The human understanding, from its peculiar nature, easily supposes a greater degree of order and equality in things than it really finds.

Bacon, Novum Organum, I, 45

The self-assembly of bubble rafts, crystals, polymers, and micelles is quite impressive. But, these inorganic systems pale in comparison with nature’s organic self-assembled structures. In this chapter we examine three biological systems that are known to self-assemble and that are at least partially understood. The tantalizing possibility of using self-assembly to create and control intricate functional structures such as the biological structures discussed here is the main reason for the level of excitement surrounding self-assembly today. We begin in Section 3.2 with a discussion of proteins and the protein folding

problem. Proteins are polymers; long chain molecules composed of simple subunits known as amino acids. These basic structures are the most important building block for the cell, and consequently for all of biology. In the last chapter, we studied polymerization; the process by which long chain molecules are constructed. Here, we focus on what happens after the chain has been built - the protein folding problem. It is the ability of proteins to uniquely fold into a wide variety of shapes that makes them such useful nanoscale machines. In Section 3.3 we discuss the tobacco mosaic virus. This was one of the first

biological self-assembling systems to be understood. In 1955, H. FraenkelConrat and R.C. Williams [43] demonstrated that a fully functional active virus could be self-assembled in a test tube from a simple collection of proteins and nucleic acids. While the structure of the tobacco mosaic virus is quite simple, the process by which it self-assembles is subtle and not obvious. Finally, in Section 3.4 we examine the ribosome. The ribosome is one of the

most important intracellular machines. Following the directions of messenger RNA it captures amino acids and builds proteins. That is, it receives information, processes that information, and uses that information to build physical structures. In short, it is a nanoscale production factory. The ribosome has

also been shown to self-assemble. It can be reduced to a collection of 55 proteins and 3 RNA molecules. When placed in a test tube under the proper conditions, these components self-assemble into a fully functional nanoscale machine.