Loss of cohesion disconnects sisters

FIG. 1. The metaphase to anaphase transition. (A), Light micrographs of mitotic figures m endosperm of the African blood lily Haemanthus katherinae Bak. Microtubules are stained in light grey and chromosomes in darker, thicker lines. In metaphase (left panel) centromere regions are aligned on the spindle equator, whereas in anaphase (right panel) the arms of separated sister chromatids trail behind centromere regions which move poleward. Size bar, 10 ,um. Reprinted, with permission from Khodjakov et al (1996). (B), A model depicting how cohesion structures (dots) physically connect sister chromatids during metaphase. Cohesion antagonizes the pulling forces exerted by spindle microtubules (horizontal lines) on kinetochores (attached to the sister chromatids). During anaphase, loss of cohesion liberates sister chromatids for poleward movement.

cytological in nature, concentrated on the mechanics of chromosome segregation (Flemming 1879, Wilson 1925, Schrader 1944, Mazia 1961). What, for example, was 'the nature of the initial act of doubling of the spireme thread (chromosome)' (Wilson 1925) and how were the sister threads moved to opposite poles of the cell during mitosis?

The elucidation of DNA's structure largely answered the first of these questions (Watson & Crick 1953) and work on cytoskeletal proteins like tubulin and the spindle fibres assembled from it, has gone a long way towards solving the mystery of chromosome movement. In contrast, until recently the mechanisms by which sister chromatids are tied together after chromosome duplication and then separated at the metaphase to anaphase transition was largely neglected, despite being equally crucial for the mitotic process (Maguire 1990, Miyazaki & Orr-Weaver 1994).

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