How the Motor of Life Pulls DNA into Two Identical Sets During Mitosis

In his research, Forth uses an optical trapping technique to quantify the forces exerted by components of cellular structures like the mitotic spindle.

Many key cellular processes rely on the physical interplay between biological components to perform functions. The “parts list” in these processes is similar: Microtubules, semi-rigid tubes of protein, can serve within the cell as scaffolding, roadways, and a building material for machinery; some proteins serve as fasteners, binding and releasing other materials; and motor proteins use chemical energy to push and pull materials along microtubules, or move the microtubules themselves.

To form the mitotic spindle, two organelles called centrosomes move into position on opposite sides of two identical sets of chromosomes massed near the center of the cell. From each of the centrosomes, a dense network of microtubules is assembled, reaching toward and around the mass of chromosomes. Some of the microtubules connect to the chromosomes, while others connect the two centrosomes, forming a cage around the chromosomes. Ideally, microtubules from each centrosome connect exclusively with one of each of the chromosomes in the set. Then the microtubules – aided by proteins and motor proteins – begin to shorten and move, pulling the chromosomes toward the centrosomes, until the two sets have been separated.

To learn more about the mechanics, Forth investigates the forces exerted within subsections of the spindle, using simple “building blocks” composed of a single type of protein and microtubules. For example, the protein PRC1 binds with microtubules, creating bundles of microtubules and cross-links between them. PRC1 works along with a motor protein, which slides microtubules, and PRC1 slows the motion of that slide through resistance. If PRC1 is knocked out of place, chromosomes will move too quickly apart from each other, foiling the intricate timing of mitosis.

“Within this simple system, we have both a pushing force and a resisting force, and the question is – how do these forces add up to move the microtubules at the proper rate?” Forth said. “This is something we can measure in my lab.”

To do so, Forth takes advantage of the relationship between a polystyrene bead and laser light. When a beam of light is focused on a polystyrene bead, the bead becomes polarized and is attracted to the field of the light beam. Any attempt to move the bead out of position bends the light, and the light pushes back on the bead in turn. Changes in the light’s direction can be measured with a photodetector and converted into a measure of physical force. By attaching cellular structures – like a PRC1 and microtubule assembly – to the polystyrene bead and allowing the structure to function, Forth can measure the physical force generated by the structure.

“The idea is to build key features of the spindle, and measure those forces,” Forth said. “Can we watch and see what’s going on with a given set of proteins, and can understanding the mechanics help us understand how the cell is functioning? That’s the goal.”