in viro DNA condensation (6th of May 2011)

In a paper just published in Biophysical Journal I proposed a model for condensation of a DNA molecule in a virus (bacteriophage) and I solved it numerically. The paper >> can be downloaded HERE. DNA molecule can be condensed by application of different chemicals, small peptides, polyethylene glycol (PEG), spermine, and other highly positively charged ions. In such a, condensed, state, the separation between the DNA strands is smaller than in the usual, non-condensed DNA state. Therefore, the total volume occupied by the DNA gets reduced.

The condensation process can take place also in the interior of a virus, and in that case, it is complicated by the fact that the viral DNA and proteins are in a direct contact. An interesting question is that of the shape that the condensed DNA adopts. We concluded that the DNA is toroidally ordered, but also that the cross-section of the toroid depends on the nature of the condensing medium (PEG or spermine). In our interpretation, there is an effective modification of DNA-protein interactions brought about by the condensing medium. The DNA-protein interaction becomes very attractive in the case of condensation with PEG, while the same effect cannot be seen in the case of spermine induced condensation.

In the image above, in the two "Polaroid photographs", experimental cryo-microscopic images of viruses are shown in the two cases of condensation. In the PEG induced condensation, the DNA toroid "adheres" to the virus protein coating, so that in the cross-section it looks like a crescent moon. The two spheres with the toroids inside them are results of my numerical calculation whose details are specified in >> the published paper. As can be seen, the model predicts shapes similar to those observed experimentally, and the shape of the crescent moon is obtained in the case of strong interaction between the DNA toroid and virus proteins.

The image which opened this post was made as a proposal for the title page of Biophysical Journal, but the editorial board didn't choose it. The image shows the "model viruses" on the table, next to "Polaroid photographs" made on the basis of experimental, microscope images. There are also "theoretical sketches" of viruses, blueprints for 3D models (see the image below). One can also see an old-fashioned technical pen (I had one of those, I don't know whether these still exist), eraser and a ruler.

The paper is a result of the first collaboration of mine with the experimentalists researching viruses. That is also the first paper I published in Biophysical Journal.

I have already wrote about DNA toroids, outside viruses, in >> a previous post, more than a year ago (...).

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Last updated on 6th of May 2011.