Think back to sophomore biology and recitations of all the six phases of mitosis. Recall the onion peel lab where you were able to see cells in varying states of mitotic division.
Remember the one phase in particular -- metaphase -- where the chromosomes lined up perfectly in the center of the cell before being pulled to the sides when cytokinesis -- cell division -- took place.
You may not have wondered what caused these chromosomes to line up just right, but scientists sure did. For the past 50 plus years, researchers have been trying to figure out what internal signals were at work to cause the chromosomes to line up along a center axis.
Tomomi Kiyomitsu with the Whitehead Institute of MIT found that it is linked to a motor protein called dynein, which usually "walks" molecular components of the cell along microtubules to a given destination. In the case of mitosis, the dynein is stationary -- to an extent -- "[acting] as winch to pull on the spindle pole, and the microtubules and chromosomes attached to it". You may remember that at each end of the dividing cell is a spindle pole from which extends microtubules that attached to the chromosomes and direct their movement. Eventuall, these spindles will help pull the duplicated chromosome apart so each cell gets its own full set of DNA.
Kiyomitsu saw that when the spindle comes close to the edge of the cell, the dynein is signaled to hop off, move to the other end of the cell and pull from this other side. This switch up continues until the spindle aligns in the center. Kiyomitsu also found that dynein can also only attach certain points near the chromosome, contributing to its alignment.
"People have been looking at these proteins and players in mitosis for decades, and no one ever saw what Tomomi observed," says Whitehead Institute Member Iain Cheeseman. "And it's very clear that these things are happening. These are very strong regulatory paradigms that are setting down these cell division axes. And careful cell biology allowed him to see that this was occurring. People have been looking at this for a long time, but never with the careful eyes he brought to it."
While you may not consider alignment of the chromosomes along the axis of the utmost importance in cell division, it actually is. An improperly divided cell can lead to cell death, disorders and cancer. Understanding what exactly helps the chromosomes align for proper division could help researchers with treatments for certain disorders in the future.
"The spindle orientation is critical for maintaining the balance between stem cells and mature cells during development," Kiyomitsu said in a statement. "And if this orientation becomes dysregulated or misregulated, it is reported that this may contribute to causing cancer even if chromosomes are properly segregated."