During meiosis, pairs of homologous chromosomes often twist around each other and remain in contact during the process of separation. The point at which paired chromosomes remain in contact is known as a chiasma, because it has a cross-shaped appearance under the microscope. Breakage and rejoining of the chromosomes at these points results in crossing over, that is, the exchange of material between the chromosomes. The reciprocal genetic exchange between homologous chromosomes, which is called crossing over, changes the pattern of genes in the chromosomes. Genes that were originally linked become separated, while genes that were on different chromosomes become linked. Thus, chiasmata are associated with the separation and recombination of genetic factors. The rearrangement of genes results in differences between the chromosomes of parents and their offspring.
Cytologists first saw the points of contact between pairs of chromosomes during meiosis in the 1890s, but the descriptions were vague and these observations were generally ignored. In 1900, cytologists reported the pairing between the chromosomes of grasshoppers. The first serious study of chiasmata was carried out by Frans Alfons Janssens (1863-1924), a Jesuit professor at the University of Louvain. Janssens proposed that each chiasma observed under the microscope was the result of a reciprocal exchange between homologous chromosomes during the early phase of meiosis. Cytologists knew that meiosis involved two divisions of the mother cell and its nucleus to produce four daughter cells.
In 1897, Janssens began to investigate the question of what the chromosomes did during the special cell divisions that produced sperm and eggs. By 1909, Janssens had become convinced that the chromosomes in the four cells produced by meiosis must always be different.
He noticed that when the chromosomes separated they seemed to stick together at particular points. This led him to suggest that the cross-like figures he observed might represent places where some exchange between the paired chromosomes had occurred. Janssens named the cross-like figures chiasma. Because of the exchanges, Janssens asserted, each of the cells produced by the reduction divisions would have chromosomes that were different from its parents, as well as different from the other three cells. Janssens was primarily interested in the possibility of the unique exchanges between chromosomes. He was not clear about the structure of the chiasmata, or the mechanism of their formation. According to Janssens' "chiasmatype hypothesis," chiasma might represent sites of chromosome breakage and healing. This concept suggested that, if the physical exchange of segments of chromosomal material actually occurred at the chiasmata, the study of genetic recombinants would represent a useful bridge between cytology and Mendelian genetics. Janssens' hypothesis created a great deal of confusion and controversy when proposed in 1909, but as supporting evidence gradually accumulated, the chiamatype hypothesis was accepted by cytologists and geneticist. Geneticists proved that the correlation between the occurrence of genetic crossing over and the frequency of cytologically observable chiasmata was very strong. Thomas Hunt Morgan and his associates provided solid confirmation of the chiasmatype theory. Working with the fruit fly, they proved that crossing over could be used to produce maps of the linear order of the genetic factors on the chromosomes of the fruit fly. Direct evidence that crossing over occurs at chiasmata during meiosis was obtained by Calvin Bridges, using special stocks of Drosophila.
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