UC Berkeley

During cellular division, chromosomes undergo a series of cytological events that allow for the maintenance and inheritance of genetic information from one generation to the next. While the necessity of these morphological changes has been appreciated for over a hundred years, we still have but a rudimentary understanding of the basic principles underlying how chromosomes are formed.

Over the past two decades, we have gained an appreciation for the integral role of two related, multi-subunit Structural Maintenance of Chromosomes (SMC) protein complexes, cohesin and condensin, play in chromosome condensation. The necessity of both protein complexes is particularly evident in the budding yeast, Saccharomyces cerevisiae, where loss of either cohesin or condensin leads to cytologically indistinguishable loss of chromosome condensation.

In this dissertation, I address some of the gaps in our knowledge of the mechanisms governing mitotic chromosome condensation. This body of work was sparked by the fundamental question of how condensin and cohesin orchestrate their functions to ensure that chromosome condensation occurs in a tightly regulated spatio-temporal manner. To that end, we used a combination of yeast genetics, cytology, sequencing and chromosome conformation capture methodologies to dissect the relative contributions of cohesin and condensin to mitotic chromosome condensation.

First, we describe the role of a kinase, Polo (Cdc5), as part of an interactome of early condensation factors along with cohesin and condensin. In particular, we describe how cohesin modulation of this kinase may impact condensin function. Next, we utilize a chromosome conformation capture technique, Micro-C, to elucidate features of yeast mitotic structures at nucleosome resolution. There, we also begin to dissect the contributions of mitotic factors in these structures. Lastly, we investigate the molecular defects of a unique cohesin mutant that allow us to dissect cohesin’s role in condensation.

Altogether, these findings reshape our understanding how higher order chromosome structures are formed and inspire us to reevaluate some of our fundamental assumptions about how SMCs coordinate their function. Additionally, this work highlights how yeast can serve as a genetically-tractable model for studying the formation of chromosome loops.