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| milestone 2
Polar expedition
"There is an important difference... between ordinary mitosis and the first division of meiosis, a difference which (as far as I know) has never been pointed out by any investigator, although it must have been observed many times, as is clear from drawings published by various authors", wrote Gunnar Östergren in his seminal paper in Hereditas - a study that shed light on one of the most fundamental processes in cell biology, but one that had puzzled scientists for decades.
His 'theory of co-orientation by pulling', published in 1951, took a great step towards understanding the complex and remarkable chromosomal gymnastics that makes meiosis such a unique process. As far back as the late-1800s, van Beneden had discovered the true nature of fertilization - that is, the fusing male and female half-nuclei (pronuclei) have only half the total number of chromosomes, so the number of chromosomes must be reduced during the formation of the gametes. He also showed that half of the chromosomes are lost within the polar bodies - a process that was called maiosis (meiosis) by Farmer and Moore in 1905. But as this process is markedly different to ordinary mitotic divisions - where the copy number of chromosomes remains constant - the big mystery was how chromosomes could physically behave in a different way during meiosis to produce this controlled reduction in chromosome number. At the time, many scientists were trying to identify the mechanism behind this process (most notably Janssens, Darlington and Geitler). But it was Östergren who provided a clear concept for the movement of the chromosomes during meiosis, by drawing together observations from these and other scientists, as well as his own findings. Earlier studies had indicated that the secret might lie in the first division of meiosis. One difference that had been observed was that chromosomes form links with each other, known as chiasmata, at homologous regions. But Östergren argued that these connections could not fully account for the behaviour of the chromosomes during meiosis. "The essential difference between these two types of division... must be a difference in the structure of the organ of movement itself", he reasoned. To find out if this was true, Östergren carried out a meticulous investigation of the first meiotic division in several plants and he found that a key difference lay in the arrangement of the kinetochore regions. In mitosis, the kinetochore is arranged on two opposite sides of the chromosome, but during the first meiotic division, the whole kinetochore is arranged on one side of the chromosome body. This seemed to be caused by a less restricted coiling of the twisted chromatid thread within the chromosome, known as the chromonemata. The result of this is that the chromosome body screens off the kinetochore so that it can interact with the spindle in one direction only, explained Östergren. Because of this, the kinetochore finds it difficult to orientate simultaneously towards both poles so will always attach to a single one. Östergren used a term coined by Darlington 14 years previously, called polarization, to describe this phenomenon - with the meiotic kinetochores being described as unpolarized and mitotic kinetochores being polarized. But how does this orientate sister chromatids in a defined way towards the two opposite poles during full metaphase? This, revealed Östergren, is caused by a phenomeon called co-orientation (again a term coined by Darlington), which involves a unique interaction between the paired chromosomes. "Both the kinetochores of a bivalent can sometimes be seen to move actively towards the same spindle pole (during metaphase)", he wrote. "During these random movements the kinetochores of the paired chromosomes, sooner or later, happen to pull on their partners. This pull, in connection with the one-sided arrangement of the kinetochore on the chromosome body, results in an orientation of the partner kinetochores towards opposite spindle poles, i.e. it produces the co-orientation".
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