Unravelling meiotic chromosomes: topoisomerase II and other proteins.

With the advent of commercial electron microscopes, it was discovered by M.J. Moses (1956) and D.W. Fawcett (1956) that chromosomes at prophase of meiosis have welldefined axial elements that synapse homologously to form the synaptonemal complex, SC. This arrangement would appear to be eminently suited to the analysis of chromosome structure and function because each bivalent is distinct and the chromatin is still in a relatively decondensed interphase state. Indeed, much was learned from observations of meiocytes in a large number of sexually reproducing species (von Wettstein et al. 1984; John, 1990). However, it proved to be difficult to isolate and purify these chromosomes for molecular analysis (Walmsley and Moses, 1984; Ierardi et al. 1983), not least because of the attachment of the SC ends to the nuclear envelope. Consequently, initial progress in studying chromosome structure was made with the analysis of somatic metaphase chromosomes rather than meiotic prophase chromosomes.