Micromechanical studies of mitotic chromosomes

Abstract

Mitotic chromosomes respond elastically to forces in the nanonewton range, a property important to transduction of stresses used as mechanical regulatory signals during cell division. In addition to being important biologically, chromosome elasticity can be used as a tool for investigating the folding of chromatin. This paper reviews experiments studying stretching and bending stiffness of mitotic chromosomes, plus experiments where changes in chromosome elasticity resulting from chemical and enzyme treatments were used to analyse connectivity of chromatin inside chromosomes. Experiments with nucleases indicate that non-DNA elements constraining mitotic chromatin must be isolated from one another, leading to the conclusion that mitotic chromosomes have a chromatin Fnetwork_ or Fgel_ organization, with stretches of chromatin strung between Fcrosslinking_ points. The as-yet unresolved questions of the identities of the putative chromatin crosslinkers and their organization inside mitotic chromosomes are discussed.

Condensin Regulates the Stiffness of Vertebrate Centromeres

“…Loss of topoisomerase II (topo II) can reduce the interkinetochore distance in metaphase (Spence et al, 2007), and topo II can influence the longitudinal elasticity of mitotic chromosome arms (Kawamura and Marko, personal communication; Marko, 2008). Indeed, treatment with topo II inhibitor ICRF 159 slightly reduced the spacing between sister kinetochores, even in the presence of condensin (Figure 4e).…”

“…Centromere and kinetochore tension and stretch are important for maintaining chromosome alignment (McIntosh et al, 2002), stabilizing kinetochore microtubule (kMT) attachments (Nicklas and Koch, 1969), spindle checkpoint signaling (Musacchio and Salmon, 2007;McEwen and Dong, 2009), and also for the back-to-back orientation of sister kinetochores (Loncarek et al, 2007). At least three independent factors have roles in the establishment of centromeric tension in metaphase: sister chromatid cohesion (Yeh et al, 2008), the elastic properties of chromatin (Houchmandzadeh et al, 1997;Almagro et al, 2004;Marko, 2008), and the higher order structure of the centromeric chromatin.…”

“…Centromere and kinetochore tension and stretch are important for maintaining chromosome alignment (McIntosh et al, 2002), stabilizing kinetochore microtubule (kMT) attachments (Nicklas and Koch, 1969), spindle checkpoint signaling (Musacchio and Salmon, 2007;McEwen and Dong, 2009), and also for the back-to-back orientation of sister kinetochores (Loncarek et al, 2007). At least three independent factors have roles in the establishment of centromeric tension in metaphase: sister chromatid cohesion (Yeh et al, 2008), the elastic properties of chromatin (Houchmandzadeh et al, 1997;Almagro et al, 2004;Marko, 2008), and the higher order structure of the centromeric chromatin.Condensin is important for the architecture of mitotic chromosome arms (Coelho et al, 2003;Hudson et al, 2003;Hirota et al, 2004;Hirano, 2006), but it also localizes to centromeres (Saitoh et al, 1994;Gerlich et al, 2006), where condensin I, but not condensin II was reported to have a role in stabilizing the structure (Gerlich et al, 2006). It has recently been suggested that condensin could have a role in regulating the elastic behavior of centromeric chromatin.…”

“…However, SMC2 S1086R , a mutant capable of assembling a condensin complex that targets to chromosomes (Supplementary Figure S1) but that lacks ATPase activity (Hudson et al, 2008) failed to rescue the spacing between sister kinetochores (Figure 4e). Therefore, normal stiffness of the centromeric chromatin requires SMC2 ATPase activity.Loss of topoisomerase II (topo II) can reduce the interkinetochore distance in metaphase (Spence et al, 2007), and topo II can influence the longitudinal elasticity of mitotic chromosome arms (Kawamura and Marko, personal communication;Marko, 2008). Indeed, treatment with topo II inhibitor ICRF 159 slightly reduced the spacing between sister kinetochores, even in the presence of condensin (Figure 4e).…”

Shape Transitions and Chiral Symmetry Breaking in the Energy Landscape of the Mitotic Chromosome

“…Microscopy reveals the overall morphology of the mitotic chromatin, but its internal structure remains controversial [2][3][4]. A complete understanding of chromosomal organization is challenging since the condensation of the chromosome into its dense mitotic form permits many different orderings and phase transitions.…”