Virus: Like DNA in a Hard Plastic Shell
A European team of biophysicists studied the mechanical properties of a virus and found the shell, made of protein, to act like hard plastic. Writing in PNAS,1 they described the coat of a bacteriophage they studied:
The protective proteinaceous shells (capsids) of viruses are striking examples of biological materials engineering. These highly regular, self-assembled, nanometer-sized containers are minimalistic in design, but they combine complex passive and active functions. Besides chemical protection, they are involved in the selective packing and the injection of the viral genetic material.
The capsids look like oblong, geometric shapes with pointy ends. The DNA is packed inside under pressure, and the coat can withstand indentations of 30%. “The measured Young’s modulus,” they found, “is comparable with that of hard plastic.” They seemed to admire the little cases: the bacteriophage capsid is
remarkably dynamic yet resilient and tough enough to easily withstand the known packing pressure of DNA (~60 atmospheres). These capsids, thus, not only provide a chemical shield but also significant mechanical protection for their genetic contents. Viral shells are a remarkable example of nature’s solution to a challenging materials engineering problem: they self-assemble to form strong shells of precisely defined geometry by using a minimum amount of different proteins.
The team is looking at these miniaturized packages for inspiration in the burgeoning field of nanotechnology.
1Ivanovska et al., “Bacteriophage capsids: Tough nanoshells with complex elastic properties,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0308198101, published online before print May 7, 2004.
Here is observational evidence that leads to interesting questions. It shows that living things need to overcome the same kinds of physics problems that engineers face. Yet viruses are not, by definition, alive; they rely on a host for replication. How could such precision bio-nanotechnology evolve? Why do viruses exist? Did they ever have a beneficial role, considering that the vast majority are harmless? We may never be able to explain such things completely, but we can marvel at the biophysics capabilities found in nature, and deduce that such things don’t just happen. Now read about the little motor that packs the contents (see 10/18/2001 headline).