33,57-63(1930). 10 Shwartzman, G.,"Phenomenon ofLocalTissueReactivity,"N. Y.,461pp. (1937). 11 Ajo, C.,Proc. Soc.Exper.Biol.& Med.,47,500-501 (1941). 12 Sterges,A.J.,Jour.Bact.,43, 317-327 (1942);Ibid.,44,138(1942). 13 Mueller,J.H.,andMiller,P.A., Ibid.,41, 301-303 (1941). THE FUSION OF BROKEN ENDS OF CHROMOSOMES FOLLOW- ING NUCLEAR FUSION By BARBARA MCCLINTOCK DEPARTMENTOF GENETICS,CARNEGIEINSTITUTIONOFWASHINGTON, COLD SPRING HARBOR CommunicatedSeptember22, 1942 When, through radiation or other causes, chromosomes are broken withinasinglenucleus,2-by-2fusionsmay occurbetweenthebrokenends. These fusionsmay leadto rearrangementsofpartsofthechromatin com- plement, giving rise to various chrorrosomal aberrations which are de- tected as reciprocal translocations, inversions, deficiencies,etc. Since, in the well-investigated cases, the breakages occurred within a single nucleus,theconditionsthatlead to fusionsof brokenendscouldnot easily be ascertained. The followingquestions have been asked: (1) Must two or more chromosomes be in intimate contact at the time of breakage in order thatfusionsmay occur? (2)If no intimate contact isnecessaryat thetimeof breakage,are thebroken ends "unsaturated,"thatis,capable offusionwithany otherunsaturatedbrokenend? (3)If question (2) can be answered inthe affirmative, what forcesareinvolvedwhichleadtothe contact and subsequent fusions of the two unsaturated broken ends? Likewise, (4) how long willthese broken ends remain unsaturated, i.e., capableoffusion? Questions(1)and (2)couldbeansweredifthefollowingconditionswere present: Assume that fusion occurs between two nuclei each of which possesses one chromosome, one end of which has been broken. Each nucleus willthen have a single broken end. When thesenucleifuseand theirchromosomes intermixwithina singlenucleus, the chromosome with a broken end contributed by one nucleuscouldfusewiththechromosome with a broken end contributedby thesecond nucleus. The chromosome fusion should occur between these two broken ends. This experiment may easily be conducted inmaize. The two nuclei thatfusecan be the male and the female gametes, respectively. The method of obtaining 458 PROC. N. A. S. GENETICS: B. McCLINTOCK gametes having a chromosome with a single "unsaturated" broken end hasbeen reportedpreviously.' This method may bebrieflysummarized. Plantswere obtained which possessed one normal chromosome 9 and one chromosome 9with a duplicationoftheshortarm. This duplicatedarm extendedbeyondthenormalshortarm,theserialorderofpartswithinthe duplicated segment being the reverse of that of the normal short arm....
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