Cell division, the process by which a parent cell divides into two daughters, is fundamental to life. An important aspect of cell division is to ensure that genomic information is conserved; chromosome segregation errors in man can cause birth defects and contribute to cancer. In all eukaryotes, chromosome segregation is accomplished by the mitotic spindle, a bipolar assembly of dynamic microtubules. Work over the last 20 years has identified and characterized many of the molecules needed for mitosis, and we may be close to a complete list in some systems. Despite this progress, surprisingly little is known about the underlying mechanical principles that govern the assembly and function of the spindle. Here we review current biophysical understanding, with a focus on force and position in animal spindles; we refer the reader elsewhere for molecules [1
Metaphase, the state in which paired sister chromosomes balance at the center of the spindle, is a natural starting point for a consideration of spindle biophysics because it is a stable steady-state. The metaphase spindle is highly dynamic, with large fluctuations and directed fluxes in both physical and chemical processes, yet the average amount and position of all spindle components is constant over time. The stability of this steady-state is evident from the remarkable ability of metaphase spindles to correct transient fluctuations in morphology and position (), and to recover from transient physical and chemical perturbations (e.g. [2
Figure 1 Three steady-states in position are reached during metaphase. Position-dependent forces (black arrows) must help reach the steady-state positions and correct any deviations (fainter colors) from them. A) During symmetrical cell division, the spindle (green) (more ...)
The spindle is made of molecules (mostly proteins, but see [10
]) and subject to chemical influences, but here we will view it as an intrinsically mechanical object. Mechanical forces help assemble the spindle [12
], move chromosomes within it [13
], stabilize [15
] and correct [16
] the attachment of chromosomes to microtubules, and regulate anaphase entry [17
]. Spindle forces are generated by molecular motors, microtubule assembly dynamics, elastic elements and friction (); because the structure is at steady-state, the action of these forces on any mechanically independent spindle component must integrate to zero. A notable aspect of most integrated spindle forces is that they are position-dependent, which is required for them to position objects in specific places. At least three positioning tasks are accomplished to generate the metaphase spindle: the spindle positions within the cell (), typically near the center of the longest axis [18
]; the chromosomes align at the center of the spindle (), generating the arrangement called the “metaphase plate” [21
]; the poles position a certain distance from each other (or perhaps from the chromosomes), determining spindle length ().
Figure 2 Microtubule architecture and dynamics in the mitotic metaphase spindle of mammalian cells. A) Architecture of the mammalian mitotic spindle: microtubules (green), sister chromosomes (blue) and kinetochores (red) for attachment of chromosomes to microtubules. (more ...)
The shape of the spindle and its likely filamentous organization was described by Flemming more than 125 years ago [22
]. Polarization microscopy in the 1950s proved that spindles are built from filaments that run parallel to the direction of chromosome motion, which we will call the spindle axis [23
]. Rapid assembly and disassembly of these filaments in response to physical and chemical perturbations lead Inoué and Sato to propose that their polymerization dynamics produce mechanical force, for example to power chromosome motion [7
]. The filaments were identified as microtubules, non-covalent polymers of the protein tubulin, by a combination of biochemistry, pharmacology and electron microscopy [24
]. Today, we know that the main structural element of the spindle is a lattice of oppositely oriented microtubules () that undergo rapid polymerization and depolymerization powered by GTP hydrolysis. Spindle microtubules are organized in space, and their dynamics are regulated by proteins that include motor proteins [26
] and microtubule-binding proteins [27
]. We will use the term “motor protein” to refer to molecules in the kinesin and dynein families that use ATP hydrolysis energy to walk along microtubules.
These generate sliding force between microtubules and other objects, and play a major role in force production ().
Figure 3 Molecular force generators and their sites of action in the mammalian metaphase spindle. Arrows depict object (square) direction of movement (small arrows) and experienced force (large arrows). A) Microtubules (green) assembling (top) and disassembling (more ...)