Much of the information about virus structures is highly visual in nature and is difficult to represent adequately in print. It is strongly recommended that the reader view the virus structure resources on the accompanying CD. Also, Figure 2.1 illustrates the approximate shapes and sizes of different families of viruses.
Why bother to form a virus particle to contain the genome? In fact, some infectious agents, such as viroids, do not (see Chapter 8); however, the fact that viruses struggle with the genetic and biochemical burden entailed in encoding and assembling the components of a particle indicates that this strategy must offer some positive benefits. At the simplest level, the function of the outer shells of a virus particle is to protect the fragile nucleic acid genome from physical, chemical, or enzymatic damage. After leaving the host cell, the virus enters a hostile environment that would quickly inactivate the unprotected genome. Nucleic acids are susceptible to physical damage, such as shearing by mechanical forces, and to chemical modification by ultraviolet light (sunlight).The natural environment is heavily laden with nucleases either derived from dead or leaky cells or deliberately secreted by vertebrates as defence against infection. In viruses with single-stranded genomes, the breaking of a single phosphodiester bond or chemical modification of one nucleotide is sufficient to inactivate that virus particle, making replication of the genome impossible. How is protection against this achieved? The protein subunits in a virus capsid are multiplyredundant (i.e., present in many copies per particle). Damage to one or more subunits may render that particular subunit nonfunctional, but rarely does limited damage destroy the infectivity of the entire particle. This makes the capsid an effective barrier.
The protein shells surrounding virus particles are very tough, about as strong as a hard plastic such as Perspex® or Plexiglas®, although, of course, they are only a billionth of a metre or so in diameter; however, they are also elastic and are able to deform by up to a third without breaking. This combination of strength, flexibility, and small size means that it is physically difficult (although not impossible) to break open virus particles by physical pressure.
The outer surface of the virus is also responsible for recognition of and the first interaction with the host cell. Initially, this takes the form of binding of a specific virus-attachment protein to a cellular receptor molecule. However, the capsid also has a role to play in initiating infection by delivering the genome in a form in which it can interact with the host cell. In some cases, this is a simple process that consists only of dumping the genome into the cytoplasm of the cell. In other cases, this stage is much more complex; for example, retroviruses carry out extensive modifications to the virus genome while it is still inside the particle, converting two molecules of single-stranded RNA to one molecule of doublestranded DNA before delivering it to the cell nucleus. Hence, the role of the capsid is vital in allowing viruses to establish an infection.
To form infectious particles, viruses must overcome two fundamental problems. First, they must assemble the particle utilizing the information available from the components that make up the particle itself. Second, virus particles form regular geometric shapes, even though the proteins from which they are made are irregularly shaped. How do these simple organisms solve these difficulties? The solutions to both problems lie in the rules of symmetry.
Figure 2.1 A diagram illustrating the shapes and sizes of viruses of families that include animal, zoonotic, and human pathogens. The virions are drawn to scale, but artistic license has been used in representing their structure. In some, the crosssectional structures of capsid and envelope are shown, with a representation of the genome. For the very small virions, only their size and symmetry are depicted. (Courtesy of F.A. Murphy, School of Veterinary Medicine, University of California, Davis.)