The Structure and Function of Viruses
Viruses are somewhat of an anomaly in that they are technically not classified as a living thing due to their dependence on a host cell in order to reproduce and metabolize. Various groups within the scientific community hold various views as whether or not to classify viruses as living. In order to reproduce, a virus must hijack the internal machinery of a host cell in order to produce more viral particles (Phillips, 2002). Once the cell is infected, the cell now contains the viral genetic information to produce the proteins necessary for viral reproduction and provides the proteins the virus lacks for metabolic activity (Tortora, 2002). The target cell can be classified as infected once the virus has attached to the surface receptors, passed through the cell membrane, and the capsid disassembles within the cell releasing the viral genome (Phillips, 2002). However, depending on the environmental conditions and the type of virus, the events that take place within the intracellular space of the host cell may vary. The virus can be considered "alive" once it has managed to pass through the cell membrane.
Viruses are classified as obligatory intracellular parasites due to their need of a host and inability to produce proteins necessary for reproduction (Tortora, 2002). Most biologists cite this reason for not classifying viruses as living. In order to be classified as a living thing the following criteria must be met; composed of a cell or multiple cells, has the ability to reproduce, conducts chemical processes that provide for metabolic functions, maintains an internal environment, and are capable of passing on traits to offspring (Johnson, 2004).
Viruses are considered to be one of the most diverse and adaptable parasites in the living world. They have adapted to a wide range of environments and can infect an entire range of unicellular and multicellular hosts (Tortora, 2002). This unit will focus on the viruses that infect humans with a smaller emphasis on the viruses that infect specifically bacteria. I am introducing the bacteriophage due it being my favorite viral species because of its distinct structure. By using these two different groups of viruses the students will be able to compare and contrast the unique morphologies associated with the specificity in hosts cells each group is adapted to infecting.
The presence of viruses on the earth is formidable, but their size dwarfs in comparison to the damage they can cause to a population. Viruses are too small to see with the naked eye, or even with conventional optical microscopes; electron microscopy is used to provide images of viruses that allow for scientists to determine their size and structure. Viral size is measured in nanometers, a unit that is one-billionth of a meter. The lengths of various viruses have been measured from 20 to 14,000 nm (Tortora, 2002). The sizes of the largest viruses are equivalent to some of the smallest bacterial cells (Tortora, 2002). I recommend showing students graphics comparing the size of bacterial and animal cells with that of viruses. I have included in my annotated teachers bibliography textbooks and websites that provide such graphics.
Viral structures can be considered quite simplistic in comparison with eukaryotic cells. Although the structure of some viruses appears simplistic with only a protein coat housing the nucleic acid of the virus, there are other viruses with more complexity in their morphology. This unit will not go into these details due to time constraints and the general level of biology this unit is intended to be used at. The protein coat of a virus serves a variety of functions. The protein coat serves as a scaffold that holds the virus together and to act as a barrier to the outside environment. The protein coat also acts to transport the viral nucleic acid to the cells of potential hosts. There are various types of viruses and they are classified into various species based on their protein coat, the type of nucleic acid they contain, and the functional role the virus plays within an ecosystem (Tortora, 2002).
Viruses are further classified by the presence of an envelope or lack of an envelope surrounding their protein coat. The term virion is used to refer to developed viral particles that contain nucleic acid encased by a capsid (Tortora, 2002). The properties and composition of the protein coat or capsid is significant in classifying viruses. The capsid is composed of capsomeres, which are proteins that form the distinct shape of a virus (Tortora, 2002). It is possible to have more then one type of capsomere forming the capsid. The shape of a virus is determined by the nucleic acids that it is protecting and shuttling. The envelope surrounding the capsid may be formed from when the viral particles lyse through the plasma membrane of its host cell. The viral capsid ends up with an envelope composed of lipids, carbohydrates, and proteins, which are macromolecules that were components of the host cell's plasma membrane.
The viral capsid can come in many different forms and viruses are placed in different categories based on whether or not their capsid has a helical, polyhedral, enveloped, or complex structure (Tortora, 2002). The polyhedral and helical viruses can also be enveloped as well. The Ebola virus is an example of a helical virus with a cylinder in the shape of a helix housing its nucleic acid. The Adenovirus or common cold is an example of a polyhedral virus with an icosahedron shaped capsid. The
or flu is an example of an enveloped virus with spikes (Tortora, 2002). Bacteriophages are an example of a complex virus with a mix of the various capsid structures. The capsid of a bacteriophage has a polyhedral shape attached to a helical sheath attached to a base plate with pins and tail fibers attached (Tortora, 2002). The examples I have chosen to give of each capsid type are a few of the more commonly known viruses that students may be familiar with and may catch their attention if they are falling asleep in class! These are also the shapes that I want the students familiar with in order to construct their own virus during a hands-on activity that will help solidify the key characteristic structures of a virus.
Another distinguishing characteristic that some enveloped viruses have are spikes that are composed of a carbohydrate-protein complex (Tortora, 2002). These spikes function to provide a method of attaching the viral particles to a host cell's membrane. There are also viruses that lack an envelope with only the capsid serving to protect the viral genome and act as the docking site for the virus to specific host cells. The surface proteins of viruses will be highlighted throughout this unit as these proteins provide the means by which viruses attach to host cells in order to infect the cell by inserting their nucleic acid.
Viral Genetic Material
The genetic material of a virus can be composed of RNA or DNA. The nucleic acids of viruses are imperative to the existence of viruses as they are the blueprints for producing the protein components of various viral particles. Viral genetic material indicates an evolutionary connection to the prokaryotic and eukaryotic cells as they both use the same nucleic acids and code to retain their genetic information. However, viruses are unique in that some types carry RNA as the genetic material instead of DNA. Viral DNA and RNA can either be single stranded or double stranded (Tortora, 2002). This could be a point of confusion for students as they will be familiar with the way in which these two nucleic acids exist in prokaryotic and eukaryotic cells. Due the possible confusion that could arise from this distinction between viruses and cells I only highlight viruses that store their genetic material as either DNA or RNA throughout the unit.
Students should have a familiarity with the two types of nucleic acids at this point in the school year and this is a good time to review with the students the components of a nucleotide and the distinct qualities of DNA and RNA. The students should also review DNA replication and the process of making a protein through transcription and translation starting with the DNA base sequence. This review will help to highlight how viruses that store their genetic material as RNA (such as HIV) must use the enzyme reverse transcriptase to convert their RNA-based genes into DNA. During the transfer from RNA to DNA massive mutations take place in the base sequences due to the lack of proofreading that takes place during this process. This is why viruses containing RNA are considered sloppy during replication due to their lack of proof reading enzymes such as DNA polymerase (Campbell, 2004). The rate of mutation that takes place within a viral genome provides one of the largest obstacles for scientists to effectively prepare a vaccine against a virus such as Human Immunodeficiency Virus (HIV).