Viruses are not classified as living organisms because they do not posess the properties of life as discussed in chp. 1. It has only been in the past 50 years that we have begun to identify, understand and attempt to control the little chemical particles contained in protein coats called viruses. This section of material will discuss the origin of our understanding about viruses and how we are presently attempting to deal with them.

    Biologist first suspected the existance of viruses or something other than bacteria in the Tobacco Mosaic Disease which caused blotchy leaves and stunted growth of tobacco plants in the earyl 1900's. The porcelain filter  strain was used to show that whatever was causing the disease was smaller than bacteria. The porcelain filter was known to filter out even the smallest bacteria, so as extracts were produced from the filtering process, if it produced the disease, then it must be something smaller than bacteria. But what was it.

    Wendell Stanley, in 1933 discovered the true nature of viruses. In attempting to purify an extract of the tobacco mosaic disease, he found the extract formed crystals, which is a property of chemicals. These crystals were able to infect healthy tobacco plants, and were therefor as Stanley concluded the virus itself.

    Within a few years, scientist were able to disassemble the TMV ( Tobacco Mosaic Virus) and confirm Stanley's conclusion. In fact they found that TMV is made of only two kinds of molecules; a nucleic acid RNA and a protien coat.

    Virus-means poision in latin and a virus is a strand of nucleic acid (RNA or DNA) covered by a protein material and can infect cells and replicates( reproduces) within them. Most viruses are DNA viruses, but many plant viruses and some animal viruses such as HIV are RNA viruses. There are several parts to a virus;

    Capsid- a protein sheath, that surrounds the core of genetic material in a virus.
    Envelope- This is a membrane that surrounds the capsid and helps the virus gain entry into the cells because it contains proteins, lipids and glycoproteins that come from the host.
 

    Viruses lack the enzymes for metabolism( breaking down energy) and have no ribosomes for protein production, therefore must rely  on living cells for reproduction.
Therefore viruses are obligate intracellular parasites, which means they have to have a host cell to operate or take on properties of living organsisms.
There are several ways viruses enter host cells. We will discuss a couple for you below.
        Injection- some viruses are constructed like a hypodermic needle and they will inject their genetic material into the host cell. Once inside the virus interrupts the activities of the host cell and the host cell begins working for the virus.
        Endocytosis- in this process the host cell engulfs or surrounds the virus with its cell membrane and takes it to the cytoplasm of the host cell.

    Once the virus enters the host cell it becomes a pathogen ( an agent which causes a disease).

    Viruses are cell specific which means that they will only attack or take over certain types of cells. Why are viruses cell specific.
    This is due the the chemical affinity  between the virus and the host cell. By chemical affinity, we mean that the glycoprotein material that makes up the capsid of the virus and the cell membrane of the host cell are so simialar, which allows the virus the ablility to be able to attatch to the host cell. Remember if the virus does not attatch to the host cell, it cannot enter the host cell. Most scientist feel that viruses are escaped fragments of host cells, or that they were once a part of the host cell that they now infect. This is why they may be so similar to each other, the host cell and the virus that is.

    Bacteriophages are viruses that infect bacteria, and are used to show us the various stages that viruses go through in controlling host cells. These stages are called the Lytic and Lysogenic Life cycles of viruses.

    The Lytic cycle is for virulent viruses. Virulent- means a virus will cause disease right away within the host organism. There are basically 5 stages to this cycle as explained below.
    Adsorption- This is when a virus attatches to the host cell membrane. It does this by means of chemical affinity as explained above.
    Entry- This is the process of the virus entering the host cell by either injection or endocytosis.
    Replication- The host cell will be instructed to take over the production of new viral parts. Basically the host cell produces the viral nucleic acids and proteins to form new capsids or envelopes.
    Assembly- Here is where the cell helps in putting the newly assembled parts together to produce the new viruses.
    Release- During this phase the virus will cause the host cell membrane to rupture which releases the new viruses so they may infect other host cells or the new virus particles will be released through the cell membrane. This causes the destruction of the host cell which is what causes the disease characteristics in most cases.

Viruses Taking Over the Host Cells

Many DNA viruses will go to the host cell nucleus and inactivate the host cell DNA so the host cell will be working for the viral DNA.

Other DNA viruses will controll cell organelles  while it is in the cell cytoplasm, so the host cell is working for the virus as well as the cell itself.

Other viruses are called Retroviruses- which are RNA viruses that use a  enzyme called Reverse Transcriptase  which converts Viral RNA into Viral DNA. Then the viral DNA will take over the operations of the cell.