Prepared by Max Darnell - AP225 Fall 2011
Viruses are very small biological constructs which contain either DNA or RNA. As they lack cellular machinery and rely on an infected cell to actually replicate their viral genomes, there is debate as to whether viruses should be considered "living." A virus consists of three main parts.
1) Genetic Material - This can be either DNA or RNA. Upon a viral infection, the virus inserts its genome into the host cell, where it is processed by various polymerases.
2) Protein capsid - this is a simple protein "shell" which envelops the genetic material and gives the virus structure.
3) Coat - there may exist certain proteins or lipids on the surface of the virus that identify the virus and aid in receptor binding to the cell surface. These surface modifications to the virus can induce an immune response in the host organism.
The figure to the right is a schematic of a bacteriophage (virus that infects bacteria) inserting its DNA . Viral replication is essentially a positive feedback loop in which the viral genome is replicated inside a cell, and that genome encodes for the capsid and envelope proteins that can make a new virus.
There are many variations of viruses, many of which can be leveraged. For instance, the shape of the capsid can fall into the following categories: helical, icosahedral, prolate, envelope, or complex. The genetic material can be either one or two-stranded. A very important feature of certain viruses is whether the viral genome integrates into the host genome (as in a lentivirus) or whether the information is processed transiently (as in an adenovirus). 
Applications/Connections to Soft Matter
In medicine, viruses are modified and then used as vaccines, where they elicit an immune response and confer immunological "memory" without being infectious. In molecular biology, viruses are used as a means of delivery for genetic material. There are currently a number of clinical trials using viruses as the delivery vehicle for gene therapy.
There is growing interest, however, in viruses as a nanotechnology building block. The ability of viruses to enter cells is valuable if one considers a virus as a biologically-relevant nanoparticle. Other examples include Angela Belcher's lab at MIT using viruses as a structural component of new bio-based batteries, as well as using viruses as a building block for self assembly[2,3].
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Blum AS, Soto CM, Wilson CD et al. (2005). "An Engineered Virus as a Scaffold for Three-Dimensional Self-Assembly on the Nanoscale". Small 7: 702.
Neltner, B., Peddie, B., Xu, A., Doenlen, W., Durand, K., Yun, D. S., Speakman, S., et al. (2010). Production of hydrogen using nanocrystalline protein-templated catalysts on m13 phage. ACS nano, 4(6), 3227-3235.