Walther Flemming’s very careful observations of cell division in the 1870s showed that nuclei do not divide by simple fission, as had been widely believed, but instead undergo a transformation in which the “threads” appear. German scientists also referred to them as little rods (“Stäbchen”) or as “Fäden” (threads).

Edouard van Beneden saw mitotic chromosomes and called them “bâtonnets” (little rods) when he described cell division in 1875. Edouard Balbiani clearly observed chromosomes and described and illustrated metaphase plates in 1861, though he misinterpreted what they were. The discovery of eukaryotic chromosomes arose from studies of cell division in the second half of the 19th century and was made possible by the development of improved compound microscopes by Zeiss in Jena.

Condensin I then forms loops nested within these larger condensin II loops, thereby giving rise to the final compaction of the mitotic chromosome in a process that requires Topo IIα.ġ. Mitotic chromosomes and the chromosome structure problem Modelling of Hi-C data suggests that condensin II adopts a spiral staircase arrangement with an extruded loop extending out from each step in a roughly helical pattern. Condensins I and II are primarily responsible for shaping the chromosome and the scaffold, and they produce the loops of DNA by an ATP-dependent process known as loop extrusion. These proteins are concentrated along the axis of the chromatid. Only a few key non-histone proteins and protein complexes are required to form the chromosome: topoisomerase IIα, cohesin, condensin I and condensin II, and the chromokinesin KIF4A. It is now well established that each mitotic chromatid consists of a central organizing region containing a so-called “chromosome scaffold” from which loops of DNA project radially. We also discuss some of the remaining questions. In this chapter, we start with the early history of chromosome studies and then describe the path that led to our current understanding of the formation and structure of mitotic chromosomes. Our understanding of the structure and function of mitotic chromosomes has come a long way since these iconic objects were first recognized more than 140 years ago, though many details remain to be elucidated.

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