Surprises in Cell Division

As promised, this summary as well as this article are shorter than those of my previous post. So, without further ado...

A research group at the California Institute of Technology has been able to capture the first three-dimensional image of cell whose nucleus is dividing in high resolution. A new method of cell slicing and imaging were used that allowed for the image to be obtained. While conventional electron microscopy involves much handling of the specimen prior to viewing, a technique called electron cryptomography, or ECT, was used trap the specimen in a more native manner, encasing it in a thin layer of ice. A limitation is that ECTs require the sample to be 500 nanometers thick at the most. The smallest known eukaryote was used and even that needed to be sliced carefully using a machine and a diamond knife.

Now that we know why what the researchers at CalTech did allowed them to capture the image while others previously could not, let's move on to the cell division aspect of this. I will refrain from going in depth regarding how mitosis works, as we discussed this quite thoroughly in class. During metaphase, the sister chromatids line up at the center of the nucleus, where they are held by microtubules in the form of a mitotic spindle. In most fungi, plants, and animals, more than one microtubule attach to each chromosome prior to the separation of the chromosome sets. In the image taken by the researchers at CalTech, a cell with 20 chromosomes is present, but only 10, somewhat incomplete microtubules were found. I was quite surprised when I read this because in all the pictures in our bio books and the pictures we drew in class, there was at least one kinetechore microtubule per sister chromatid set.
This is sort of how I have always pictured mitotic metaphase:

Note that I pictured it with one microtubule per sister chromatids

This leads to the question of how the sister chromatids separated. The researchers hypothesized that perhaps the chromosomes bundled up so that they could be separated by a fewer number of microtubules. Further studies using this slicing and imaging technique might lead to new discoveries in several kinds of cells, including human cells.

The top image shows multiple microtubules per set while the bottom picture show the bundling of chromosomes idea that fits with the information found in the study

Click here to view the article.