# Dividing Line: not so simple division in ctenophores

This wonderful movie has repeatedly popped up into my twitter feed.

It was taken by Tessa Montague and is available here (tweet is here).

The movie is striking because of the way that cytokinesis starts at one side and moves to the other. Most model systems for cell division have symmetrical division.

Rob de Bruin commented that “it makes total sense to segregate this way”. Implying that if a cell just gets cut in half it deals with equal sharing of components. This got me thinking…

It does make sense to share n identical objects this way. For example, vesiculation of the Golgi generates many equally sized vesicles. Cutting the cell in half ensures that each cell gets approximately half of the Golgi (although there is another pathway that actively segregates vesicular material, reviewed here). However, for segregation of genetic material – where it is essential that each cell receives one (and exactly one) copy of the genome – a cutting-in-half mechanism simply doesn’t cut it (pardon the pun).

The error rate of such a mechanism would be approximately 50% which is far too high for something so important. Especially at this (first) division as shown in the movie.

I knew nothing about ctenophores (comb jellies) before seeing this movie and with a bit of searching I found this paper. In here they show that there is indeed a karyokinetic (mitotic) mechanism that segregates the genetic material and that this happens independently of the cytokinetic process which is actin-dependent. So not so different after all. The asymmetric division and the fact that these divisions are very rapid and synchronised is very interesting. It’s very different to the sorts of cells that we study in the lab. Thanks to Tessa Montague for the amazing video that got me thinking about this.

Footnote: the 50% error rate can be calculated as follows. Although segregation is in 3D, this is a 1D problem. If we assume that the cell divides down the centre of the long axis and that object 1 and object 2 can be randomly situated along the long axis. There is an equal probability of each object ending each cell. So object 1 can end in either cell 1 or cell 2, as can object 2. The probability that objects 1 and 2 end in the same cell is 50%. This is because there is a 25% chance of each outcome (object 1 in cell 1, object 2 in cell 2; object 1 in cell 2, object 2 in cell 1; object 1 and object 2 in cell 1; object 1 and object 2 in cell 2). It doesn’t matter how many objects we are talking about or the size of the cell. This is a highly simplified calculation but serves the purpose of showing that another solution is needed to segregate objects with identity during cell division.

The post title comes from “Dividing Line” from the Icons of Filth LP Onward Christian Soldiers.

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