Beyond the code: the mechanical properties of DNA as they relate to mitosis

As students of mitosis, we seek to identify the DNA and protein components required for chromosome segregation and to design experiments in order to test our latest theory on how these components fit together. The identification of what is approaching a complete parts list has provided major advances in the past few years. The problem on the horizon is to understand how the parts fit together to segregate the complete genetic complement to daughter cells with the accuracy required to form complex organisms. Historically, science is guided by our observations, thus much of our intuition stems from how we interact with our environment. We make analogies based on our experiences and use these to help us begin to think about processes inside a cell. The problem lies in that life inside a cell is nothing like our life. There are several physical concepts to consider as we delve into the life of a cell. One is the sense of scale. As we think small (i.e., micrometers), consider that structures do not scale in linear proportion upon miniaturization. As described in a classic lecture by Richard Feynman (There’s plenty of room at the bottom, 1959 in Engineering and Science, Caltech, 1960), if you build a billion structures at 1/4,000 the scale of the original, the sum of the new structure occupies less than 2% of the original volume. This reflects the fact that volume scales to the power of 3 (the volume of a sphere being 4/3πr3). Secondly, molecules stick together. Experimental scientists spend a great deal of time trying to identify binding partners (e.g., yeast two-hybrid, co-immunoprecipitation), to identify functional complexes and deciphering networks. What might be overlooked is that van der Waals forces (0.01–1 kBT) are significant on the force scales inside a cell (see below). For instance, it is important to use the right nut and bolt combination (angle, diameter, pitch) in construction. However, were you to unscrew a nut from a bolt in the cell, you would not have to worry about losing it. The problem will be that you will not be able to get the nut off the bolt. The point is that viscous forces dominate; it is thick and crowded in the cell. Finally, cellular structures are in constant flux. Try to imagine a racecar where the pit crew is following the car around, changing tires, refueling, and hammering bent fenders at 200 mph. This is what it is like in the cell. Our challenge is to design experiments to test mechanisms that move billions of nucleotides on timescales of a few minutes in a very small world. What is it really like to segregate a chromosome? If there is no weight, what does it mean that the chromosome is long? How does this influence our thinking about the mechanism of mitosis? How much force is required to segregate a chromosome? Why does not a chromosome fragment as it gets dragged across the cell? In this review, we consider mitosis in a world very foreign to us. We need to know the composition of the materials, the scale of force required, and how these parameters shape questions for future studies. The review is focused on chromosome segregation in budding yeast where we know the exact number of kinetochore microtubule attachments per chromosome (one) and the number of spindle microtubules and can emphasize the quantitative properties of the system. It is likely that the principle derived from this simple system will lead the way for understanding complex kinetochores.