Viral Structure

There are problems with these three hypotheses and it is currently believed that virus particles are extremely ancient and pre-date the divergence of life into the three domains we use to define living things. Genetic analyses of virus particles have so far indicated that there is probably no single ancestral virus particle and that they sprung up several times in order to have multiple ancestral organisms, rather than just one.

Prions are related to viruses but are not life at all because they don’t contain RNA or DNA. These are infectious protein molecules. There are many protein-prion diseases, such as “mad cow disease” or bovine spongiform encephalopathy, scrapie in sheep, chronic wasting disease in deer, kuru in humans, and Creutzfeldt-Jakob disease in humans.

Whether or not viruses can be considered life is a matter of debate. They have genetic material and genes, they are known to evolve through natural selection, and can reproduce by creating multiple viral particles. They don’t, however, have a cellular structure or their own metabolism. They also don’t divide through mitosis and cell division but through the building up of their structure within a cell. They don’t, however, grow like crystal structures because they can inherit genetic mutations (and undergo natural selection).

VIRAL STRUCTURE

Viruses have many different viral structures. Most are extremely small and cannot be seen via typical light microscopy. Many are between 20 and 300 nanometers in diameter; however, some are long and thin. As mentioned, they have a capsid or protein coat, which is made by different capsomeres or individual protein subunits. These capsomeres are made by the genetic material of the virus but require the protein-making structures of the cells that they infect. Some viruses will make proteins that assist in the making of their protein coat and other structures.

There are several different virus shapes, including the following:

• Helical—these are viruses made from a single capsomere that are stacked around a central axis in order to form the structure of a helix. There may or may not be a central cavity (or tube). These can be short and roughly cylindrical or very long and filamentous. These are made often from single-stranded RNA but can be single-stranded DNA. The nucleic acids are negatively charged and are attracted to the capsid by the fact that the capsid is positively charged. The length of the virus depends on how long the nucleic acid chain necessarily must be covered. Figure 7 shows what a helical virus particle looks like:

• Icosahedral—these are roughly spherical, closing the virus particle within a closed structure. These have multiple triangular faces, many with more than sixty capsomeres per triangular face (with the minimum number of capsomeres per face being three). Those with large triangular faces appear spherical. Figure 8 shows a viral particle that is icosahedral in shape:

Viruses that have icosahedral structures are only released into the environment when the infected cells die off and release viral particles only when the cell itself breaks down. Examples of this include the adenovirus, poliovirus, and rhinovirus.

• Prolate—this is an elongated icosahedral viral particle that is commonly seen as part of bacteriophage heads. This is basically a cylinder that has a cap at either end. It is still icosahedral but is elongated with faces that are not the same size.

• Envelope—Certain viruses have membranes around them. These could be envelopes that come from the cell membranes of the cells they have infected, from the nuclear envelope (the lipid layer around cellular DNA) or from the lipid making up the endoplasmic reticulum (an organelle inside cells). This will create a viral envelope that has proteins on it that come from the host and from the virus itself; this same viral envelope will contain carbohydrates that come from the host. The envelope, used in the influenza virus and the HIV virus, is needed for the virus to infect the cells they get inside.

• Complex—these are complicated viral particles that have extra structures on them or a complex outer wall. These include many bacteriophages that have an icosahedral head, a helical tail and protein-containing tail fibers. The tail is

responsible for injecting the viral genome into the cell. Figure 9 shows a complex-type bacteriophage:

Poxviruses are complex, having a centralized disc structure called a nucleoid. It is a membrane-bound virus that has two lateral bodies (paired bodies) that have no known function. This is a pleomorphic virus that is roughly ovoid in shape. Figure 10 shows what the poxvirus particle looks like: